gels
Review
Droplets, Evaporation and a Superhydrophobic
Surface: Simple Tools for Guiding Colloidal Particles
into Complex Materials
Marcel Sperling and Michael Gradzielski *
Stranski Laboratorium für Physikalische & Theoretische Chemie, Institut für Chemie, Technische Universität
Berlin, Straße des 17. Juni 124, Sekr. TC7, D-10623 Berlin, Germany
*Correspondence: [email protected]; Tel.: +49-(0)30-314-24934
Academic Editor: Clemens K. Weiss
Received: 29 December 2016; Accepted: 13 April 2017; Published: 4 May 2017
Abstract:
The formation of complexly structured and shaped supraparticles can be achieved
by evaporation-induced self-assembly (EISA) starting from colloidal dispersions deposited on
a solid surface; often a superhydrophobic one. This versatile and interesting approach allows for
generating rather complex particles with corresponding functionality in a simple and scalable fashion.
The versatility is based on the aspect that basically one can employ an endless number of combinations
of components in the colloidal starting solution. In addition, the structure and properties of the
prepared supraparticles may be modified by appropriately controlling the evaporation process,
e.g., by external parameters. In this review, we focus on controlling the shape and internal structure of
such supraparticles, as well as imparted functionalities, which for instance could be catalytic, optical
or electronic properties. The catalytic properties can also result in self-propelling (supra-)particles.
Quite a number of experimental investigations have been performed in this field, which are compared
in this review and systematically explained.
Keywords:
superhydrophobic surfaces; droplets; nanoparticles; evaporation; self-assembly;
self-propelling; anisometric; colloids; supraparticles; functional materials; catalysis
1. Introduction
Self-assembly of colloids into bigger particles (from
µ
m to mm) has been in the focus of colloid
and material research for some time, as it allows for the fabrication of materials providing various
functionalities [
1
–
3
]. Their production can be accomplished by the utilization of several different
techniques that have been developed over the last few years, such as sedimentation, evaporation,
adsorption, external force fields, bio-recognition or surface and droplet templating, which have
been summarized elsewhere [
4
–
6
]. Especially, considering the ratio of estimated fabrication costs vs.
the added value in terms of provided functionality is a crucial aspect, when it comes to potential
applications or scale-up for industrial production. Hypothetically, this ratio has been estimated by
Velev and Gupta as shown in Figure 1[
4
]. They were also relating principally available techniques,
also those mentioned above, to the accessible dimensionality (1D, 2D or 3D) and scalability of the
procedure. One may state that usually, the lower cost processes are favored for scaling up [
4
]. Besides
the applied technique, also the quality of materials plays an important role. When considering
dispersed particles, especially on the micron- or even nano-meter scale, the efforts needed for their
synthesis may increase dramatically when seeking a high degree of monodispersity, as for instance
required in photonic applications [
7
]. In order to produce high quality photonic crystals [
8
], a defined
control of inter-particle spacing is needed to achieve a high sensitivity for the desired diffracted
wavelength. Thus, this sensitivity depends on the fluctuations in particle size as that determines
Gels 2017,3, 15; doi:10.3390/gels3020015 www.mdpi.com/journal/gels
Gels 2017,3, 15 2 of 27
the quality of the corresponding crystal lattice. This correlation of in-depth lattice spacing with
the resulting diffracted wavelength is expressed by Bragg’s law [
9
]. Moreover, besides pure size
aspects of the assembling particles, also their shapes, either static [
10
–
13
] or dynamic [
14
,
15
] can be of
major importance.
Gels 2017, 3, 15 2 of 26
quality of the corresponding crystal lattice. This correlation of in-depth lattice spacing with the
resulting diffracted wavelength is expressed by Bragg’s law [9]. Moreover, besides pure size aspects
of the assembling particles, also their shapes, either static [10–13] or dynamic [14,15] can be of major
importance.
Figure 1. Hypothetical qualitative estimation of the value-to-price ratio for different products
gathered by colloidal assembly. Adapted with permission from [4] (p. 7), Copyright 2009 Wiley.
There exist multiple ways of producing colloidal assemblies, such as supraparticles, that differ
with respect to dimensionality, cost and scale. Many of these ways have been reviewed by Velev and
Gupta [4]. In general, all of these methods start from confining the colloidal building blocks within a
droplet (or generally a container). This could for example be done using emulsion droplets, where
the solubilized liquid becomes removed by heating and other approaches. However, our main focus
in this work will be set on evaporation-induced self-assembly (EISA) on super-repellant solid
surfaces, typically superhydrophobic ones. In the following sections, we shall give a detailed
overview of the basics of this technique, in terms of the materials applied and the condition
parameters to be set, as well as the most recent materials fabricated by it.
In that context, EISA is a particularly simple and therefore appealing approach, in which one
simply evaporates the solvent of a colloidal dispersion leading to a situation where the ingredients
become more or less arranged during the drying process [13,14], thereby forming self-assembled
nanostructures in a simple way. In this fashion, one may access structured thin films [16,17], but also
supraparticles by the preparation from droplets deposited on a solvophobic surface. Probably the
most common version of the latter are superhydrophobic surfaces applicable to water droplets [18–
22].
In the scope of this review, we will present recent achievements in the field of supraparticle
fabrication using aqueous suspension droplets dispensed and dried on superhydrophobic surfaces,
i.e., using EISA and droplet templating. After some fundamental discussion of EISA and the droplet
templating method, as well as superhydrophobic surfaces, we will show possible ways to create
supraparticles of various shapes, hierarchical structures (like Janus-type) and functionalities.
Furthermore, we will give some general discussion about applications for supraparticles that have
recently been developed. It may be noted that in our review we focus on large supraparticles in the
size range of hundreds of µm or even mm, neglecting the abundant work on smaller-sized
assemblies.
2. Evaporation-Induced Self-Assembly and the Droplet Templating Method
The concept of evaporation-induced self-assembly (EISA) uses the controlled removal of a
volatile component to trigger the defined aggregation of contained materials. This aggregation can
Figure 1.
Hypothetical qualitative estimation of the value-to-price ratio for different products gathered
by colloidal assembly. Adapted with permission from [4] (p. 7), Copyright 2009 Wiley.
There exist multiple ways of producing colloidal assemblies, such as supraparticles, that differ
with respect to dimensionality, cost and scale. Many of these ways have been reviewed by Velev
and Gupta [
4
]. In general, all of these methods start from confining the colloidal building blocks
within a droplet (or generally a container). This could for example be done using emulsion droplets,
where the solubilized liquid becomes removed by heating and other approaches. However, our main
focus in this work will be set on evaporation-induced self-assembly (EISA) on super-repellant solid
surfaces, typically superhydrophobic ones. In the following sections, we shall give a detailed overview
of the basics of this technique, in terms of the materials applied and the condition parameters to be set,
as well as the most recent materials fabricated by it.
In that context, EISA is a particularly simple and therefore appealing approach, in which one
simply evaporates the solvent of a colloidal dispersion leading to a situation where the ingredients
become more or less arranged during the drying process [
13
,
14
], thereby forming self-assembled
nanostructures in a simple way. In this fashion, one may access structured thin films [
16
,
17
], but also
supraparticles by the preparation from droplets deposited on a solvophobic surface. Probably the most
common version of the latter are superhydrophobic surfaces applicable to water droplets [18–22].
In the scope of this review, we will present recent achievements in the field of supraparticle
fabrication using aqueous suspension droplets dispensed and dried on superhydrophobic surfaces,
i.e., using EISA and droplet templating. After some fundamental discussion of EISA and the
droplet templating method, as well as superhydrophobic surfaces, we will show possible ways
to create supraparticles of various shapes, hierarchical structures (like Janus-type) and functionalities.
Furthermore, we will give some general discussion about applications for supraparticles that have
recently been developed. It may be noted that in our review we focus on large supraparticles in the
size range of hundreds of
µ
m or even mm, neglecting the abundant work on smaller-sized assemblies.
2. Evaporation-Induced Self-Assembly and the Droplet Templating Method
The concept of evaporation-induced self-assembly (EISA) uses the controlled removal of a volatile
component to trigger the defined aggregation of contained materials. This aggregation can either occur
Gels 2017,3, 15 3 of 27
due to continuously limiting the available space for the dispersed components or simply due to their
increasing concentration. One example for the latter can be found within mixed surfactant systems,
in which the starting solution has a concentration below the cmc (critical micelle concentration),
which is surpassed during subsequent evaporation of the solvent. This leads to spontaneous formation
micellar aggregates [
23
,
24
]. In this article, we shall focus on systems of colloidal dispersions containing
insoluble particles, which are forced to assemble thereby potentially promoting colloidal crystallization
via EISA [5,8,25].
At first glance, a simple system to do so is represented by droplets with suspended colloidal
components. These droplets can either float in an immiscible second liquid, e.g., water droplets in
fluorinated oil, or be dispensed on a solid surface. Considering the latter, the resulting assemblies
strongly depend on the type of surface used, as the interaction between the droplets’ liquid phase
and the solid substrate may vary substantially. Therefore, the wetting properties constitute the
predominant influencing parameter, characterized by adhesion forces and contact angle (CA). The CA
for a droplet deposited on a solid substrate depends on the interfacial tensions between the different
phases (Figure 2) and is given by Young’s equation (Equation (1)), with
θ
being the CA and
γ
the
interfacial tension between the solid (s), gas (g) and liquid (l) phases, respectively.
cos θ=γsg −γsl
γlg
(1)
Gels 2017, 3, 15 3 of 26
either occur due to continuously limiting the available space for the dispersed components or simply
due to their increasing concentration. One example for the latter can be found within mixed surfactant
systems, in which the starting solution has a concentration below the cmc (critical micelle
concentration), which is surpassed during subsequent evaporation of the solvent. This leads to
spontaneous formation micellar aggregates [23,24]. In this article, we shall focus on systems of
colloidal dispersions containing insoluble particles, which are forced to assemble thereby potentially
promoting colloidal crystallization via EISA [5,8,25].
At first glance, a simple system to do so is represented by droplets with suspended colloidal
components. These droplets can either float in an immiscible second liquid, e.g., water droplets in
fluorinated oil, or be dispensed on a solid surface. Considering the latter, the resulting assemblies
strongly depend on the type of surface used, as the interaction between the droplets’ liquid phase
and the solid substrate may vary substantially. Therefore, the wetting properties constitute the
predominant influencing parameter, characterized by adhesion forces and contact angle (CA). The
CA for a droplet deposited on a solid substrate depends on the interfacial tensions between the
different phases (Figure 2) and is given by Young’s equation (Equation (1)), with being the CA and
the interfacial tension between the solid (), gas () and liquid () phases, respectively.
cos = −
(1)
Figure 2. Scheme describing the contact angle (CA) θ of the liquid droplet on a solid substrate and its
relation to the different interfacial tensions between the solid (s), liquid (l) and gas (g) phase, as related
to each other by Equation (1).
A well-known phenomenon related to drying suspension droplets on solid surfaces at low CA
is the so-called “coffee-ring effect” [26,27]. Closer investigation of the leading mechanism in these
drying spherical cap droplets revealed that convective liquid transport occurs from the center apex
down and outwards to the three-phase contact line (TPCL). This preserves the geometry upon strong
evaporation at the droplet edge, which is driven by pinning of the TPCL due to the contained particles
adhering to the surface, thus prohibiting transversal shrinkage during evaporation. Consequently,
colloidal material accumulates and finally precipitates at the TPCL, leaving a solid ring-like pattern.
Such a pattern can be quite characteristic for the dried sample, e.g., depending on its ionic strength
[28,29]. This effect can for example be of interest for diagnostic purposes when investigating dried
droplets of blood, as shown in Figure 3 [30,31].
The formation of “cracks” such as seen in Figure 3 is always an indication of rather small CAs
and pronounced pinning of the droplets on the substrate. In that context, it might be mentioned that
such cracks have also been observed very pronouncedly for drying droplets containing mixtures of
DNA and small silica nanoparticles (Figure 4). The final solid films showed interesting surface
patterns with unique shapes, which are induced by local segregation of the two colloidal components
of DNA and silica [32]. For more complexly-composed colloidal mixtures, such local segregation is a
further complication that may arise in the EISA process.
Figure 2. Scheme describing the contact angle (CA) θof the liquid droplet on a solid substrate and its
relation to the different interfacial tensions between the solid (s), liquid (l) and gas (g) phase, as related
to each other by Equation (1).
A well-known phenomenon related to drying suspension droplets on solid surfaces at low
CA is the so-called “coffee-ring effect” [
26
,
27
]. Closer investigation of the leading mechanism
in these drying spherical cap droplets revealed that convective liquid transport occurs from the
center apex down and outwards to the three-phase contact line (TPCL). This preserves the geometry
upon strong evaporation at the droplet edge, which is driven by pinning of the TPCL due to the
contained particles adhering to the surface, thus prohibiting transversal shrinkage during evaporation.
Consequently, colloidal material accumulates and finally precipitates at the TPCL, leaving a solid
ring-like pattern. Such a pattern can be quite characteristic for the dried sample, e.g., depending on
its ionic strength
[28,29]
. This effect can for example be of interest for diagnostic purposes when
investigating dried droplets of blood, as shown in Figure 3[30,31].
The formation of “cracks” such as seen in Figure 3is always an indication of rather small CAs and
pronounced pinning of the droplets on the substrate. In that context, it might be mentioned that such
cracks have also been observed very pronouncedly for drying droplets containing mixtures of DNA
and small silica nanoparticles (Figure 4). The final solid films showed interesting surface patterns
with unique shapes, which are induced by local segregation of the two colloidal components of DNA
and silica [
32
]. For more complexly-composed colloidal mixtures, such local segregation is a further
complication that may arise in the EISA process.
Gels 2017,3, 15 4 of 27
Gels 2017, 3, 15 4 of 26
Figure 3. Blood samples dried on a glass surface from: (a) 27-year-old healthy woman;
(b) a person with anemia; (c) a 31-year-old healthy man; (d) a person with hyperlipidemia. Adapted
with permission from [30] (p. 90). Copyright 2011 Cambridge University Press.
Figure 4. (a) SEM image of dried drops for a ratio of silica NPs (diameter: 10 nm)/DNA (20,000 bp)
ratio 1:0.5. The scale bar is 200 µm. (b) HR-SEM images of dried drops for an NP/dsDNA ratio of 1:0.5.
The scale bar is 2 µm. Adapted with permission from [32] (p. 3663). Copyright 2014 Springer Nature.
The process of coffee-ring formation is counterbalanced by the Marangoni effect [33–35].
This effect describes convection processes arising from thermally-induced gradients in the droplet’s
interfacial tension (also called Bénard–Marangoni convection). In addition, convectional flow simply
arising from a decrease in surface temperature due to evaporation is observed, which then in turn
causes density differences (see also Figure 5). Usually, the occurrence of coffee-ring deposits indicates
stronger evaporation closer to the TPCL and low or even absent Marangoni stresses [36]. It is also
reported that experimentally, Marangoni convection in water droplets is often lower than expected
from theoretical simulations, which can be related to the surface contaminants enriching at the water
interface with ease and, hence, lowering the interfacial tension [35,37]. Generally, thermal
conductivity and the latent heat of evaporation are important parameters that determine the extent
of Marangoni flow in an opposing fashion due to the generation of a thermal gradient, but also
concentration gradients of ingredients, e.g., surfactants, may induce such convective flow. However,
for the case of EISA, the thermal gradient-induced Marangoni effect should play the predominant
role. Furthermore, the Marangoni convective flow may also completely reverse the coffee-ring effect,
as reported for octane droplets on glass [35].
The basic concepts of microfluidic flow inside sessile droplets are usually discussed for the case
of a pinned droplet contact line, i.e., the “constant contact radius” (CCR) mode. Of course, one may
also observe the complementary case, in which the droplet’s contact line successively recedes at
constant CA due to the absence of any pinning effects, i.e., “constant contact angle” (CCA) mode. The
details of the evaporation process for the two different modes (CCR and CCA) are described in Figure
Figure 3.
Blood samples dried on a glass surface from: (
a
) 27-year-old healthy woman; (
b
) a person with
anemia; (
c
) a 31-year-old healthy man; (
d
) a person with hyperlipidemia. Adapted with permission
from [30] (p. 90). Copyright 2011 Cambridge University Press.
Gels 2017, 3, 15 4 of 26
Figure 3. Blood samples dried on a glass surface from: (a) 27-year-old healthy woman;
(b) a person with anemia; (c) a 31-year-old healthy man; (d) a person with hyperlipidemia. Adapted
with permission from [30] (p. 90). Copyright 2011 Cambridge University Press.
Figure 4. (a) SEM image of dried drops for a ratio of silica NPs (diameter: 10 nm)/DNA (20,000 bp)
ratio 1:0.5. The scale bar is 200 µm. (b) HR-SEM images of dried drops for an NP/dsDNA ratio of 1:0.5.
The scale bar is 2 µm. Adapted with permission from [32] (p. 3663). Copyright 2014 Springer Nature.
The process of coffee-ring formation is counterbalanced by the Marangoni effect [33–35].
This effect describes convection processes arising from thermally-induced gradients in the droplet’s
interfacial tension (also called Bénard–Marangoni convection). In addition, convectional flow simply
arising from a decrease in surface temperature due to evaporation is observed, which then in turn
causes density differences (see also Figure 5). Usually, the occurrence of coffee-ring deposits indicates
stronger evaporation closer to the TPCL and low or even absent Marangoni stresses [36]. It is also
reported that experimentally, Marangoni convection in water droplets is often lower than expected
from theoretical simulations, which can be related to the surface contaminants enriching at the water
interface with ease and, hence, lowering the interfacial tension [35,37]. Generally, thermal
conductivity and the latent heat of evaporation are important parameters that determine the extent
of Marangoni flow in an opposing fashion due to the generation of a thermal gradient, but also
concentration gradients of ingredients, e.g., surfactants, may induce such convective flow. However,
for the case of EISA, the thermal gradient-induced Marangoni effect should play the predominant
role. Furthermore, the Marangoni convective flow may also completely reverse the coffee-ring effect,
as reported for octane droplets on glass [35].
The basic concepts of microfluidic flow inside sessile droplets are usually discussed for the case
of a pinned droplet contact line, i.e., the “constant contact radius” (CCR) mode. Of course, one may
also observe the complementary case, in which the droplet’s contact line successively recedes at
constant CA due to the absence of any pinning effects, i.e., “constant contact angle” (CCA) mode. The
details of the evaporation process for the two different modes (CCR and CCA) are described in Figure
Figure 4.
(
a
) SEM image of dried drops for a ratio of silica NPs (diameter: 10 nm)/DNA (20,000 bp)
ratio 1:0.5. The scale bar is 200
µ
m; (
b
) HR-SEM images of dried drops for an NP/dsDNA ratio of 1:0.5.
The scale bar is 2 µm. Adapted with permission from [32] (p. 3663). Copyright 2014 Springer Nature.
The process of coffee-ring formation is counterbalanced by the Marangoni effect [
33
–
35
].
This effect describes convection processes arising from thermally-induced gradients in the droplet’s
interfacial tension (also called Bénard–Marangoni convection). In addition, convectional flow simply
arising from a decrease in surface temperature due to evaporation is observed, which then in turn
causes density differences (see also Figure 5). Usually, the occurrence of coffee-ring deposits indicates
stronger evaporation closer to the TPCL and low or even absent Marangoni stresses [
36
]. It is also
reported that experimentally, Marangoni convection in water droplets is often lower than expected
from theoretical simulations, which can be related to the surface contaminants enriching at the water
interface with ease and, hence, lowering the interfacial tension [
35
,
37
]. Generally, thermal conductivity
and the latent heat of evaporation are important parameters that determine the extent of Marangoni
flow in an opposing fashion due to the generation of a thermal gradient, but also concentration
gradients of ingredients, e.g., surfactants, may induce such convective flow. However, for the case of
EISA, the thermal gradient-induced Marangoni effect should play the predominant role. Furthermore,
the Marangoni convective flow may also completely reverse the coffee-ring effect, as reported for
octane droplets on glass [35].
The basic concepts of microfluidic flow inside sessile droplets are usually discussed for the case
of a pinned droplet contact line, i.e., the “constant contact radius” (CCR) mode. Of course, one may
also observe the complementary case, in which the droplet’s contact line successively recedes at
Gels 2017,3, 15 5 of 27
constant CA due to the absence of any pinning effects, i.e., “constant contact angle” (CCA) mode.
The details of the evaporation process for the two different modes (CCR and CCA) are described in
Figure 5. In contrast to the coffee-ring effect, Marangoni convection will occur for both CCR and CCA
modes [
33
,
34
]. It might be added here that it is also possible to use electric fields to manipulate the
wetting behavior of a sessile droplet [38–40].
Gels 2017, 3, 15 5 of 26
5. In contrast to the coffee-ring effect, Marangoni convection will occur for both CCR and CCA
modes [33,34]. It might be added here that it is also possible to use electric fields to manipulate the
wetting behavior of a sessile droplet [38–40].
Figure 5. Schematic description of the different modes observed for a drying droplet on a solid
substrate, with the mass flow of cooled water (blue arrows) and that due to the interfacial tension
gradients (Marangoni flow; red arrows) being indicated for: (a) constant contact angle (CCA) mode,
as observed on most superhydrophobic surfaces; (b) constant contact radius (CCR) mode.
The provided mode of wetting directly controls the kinetics of the evaporation process, which
has been semi-empirically analyzed by Picknett and Bexon [41]. It was already postulated by Maxwell
that the rate of mass loss
of an evaporating sessile droplet is attributed to the product of the vapor
diffusion coefficient in air and the difference of the vapor concentration at the droplet surface
(which can be approximated by the saturation vapor concentration), its value in the surrounding ,
as well as the shape-dependent electrostatic capacitance (Equation (2)) [42].
=4− (2)
Starting Snow’s series [43], Picknett and Bexon used a polynomial evaluation for equiconvex
lenses, being similar to those of sessile droplets at different CAs. As a result, an empiric expression
was obtained giving / as a function of the CA, represented by , with being the droplet
radius [41]. In this way, one accounts for the interfacial blockage by the solid surface, which prohibits
free vapor diffusion and thus reduces local evaporation. For 0.175 , this translates into
Equation (3).
=/ =0.000089570.64440.1160−0.088780.01033 (3)
At this point, it also becomes clear that the mode of evaporation (CCR or CCA) clearly affects
the evaporation as for the case of CCA remains constant, while for CCR, the changing CA will
also have an effect. Now, for the case of a constantly-receding contact-line, i.e., in the absence of
droplet pinning, Equation (2) translates to a simple expression (Equation (4)) for the -function,
with being the initial volume and the total drying time. However, it might be noted that in
recent investigations on sessile droplets of aqueous saline solutions (CAs between 2° and 50°), high
salt concentrations and small contact angles showed significantly lower evaporation rates than
expected from simple diffusion-controlled evaporation. Particle tracking experiments proved that
this lower evaporation has to be attributed to the Marangoni effect [44]. In contrast, it has been
observed that the presence of nanoparticles increases the evaporation rate of aqueous droplets, as
observed for anthraquinone nanoparticles (diameter: 285 nm) on a hydrophobized silica wafer (CA:
Figure 5.
Schematic description of the different modes observed for a drying droplet on a solid
substrate, with the mass flow of cooled water (blue arrows) and that due to the interfacial tension
gradients (Marangoni flow; red arrows) being indicated for: (
a
) constant contact angle (CCA) mode,
as observed on most superhydrophobic surfaces; (b) constant contact radius (CCR) mode.
The provided mode of wetting directly controls the kinetics of the evaporation process, which has
been semi-empirically analyzed by Picknett and Bexon [
41
]. It was already postulated by Maxwell
that the rate of mass loss
dm
dt
of an evaporating sessile droplet is attributed to the product of the vapor
diffusion coefficient
D
in air and the difference of the vapor concentration at the droplet surface
cs
(which can be approximated by the saturation vapor concentration), its value in the surrounding
c∞
,
as well as the shape-dependent electrostatic capacitance C(Equation (2)) [42].
dm
dt =4πDC(cs−c∞)(2)
Starting Snow’s series [
43
], Picknett and Bexon used a polynomial evaluation for equiconvex
lenses, being similar to those of sessile droplets at different CAs. As a result, an empiric expression was
obtained giving
C/r
as a function of the CA, represented by
f(θ)
, with
r
being the droplet radius [
41
].
In this way, one accounts for the interfacial blockage by the solid surface, which prohibits free vapor
diffusion and thus reduces local evaporation. For 0.175 ≤θ≤π, this translates into Equation (3).
f(θ)=C/r=0.00008957 +0.6444 θ+0.1160 θ2−0.08878 θ3+0.01033 θ4(3)
At this point, it also becomes clear that the mode of evaporation (CCR or CCA) clearly affects
the evaporation as for the case of CCA
f(θ)
remains constant, while for CCR, the changing CA will
also have an effect. Now, for the case of a constantly-receding contact-line, i.e., in the absence of
droplet pinning, Equation (2) translates to a simple expression (Equation (4)) for the
V(t)
-function,
with
V0
being the initial volume and
ttotal
the total drying time. However, it might be noted that
in recent investigations on sessile droplets of aqueous saline solutions (CAs between 2
◦
and 50
◦
),
high salt concentrations and small contact angles showed significantly lower evaporation rates than
expected from simple diffusion-controlled evaporation. Particle tracking experiments proved that this
Gels 2017,3, 15 6 of 27
lower evaporation has to be attributed to the Marangoni effect [
44
]. In contrast, it has been observed
that the presence of nanoparticles increases the evaporation rate of aqueous droplets, as observed
for anthraquinone nanoparticles (diameter: 285 nm) on a hydrophobized silica wafer (CA: 80
◦
) [
45
].
Furthermore, for evaporating droplets with an initial CA larger than 90
◦
, one may start with a CCA
mode, then switch to a CCR mode and end the evaporation in a mixed mode [
46
]. In general, it may be
concluded that the description of the evaporation processes of droplets can become quite complicated,
especially for more complex geometries. Moreover, the change in composition of the drying droplet
may also have a substantial impact on the interfacial properties and, hence, on the evaporation process.
V(t)=V01−t
ttotal 3
2(4)
Obviously, in order to produce defined assemblies of micro- or nano-particles, which after
production can be easily isolated from the surface, one will preferentially use surfaces with low
adhesion forces, thus avoiding pinning of the liquid. Superhydrophobic surfaces represent a nice,
biomimetic example for this purpose. Due to their large CA and, mostly, low adhesion forces,
they represent an ideal substrate for the preparation of particle assemblies that allow for easy separation
after drying. Hence, for the sake of completeness the basic concept of superhydrophobic surfaces will
be discussed in the following section.
3. Superhydrophobic Surfaces
Superhydrophobic surfaces represent a class of biomimetic, nanostructured materials, where usually
the term “superhydrophobicity” accounts for water contact angles (WCA) larger than 150
◦[18–22]
.
A common example from nature for water super-repellency is the “lotus-effect” [
47
] as seen for the
leaf of sacred lotus [
48
], which also has been mimicked by artificial fabrication [
49
]. In general, it can
be concluded that it is the combination of chemical hydrophobicity paired with surface texture on the
micro- or even nano-scale, which is essential for achieving such high WCAs. Consequently, the main
challenge for the artificial preparation of superhydrophobic surfaces is the introduction of surface
roughness on the nm to
µ
m scale in hydrophobic surfaces. A large variety of different technical
approaches has been developed over the past years to address this challenge [
50
], and the bare number
of publications available in the field of superhydrophobic surfaces is a statement of the success of
this concept, as well as its relevance for applications. This is one reason why scientists’ curiosity
even pushed the development further towards the creation of superamphiphobic surfaces, capable of
efficiently repelling both oil and water [
51
,
52
]. Starting Young’s equation (Equation (1)), this is quite
remarkable when considering the large range of interfacial tension values covered when comparing
water to typical oils.
3.1. Wetting Modes
For superhydrophobic surfaces, a rough surface topography is essential to amplify hydrophobicity
in order to reach WCAs of greater than 150
◦
. Thus, the solid surface will consist of grooves on the micro-
to nano-scale, where their ratio is proportional to surface roughness. Consequently, when depositing
a water droplet on such a surface, one may imagine two limiting cases: either the liquid is sitting on
top of the grooves, i.e., air stays entrapped within the grooves or the liquid is penetrating the same.
The former state is known as Cassie’s (Cassie–Baxter) [
53
] and the latter as Wenzel’s [
54
] mode of
wetting, as shown in Figure 6a,b, respectively. In the Cassie–Baxter state, the droplet is situated on
top of the surfaces structure, while air stays entrapped within the interspacing provided by surface
roughness (Figure 6a). Hence, at these regions, instead of wetting the surface, the droplet is in contact
with air, which is the most hydrophobic (solvophobic) medium. Again, considering Young’s equation
(Equation (1)), this then favors a higher CA due to
γsl =γlv
. Note that for most polymeric or waxy
substrates
γsl
is about equal to 30–50 mN/m, which is substantially lower than
γlv
(72.8 mN/m at
Gels 2017,3, 15 7 of 27
25
◦
C). Therefore, maximizing the fraction of interspatial air pockets entrapped within the solid surface
leads to CAs approaching 180
◦
. In contrast to that, in the Wenzel state, the droplet penetrates the
interspatial volume and fully wets the surface (Figure 6b).
Gels 2017, 3, 15 7 of 26
Figure 6. Wetting of superhydrophobic surfaces: (a) Cassie state with entrapped air within the surface
grooves; (b) Wenzel state with liquid filling the grooves. Dynamic wetting strongly depends on the
prevailing mode of (a) versus (b): (c) the difference of advancing
and receding
) CA is the
measure of the surface hysteresis and droplet adhesion.
Another important quantity is the CA hysteresis, especially when considering the self-cleaning
effect of superhydrophobic surfaces. CA hysteresis is determined by the difference of the advancing
() and receding () CA, as shown in Figure 6c. Low hysteresis equals a low tilting angle () being
necessary to cause the droplet to roll off the surface. Accordingly, paired with low adhesion forces,
these rolling droplets can collect dirt particles from the surface, thereby promoting self-cleaning, as
known from many plant leaves. This effect has been the inspiration for numerous materials and
applications [55], such as self-cleaning and dirt- and water-repellent coatings used on tiles, roofs or
panels (for instance, in bathrooms or kitchens). Using such coatings on glass leads to the “anti-fog”
effect, while also antimicrobial coatings based on superhydrophobic surfaces have been investigated
[55].
Usually, CA hysteresis stays lower for droplets in the Cassie–Baxter as compared to the Wenzel
state. However, this does not automatically imply low roll-off angles for the Cassie–Baxter state.
Moreover, the roll-off angle depends on the pinning forces at the contact area between solid and
liquid [56]. Generally, it may be stated that for supraparticle formation by EISA, the Cassie–Baxter
state with a CA as high as possible is to be preferred.
3.2. Production of Superhydrophobic Surfaces
For the preparation of superhydrophobic surfaces, there exists a large range of different
approaches, which also have been reviewed comprehensively in recent times [57,58]. Accordingly,
we refrain here from describing these processes in further detail, but instead will briefly describe two
typical approaches that also have been successfully employed in our work. One of them employs
superhydrophobic surfaces produced by an electrochemical deposition (ECD) method [59]. The other
is a soot-based method comprising a chemical vapor deposition (CVD) leading to surfaces that can
either be rendered superhydrophobic or even superamphiphobic, the latter meaning super-repellant
for water and oil [51].
ECD methods usually employ an electrochemical potential to deposit dissolved material from
solution onto a surface substrate. This usually leads to a uniform and highly porous surface coverage.
If the material deposited is not hydrophobic enough by itself, further chemical modification may
serve to complete superhydrophobic surface formation. For example, a suitable approach was
reported by Gu et al. utilizing activated copper surfaces. Here, activation usually just means surface
polishing to remove the passivation layer of copper oxide or sulfide (typical reactions, when left in
air) [59]. When immersed in a silver nitrate solution, a black precipitation layer of silver is formed on
Figure 6.
Wetting of superhydrophobic surfaces: (
a
) Cassie state with entrapped air within the surface
grooves; (
b
) Wenzel state with liquid filling the grooves. Dynamic wetting strongly depends on the
prevailing mode of (
a
) versus (
b
): (
c
) the difference of advancing
(θA)
and receding
(θR
) CA is the
measure of the surface hysteresis and droplet adhesion.
Another important quantity is the CA hysteresis, especially when considering the self-cleaning
effect of superhydrophobic surfaces. CA hysteresis is determined by the difference of the advancing
(
θA
) and receding (
θR
) CA, as shown in Figure 6c. Low hysteresis equals a low tilting angle (
αT
)
being necessary to cause the droplet to roll off the surface. Accordingly, paired with low adhesion
forces, these rolling droplets can collect dirt particles from the surface, thereby promoting self-cleaning,
as known from many plant leaves. This effect has been the inspiration for numerous materials
and applications [
55
], such as self-cleaning and dirt- and water-repellent coatings used on tiles,
roofs or panels (for instance, in bathrooms or kitchens). Using such coatings on glass leads to the
“anti-fog” effect, while also antimicrobial coatings based on superhydrophobic surfaces have been
investigated [55].
Usually, CA hysteresis stays lower for droplets in the Cassie–Baxter as compared to the Wenzel
state. However, this does not automatically imply low roll-off angles for the Cassie–Baxter state.
Moreover, the roll-off angle depends on the pinning forces at the contact area between solid and
liquid [
56
]. Generally, it may be stated that for supraparticle formation by EISA, the Cassie–Baxter
state with a CA as high as possible is to be preferred.
3.2. Production of Superhydrophobic Surfaces
For the preparation of superhydrophobic surfaces, there exists a large range of different
approaches, which also have been reviewed comprehensively in recent times [
57
,
58
]. Accordingly,
we refrain here from describing these processes in further detail, but instead will briefly describe two
typical approaches that also have been successfully employed in our work. One of them employs
superhydrophobic surfaces produced by an electrochemical deposition (ECD) method [
59
]. The other
is a soot-based method comprising a chemical vapor deposition (CVD) leading to surfaces that can
either be rendered superhydrophobic or even superamphiphobic, the latter meaning super-repellant
for water and oil [51].
Gels 2017,3, 15 8 of 27
ECD methods usually employ an electrochemical potential to deposit dissolved material from
solution onto a surface substrate. This usually leads to a uniform and highly porous surface coverage.
If the material deposited is not hydrophobic enough by itself, further chemical modification may
serve to complete superhydrophobic surface formation. For example, a suitable approach was
reported by Gu et al. utilizing activated copper surfaces. Here, activation usually just means surface
polishing to remove the passivation layer of copper oxide or sulfide (typical reactions, when left in
air) [
59
]. When immersed in a silver nitrate solution, a black precipitation layer of silver is formed
on the copper surface. Thus, if choosing the appropriate time of immersion and silver concentration,
highly porous, coral-like structures are achieved within the solid silver layer. Further functionalization
with an aliphatic thiol, such as 1-dodecanethiol, then leads to a well-performing superhydrophobic
surface with a CA above 150◦and low CA hysteresis [59].
Another method well-known to most of us from childhood days is that of covering a heat-stable
surface, such as a spoon, with soot by holding it into the flame of an ordinary candle. Unluckily,
the adhesion of the as-deposited carbon layer is very poor, which results in its immediate rupture
when exposed to a rolling water droplet (similar to the self-cleaning effect of lotus leaves [
48
]). Hence,
Deng et al. searched a way to preserve the structure of the soot, while at the same time increasing
its stability [
51
]. In order to do so, they covered the soot surface layer with a robust silica shell using
CVD of a silica precursor (like tetraethyl-orthosilicate (TEOS)). After subsequent removal of the soot
using calcination at high temperatures and functionalization with an appropriate aliphatic silane via
CVD of a corresponding precursor, as a result, a superhydrophobic or, in the case of a fluorinated
silane, a superamphiphobic surface could be achieved. Surfaces of this kind provide a CA above
150
◦
and low CA hysteresis. Another aspect attributing quite some elegance to this method is the
fact that the application of this layer can occur on any surface able to resist the temperatures used for
the calcination step. Moreover, if carefully done, this surface also provides high transparency [
51
],
which for example even allows experiments with an inverse optical microscope setup for observing
the inside of the droplet from below the surface [
60
]. It might be noted here that the curvature of
small-scale roughness was revealed to play a key role for achieving high resistance against wetting,
thus being of major importance for acquiring superamphiphobicity [61].
For the sake of not losing our intended focus of this review, we will not further discuss the many
other preparation methods available. Instead, the interested reader is referred to comprehensive
reviews giving a nice overview of superhydrophobic surfaces and their different preparation methods
that have been accomplished [19,22,50,52].
4. The Concept of Supraparticle Formation
Supraparticles can be created by self-assembly of colloidal components into larger, ordered
arrays [
62
]. Such supraparticles can possess a large number of combined functionalities as they may
contain many different colloidal materials of a specific nature, like proteins [
63
,
64
], photosensitive
particles, such as semi-conductors [65,66], or magnetic materials [67,68].
With respect to their preparation, a controlled guiding of the assembly process is vital in order
to create supraparticles in a defined way [
69
]. This means that typically, one employs droplets as
the confining object within another, immiscible liquid or on a solid surface. Accordingly, besides
assembling particles from freely-suspended solutions [
65
,
70
], several methods have been developed
employing droplets as confining geometry. There are different methods suitable for generating droplets,
such as emulsion techniques [
71
–
74
], microfluidics [
75
–
78
] or ink-jet printing [
79
–
82
], that besides
precise control on the droplet size, also offer potential for scalability towards mass production.
Apart from just serving as a container for the contained particles, the droplets may also actively
promote the assembly process by drying induced shrinkage that constrains the colloidal components.
One way to do so is dispersing aqueous colloidal suspension droplets in hydrocarbon or fluorinated
oil with subsequent removal of the aqueous phase [
25
]. In such a setup, the position of the droplets can
also be controlled using separately addressable electrodes constructed in an array allowing for defined
Gels 2017,3, 15 9 of 27
droplet dielectrophoresis [
83
]. Some examples for resulting supraparticles achieved by such processes
are illustrated in Figure 7for the case of polystyrene (PS) latex particles dispersed in aqueous droplets
floating in fluorinated oil.
The supraparticles shown in Figure 7a,b show a remarkably smooth and regular structure,
which is also indicated by the colored appearance under top-light illumination. This color effect
requires long-range ordering of the particles, which was corroborated by analyzing the colloidal
structure with scanning electron microscopy (SEM). Here large arrays of regularly-ordered particles
within hexagonal closely-packed planes and face-centered cubic lattices were observed, as illustrated
in Figure 7c,d [
25
]. The driving force for this high degree of ordering was addressed by Denkov et al.,
who investigated 2D arrangements of monodisperse and micron-sized PS-latex particles on a solid
surface [
84
,
85
]. They found that crystallization, i.e., the ordering process, always occurred when
the height of the liquid’s meniscus reached the particle diameter. Furthermore it was found that
this fact neither depends on the ionic strength of the suspension, nor on the surface charge or initial
concentration of the particles. Thus, electrostatic and van der Waals interactions could be excluded as
governing factors [
85
]. Instead, capillary forces between the particles caused by the menisci formed and
convective particle transport within the solution were found as the predominant control parameters.
This extraordinarily high degree of long-range ordering also leads to a precise control of the porosity,
e.g., by tuning the size of the particles or applying additives, such as glucose [
86
] or DNA to drying
silica suspensions [
32
]. Using a microfluidic setup and analyzing the time-dependent solute release
profile from similar sub-mm-sized supraparticles containing 320-nm diameter PS latex microspheres
using a dye, Rastogi et al. showed that such assemblies provide a permeable matrix allowing for
a more homogeneous and slower dye release compared to normal pellets [
87
]. The internal structuring
of colloidal spheres within liquid droplets also was discussed based on geometrical constraints,
surface tension and the interparticle potential. Depending on the detailed conditions, one may expect
the formation of colloidal clusters, colloidosomes or Pickering emulsions [
88
]. Here, the latter two
structures are most likely to be observed upon the conditions prevalent if the second phase is another
liquid, i.e., in an emulsion.
Gels 2017, 3, 15 9 of 26
al., who investigated 2D arrangements of monodisperse and micron-sized PS-latex particles on a
solid surface [84,85]. They found that crystallization, i.e., the ordering process, always occurred when
the height of the liquid’s meniscus reached the particle diameter. Furthermore it was found that this
fact neither depends on the ionic strength of the suspension, nor on the surface charge or initial
concentration of the particles. Thus, electrostatic and van der Waals interactions could be excluded
as governing factors [85]. Instead, capillary forces between the particles caused by the menisci formed
and convective particle transport within the solution were found as the predominant control
parameters. This extraordinarily high degree of long-range ordering also leads to a precise control of
the porosity, e.g. by tuning the size of the particles or applying additives, such as glucose [86] or DNA
to drying silica suspensions [32]. Using a microfluidic setup and analyzing the time-dependent solute
release profile from similar sub-mm-sized supraparticles containing 320-nm diameter PS latex
microspheres using a dye, Rastogi et al. showed that such assemblies provide a permeable matrix
allowing for a more homogeneous and slower dye release compared to normal pellets [87]. The
internal structuring of colloidal spheres within liquid droplets also was discussed based on
geometrical constraints, surface tension and the interparticle potential. Depending on the detailed
conditions, one may expect the formation of colloidal clusters, colloidosomes or Pickering emulsions
[88]. Here, the latter two structures are most likely to be observed upon the conditions prevalent if
the second phase is another liquid, i.e., in an emulsion.
Figure 7. Typical examples for supraparticles prepared by drying of aqueous droplets containing
polystyrene (PS) latex particles dispersed in fluorinated oil. Spherical supraparticles are formed
showing different color patterns based on the size of the PS latex particles: (a) 270 nm and (b) 320 nm,
scale bars = 500 µm. These patterns arise from light-diffraction due to long-range ordering of the
particles as shown in (c) at the surface and (d) along the vertically-broken edge of a similar
supraparticle; scale bars = 1 µm. Adapted with permission from [25] (p. 2241). Copyright 2000 Science.
5. Supraparticles by EISA on Superhydrophobic Surfaces
In the following, we will focus on supraparticles obtained by drying aqueous suspension
droplets on (solid) superhydrophobic surfaces. Recent work in that field has shown how simple
procedures grant precise control over the shape and internal hierarchical structure in these
supraparticles, thereby leading to new kinds of functional materials.
One major drawback of using a two-liquid system, as often employed, is the difficult isolation
of the obtained products and their purification. This fact gave rise to the utilization of
superhydrophobic surfaces. Due to their high WCA, these surfaces also provide spherical droplets
and thus isometric surrounding conditions, but avoid the need of subsequent separation of a second
(oil) phase from the dried supraparticles. Instead, those can readily be collected from the surface due
to low adhesive forces (as evident from the high WCA and low roll-off angle). Note, that despite
Figure 7.
Typical examples for supraparticles prepared by drying of aqueous droplets containing
polystyrene (PS) latex particles dispersed in fluorinated oil. Spherical supraparticles are formed
showing different color patterns based on the size of the PS latex particles: (
a
) 270 nm and (
b
) 320 nm,
scale bars = 500
µ
m. These patterns arise from light-diffraction due to long-range ordering of the
particles as shown in (
c
) at the surface and (
d
) along the vertically-broken edge of a similar supraparticle;
scale bars = 1 µm. Adapted with permission from [25] (p. 2241). Copyright 2000 Science.
Gels 2017,3, 15 10 of 27
5. Supraparticles by EISA on Superhydrophobic Surfaces
In the following, we will focus on supraparticles obtained by drying aqueous suspension droplets
on (solid) superhydrophobic surfaces. Recent work in that field has shown how simple procedures
grant precise control over the shape and internal hierarchical structure in these supraparticles, thereby
leading to new kinds of functional materials.
One major drawback of using a two-liquid system, as often employed, is the difficult isolation of
the obtained products and their purification. This fact gave rise to the utilization of superhydrophobic
surfaces. Due to their high WCA, these surfaces also provide spherical droplets and thus isometric
surrounding conditions, but avoid the need of subsequent separation of a second (oil) phase from the
dried supraparticles. Instead, those can readily be collected from the surface due to low adhesive forces
(as evident from the high WCA and low roll-off angle). Note, that despite being superhydrophobic,
these surfaces can be designed for both adhesive and non-adhesive properties by tuning pitch
values and the density of micro- and nano-structures, respectively [
89
,
90
]. Using EISA on such
superhydrophobic surfaces in combination with the droplet templating method, several new types
of supraparticles of different structures and functionality have been produced. However, in order to
be easily isolable, the droplets must not penetrate the surface texture. Consequently, the size range
of supraparticles produced on such surfaces is limited to several
µ
m [
91
] as a lower size limit and
may reach about mm size [
92
–
97
], while for still larger droplets gravitational effects become dominant
leading to their shape being no longer spherical.
In the following, we shall present recently developed methods for the creation of supraparticles of
various kinds of shapes and functionalities based on their preparation on superhydrophobic surfaces.
5.1. Shaping of Supraparticles
In the simple case, the shape of the droplet will directly determine that of the finally obtained
supraparticle, since the contained particles remain trapped within the provided geometry, i.e., one
simply templates the initial shape. Therefore, the confining geometry is retained. As this geometry on
superhydrophobic surfaces is to a first order spherical (except for gravitational effects), the resulting
supraparticles for the ordinary case are also of a similar shape. Hence, in order to alter the final
shape of the supraparticles, one has to change the droplet geometry. For the case of aqueous droplets
dispensed and floating on fluorinated oil and containing PS-latex microspheres, this was done by
using additional fluorinated surfactant. This led to the formation of dimpled, red blood cell-like or
even doughnut-shaped supraparticles at higher concentrations [
25
], which is caused by the change
of the interfacial tension between the liquids promoted by the surfactant. Lastly, a pronounced
deviation from spherical geometry also occurred when lowering the particle concentrations below
20%, and a continuous transition from a spherical to disc-like shape was observed [25].
Superhydrophobic surfaces, for the case of up to
µ
L-volumes, also provide spherical droplets, with
some slight, but mostly not substantial bottom deformation due to gravitational effects. This can be
deduced from the high CA and low Eötvös or Bond number B
0
(Equation (5)), which is a dimensionless
number describing the relative influence of gravity and surface tension on the shape of a liquid drop.
Here,
∆ρ
represents the density difference between the droplet and its surroundings,
g
the gravitational
constant, rthe droplet radius and γthe interfacial tension [98].
B0=∆ρgr2
γ(5)
Rastogi et al. used superhydrophobic surfaces to create supraparticles of a reduced symmetry,
“doughnut”-like shape, as shown in Figure 8[
92
]. The morphological loss of symmetry corresponds to
the geometric alteration from spherical
R3
- to cylindrical
D∞
-symmetry. This interesting overall shape
is accompanied by light diffraction effects that are similar as for the spherical supraparticles arising
from the segregation of the different colloids contained in the supraparticle.
Gels 2017,3, 15 11 of 27
It has been reported for aqueous polymer solution droplets that a thin shell of concentrated
polymer is formed at the water-air interface, due to internal transport flux. This flux is directed radially
outward and significantly influences the droplet’s shape upon evaporation due to increased surface
elasticity promoted by the concentration gradient [
29
,
98
–
101
]. If this accumulation is stronger at the
three-phase contact line (TPCL) than at the apex, self-pinning of the droplet occurs. This means that
the transversal shrinkage is suppressed, which in turn results in exclusive shrinkage along the droplet
height. Note that the origin of this self-pinning is notably different from the coffee-ring effect, where the
droplets are pinned due to the surface wetting properties [
26
,
27
]. The importance of the CA for the
shape of the formed dry supraparticles has been demonstrated by assembling silica microspheres of
300 nm via EISA on substrates with CAs ranging from 28
◦
–105
◦
. Depending on the CA, ring-like
structures, doughnut-like structures or hemispheres were formed [
102
]. Such hemispherical assemblies
could also be achieved by EISA on a substrate with a CA of 100
◦
when employing monodisperse latex
particles with diameters varying in the range of 300–1100 nm. Here, a very high degree of ordering of
the latex particles was locally observed, which created pores of relatively high uniformity in theses
supraparticles [103].
Gels 2017, 3, 15 11 of 26
employing monodisperse latex particles with diameters varying in the range of 300–1100 nm. Here,
a very high degree of ordering of the latex particles was locally observed, which created pores of
relatively high uniformity in theses supraparticles [103].
Figure 8. Top: Illustration of the evaporation-induced formation of “doughnut” supraparticles; the
inset is showing an SEM of the particle lattice built by 330-nm diameter silica particles. Bottom:
Optical micrographs of the final supraparticle from top- (left) and side-view; scale bars are 500 µm.
Adapted with permission from [92] (p. 192). Copyright 2010 Wiley.
Large particles contained within the drying solution similarly undergo surface collection in
analogy to polymers [28,60]. In the case of the doughnut silica supraparticles, the colloidal particles
(330-nm diameter silica microspheres) get collected at the interface of the precursor droplets due to
the shrinking surface, which is propagating faster towards the interior than can be counterbalanced
by particle diffusion [92]. As the particles predominantly collect close to the TPCL, the droplets
deform in a fashion that yields dimpled supraparticles for volume fractions of silica lower than 15%.
However, when working at a silica volume fraction of 2.5%, the dimple evolved into a complete hole,
hence yielding doughnuts. This effect vanishes when using 1000-nm instead of 330-nm silica
particles, as in that case early sedimentation prohibited the doughnut-hole formation. Furthermore,
the effect of surface concentration and the way the drying process affects the internal structure of the
drying droplet has been studied by means of microbeam small-angel x-ray scattering (SAXS) on a
hanging droplet of a colloidal suspension containing silica nanoparticles of ~10-nm diameter. These
experiments showed that isotropic assembly is still possible for Péclet numbers significantly higher
than one and an accumulation of silica at the droplet surface was only observed for high initial
concentrations [104]. This observation is to be expected, as shell accumulation may only occur if the
colloid movement by diffusion is slower than the rate by which the droplet surface is receding due
to evaporation.
The symmetry of the supraparticles can substantially reduced be further towards anisometric,
ellipsoidal shapes when fumed silica (FS) is used in aqueous colloidal suspension droplets instead of
compact spherical silica nanoparticles. In contrast to the latter, FS has a very open and fluffy fractal
structure of polydisperse particles with hydrodynamic radii of 100–200 nm and correspondingly
large specific surface areas that typically are in the range of 50–400 m
2
/g [105]. However, the
formation of anisometric supraparticles is only observed once a certain concentration of salt is present
and the extent of anisometry can be controlled by adjusting the ionic strength [93,96]. Typical
examples for different FS particle concentrations are illustrated for the case of no deformation in
Figure 9a at 0.001 mM and for the maximum anisometry in Figure 9b obtained at 25 mM ionic
strength of the suspensions at the start of the drying process.
Figure 8.
Top: Illustration of the evaporation-induced formation of “doughnut” supraparticles; the inset
is showing an SEM of the particle lattice built by 330-nm diameter silica particles. Bottom: Optical
micrographs of the final supraparticle from top- (left) and side-view; scale bars are 500
µ
m. Adapted
with permission from [92] (p. 192). Copyright 2010 Wiley.
Large particles contained within the drying solution similarly undergo surface collection in
analogy to polymers [
28
,
60
]. In the case of the doughnut silica supraparticles, the colloidal particles
(330-nm diameter silica microspheres) get collected at the interface of the precursor droplets due to
the shrinking surface, which is propagating faster towards the interior than can be counterbalanced
by particle diffusion [
92
]. As the particles predominantly collect close to the TPCL, the droplets
deform in a fashion that yields dimpled supraparticles for volume fractions of silica lower than 15%.
However, when working at a silica volume fraction of 2.5%, the dimple evolved into a complete hole,
hence yielding doughnuts. This effect vanishes when using 1000-nm instead of 330-nm silica particles,
as in that case early sedimentation prohibited the doughnut-hole formation. Furthermore, the effect of
surface concentration and the way the drying process affects the internal structure of the drying droplet
Gels 2017,3, 15 12 of 27
has been studied by means of microbeam small-angel x-ray scattering (SAXS) on a hanging droplet
of a colloidal suspension containing silica nanoparticles of ~10-nm diameter. These experiments
showed that isotropic assembly is still possible for Péclet numbers significantly higher than one and
an accumulation of silica at the droplet surface was only observed for high initial concentrations [
104
].
This observation is to be expected, as shell accumulation may only occur if the colloid movement by
diffusion is slower than the rate by which the droplet surface is receding due to evaporation.
The symmetry of the supraparticles can substantially reduced be further towards anisometric,
ellipsoidal shapes when fumed silica (FS) is used in aqueous colloidal suspension droplets instead of
compact spherical silica nanoparticles. In contrast to the latter, FS has a very open and fluffy fractal
structure of polydisperse particles with hydrodynamic radii of 100–200 nm and correspondingly large
specific surface areas that typically are in the range of 50–400 m
2
/g [
105
]. However, the formation of
anisometric supraparticles is only observed once a certain concentration of salt is present and the extent
of anisometry can be controlled by adjusting the ionic strength [
93
,
96
]. Typical examples for different
FS particle concentrations are illustrated for the case of no deformation in Figure 9a at 0.001 mM and
for the maximum anisometry in Figure 9b obtained at 25 mM ionic strength of the suspensions at the
start of the drying process.
Gels 2017, 3, 15 12 of 26
Figure 9. Examples for anisometric supraparticles obtained from drying fumed silica (FS) suspensions
(from left to right 1.75%, 3.50%, 5.25% w/v) for an initial ionic strength of (a) 0.001 mM and (b) 25 mM
using NaCl; the last image on the right side shows a side-view perspective. The scale bars are 500 µm.
Adapted with permissions from [93,96] (pp. 587, 598). Copyright 2014 Wiley.
Similar to previous findings for doughnut-shaped supraparticles, particle accumulation at the
water-air interface occurs. This leads to a modification of the surface rigidity, which depends on the
ionic strength, resulting in a higher rigidity for higher ionic strength. This could also be verified using
time-resolved confocal laser scanning fluorescence microscopy. By labeling the FS and the water
phase with different fluorescent dyes, the density profiles of the FS perpendicular to the droplet
surface, i.e., in the radial direction, could be deduced throughout the drying process. Thereby, it was
shown that the FS particles accumulate at the droplet surface and that the deformation of the droplet
takes place once a certain effective thickness and density of this shell is reached [60]. In a somewhat
related study, the evaporation process of an aqueous lysozyme solution on a superhydrophobic
PMMA surface has been followed by microbeam SAXS. The evaporation process leads to hollow
spherical residues, and the SAXS experiments show the increasing concentration at the droplet
surface which leads to the precipitation of crystalline lysozyme nanoparticles towards the end of the
drying process [106].
Coming back to the formation of the anisometric supraparticles, the main difference between
the spherical silica microspheres used before and the amorphous ones of FS is the capability of
intercalating and interconnecting during their aggregation, wherein this capability does not apply
for spheres. In turn, FS can interact in a much more pronounced and cohesive way than silica spheres,
where this attractive interaction will become dominant for sufficiently strong electrostatic screening.
With increasing ionic strength and thereby lower Debye-screening length, the electrostatic repulsion
between the FS particles becomes reduced. As an effect thereof, the shell rigidity arising from the
intercalated FS particles at an ionic strength of above 0.5 mM increases such that the droplets can no
longer shrink, while at the same time retaining their spherical shape. Accordingly, they become
anisometrically deformed, just in the same fashion as a (spherical) football changes its shape when
becoming deflated. The deformation leads to a droplet elongation, whose direction varies
statistically, as it depends on the weakest spots of the as-formed shell. The resulting supraparticle
anisometry, i.e., the ratio of principal axes lengths, could be precisely tuned within the range of 0.5–
25 mM of ionic strength, and the observed anisometry is directly proportional to the logarithm of the
ionic strength and reaches values of up to 1.6 [93,96]. A closer look into the formation mechanism
revealed that the deformation occurs at a constant surface excess concentration of FS [95] and for an
effective shell-thickness of about 22 µm, which corresponds to an interfacial FS volume fraction of
0.17 [60]. It might be noted that shell formation during evaporation can be related to buckling of the
drying droplets. For instance, for µm-sized silica supraparticles obtained by spray-drying, the
formation of doughnuts and dimpled/grainy spheres of buckyball appearance has been attributed to
such surface instabilities. The buckling phenomenon could be arrested by the addition of PEO, which
then allows for shape control [107].
Figure 9.
Examples for anisometric supraparticles obtained from drying fumed silica (FS) suspensions
(from left to right 1.75%, 3.50%, 5.25% w/v) for an initial ionic strength of (
a
) 0.001 mM and (
b
) 25 mM
using NaCl; the last image on the right side shows a side-view perspective. The scale bars are 500
µ
m.
Adapted with permissions from [93,96] (pp. 587, 598). Copyright 2014 Wiley.
Similar to previous findings for doughnut-shaped supraparticles, particle accumulation at the
water-air interface occurs. This leads to a modification of the surface rigidity, which depends on the
ionic strength, resulting in a higher rigidity for higher ionic strength. This could also be verified using
time-resolved confocal laser scanning fluorescence microscopy. By labeling the FS and the water phase
with different fluorescent dyes, the density profiles of the FS perpendicular to the droplet surface,
i.e., in the radial direction, could be deduced throughout the drying process. Thereby, it was shown
that the FS particles accumulate at the droplet surface and that the deformation of the droplet takes
place once a certain effective thickness and density of this shell is reached [
60
]. In a somewhat related
study, the evaporation process of an aqueous lysozyme solution on a superhydrophobic PMMA surface
has been followed by microbeam SAXS. The evaporation process leads to hollow spherical residues,
and the SAXS experiments show the increasing concentration at the droplet surface which leads to the
precipitation of crystalline lysozyme nanoparticles towards the end of the drying process [106].
Coming back to the formation of the anisometric supraparticles, the main difference between
the spherical silica microspheres used before and the amorphous ones of FS is the capability of
intercalating and interconnecting during their aggregation, wherein this capability does not apply for
Gels 2017,3, 15 13 of 27
spheres. In turn, FS can interact in a much more pronounced and cohesive way than silica spheres,
where this attractive interaction will become dominant for sufficiently strong electrostatic screening.
With increasing ionic strength and thereby lower Debye-screening length, the electrostatic repulsion
between the FS particles becomes reduced. As an effect thereof, the shell rigidity arising from the
intercalated FS particles at an ionic strength of above 0.5 mM increases such that the droplets can
no longer shrink, while at the same time retaining their spherical shape. Accordingly, they become
anisometrically deformed, just in the same fashion as a (spherical) football changes its shape when
becoming deflated. The deformation leads to a droplet elongation, whose direction varies statistically,
as it depends on the weakest spots of the as-formed shell. The resulting supraparticle anisometry,
i.e., the ratio of principal axes lengths, could be precisely tuned within the range of 0.5–25 mM of
ionic strength, and the observed anisometry is directly proportional to the logarithm of the ionic
strength and reaches values of up to 1.6 [
93
,
96
]. A closer look into the formation mechanism revealed
that the deformation occurs at a constant surface excess concentration of FS [
95
] and for an effective
shell-thickness of about 22
µ
m, which corresponds to an interfacial FS volume fraction of 0.17 [
60
].
It might be noted that shell formation during evaporation can be related to buckling of the drying
droplets. For instance, for
µ
m-sized silica supraparticles obtained by spray-drying, the formation of
doughnuts and dimpled/grainy spheres of buckyball appearance has been attributed to such surface
instabilities. The buckling phenomenon could be arrested by the addition of PEO, which then allows
for shape control [107].
The major drawback of producing anisometric supraparticles on flat surfaces is their statistical
distribution of orientation after drying with respect to the surface placement. This renders it
impossible to position a patch in a controlled fashion, as for instance is possible by using additional
magneto-responsive ingredients within the precursor suspension droplets [
92
]. In a recent approach
the process of anisometric supraparticle formation using FS [
93
,
95
,
96
] was modified by manipulation of
the evaporation conditions using distinct surface geometries of the solid substrate [
108
]. More precisely,
similar superhydrophobic substrates as utilized in the original experiments were bent at different angles
into a V-shape. Monitoring the evolution of the droplet shape, a directed anisotropy perpendicular
to the surface’s bending axis was observed. This effect of orientation is caused by the evaporation
rate being higher at the free sides of the droplet compared to the blocked ones. This anisotropic
evaporation, indicated by the red arrows in Figure 10a, leads to more accumulation of FS particles,
which at ionic strengths above the threshold of 0.5 mM promotes the controlled droplet elongation
perpendicular to the bending axis. It might be noted that this directional orientation then is
reproducible with high precision, i.e., within a few degrees with respect to the bending axis of
the surface. With this well-predictable orientation of the supraparticles it is possible to position patches
of colloidal assemblies at will at any point(s) of the as-produced supraparticle (e.g., for magnetic
colloids by an external magnetic field, but also other external influences could be imagined). Thus,
this achievement constitutes a substantial advance for accessing increasingly more complex and
functional supraparticles.
Accordingly, besides a general improvement of the extent and reproducibility of the anisometry
values, by taking advantage of the predictable orientation of the final anisometric supraparticles, it is
possible to render them patchy in a sophisticated and defined manner. For instance, supraparticles
with distinct patch locations can be obtained by employing additional magnetic colloidal components
(Fe
3
O
4
) and positioning an external magnet parallel (Figure 10b) or perpendicular (Figure 10c) to
the bent channel. Such anisometric patchy supraparticles can be of potential use for designing
self-propelling systems or applications requiring orientation functionality.
Gels 2017,3, 15 14 of 27
Gels 2017, 3, 15 13 of 26
The major drawback of producing anisometric supraparticles on flat surfaces is their statistical
distribution of orientation after drying with respect to the surface placement. This renders it
impossible to position a patch in a controlled fashion, as for instance is possible by using additional
magneto-responsive ingredients within the precursor suspension droplets [92]. In a recent approach
the process of anisometric supraparticle formation using FS [93,95,96] was modified by manipulation
of the evaporation conditions using distinct surface geometries of the solid substrate [108]. More
precisely, similar superhydrophobic substrates as utilized in the original experiments were bent at
different angles into a V-shape. Monitoring the evolution of the droplet shape, a directed anisotropy
perpendicular to the surface’s bending axis was observed. This effect of orientation is caused by the
evaporation rate being higher at the free sides of the droplet compared to the blocked ones. This
anisotropic evaporation, indicated by the red arrows in Figure 10a, leads to more accumulation of FS
particles, which at ionic strengths above the threshold of 0.5 mM promotes the controlled droplet
elongation perpendicular to the bending axis. It might be noted that this directional orientation then
is reproducible with high precision, i.e., within a few degrees with respect to the bending axis of the
surface. With this well-predictable orientation of the supraparticles it is possible to position patches
of colloidal assemblies at will at any point(s) of the as-produced supraparticle (e.g., for magnetic
colloids by an external magnetic field, but also other external influences could be imagined). Thus,
this achievement constitutes a substantial advance for accessing increasingly more complex and
functional supraparticles.
Accordingly, besides a general improvement of the extent and reproducibility of the anisometry
values, by taking advantage of the predictable orientation of the final anisometric supraparticles, it is
possible to render them patchy in a sophisticated and defined manner. For instance, supraparticles
with distinct patch locations can be obtained by employing additional magnetic colloidal components
(Fe
3
O
4
) and positioning an external magnet parallel (Figure 10b) or perpendicular (Figure 10c) to the
bent channel. Such anisometric patchy supraparticles can be of potential use for designing self-
propelling systems or applications requiring orientation functionality.
Figure 10. Controlled supraparticle orientation after drying: (a) anisotropic evaporation due to the
surface bending. This allows for the creation of patchy anisometric particles with defined location of
magnetic components (Fe
3
O
4
) by appropriately placing external magnets: (b) along the transversal or
(c) longitudinal diameter of the ellipsoid. In (b) and (c), the images on the right side present zoomed-
in excerpts; scale bars = 0.5 mm; other scale bars are 1 mm. Composed with permission from [108] (p.
5). Copyright 2016 Wiley.
Figure 10.
Controlled supraparticle orientation after drying: (
a
) anisotropic evaporation due to the
surface bending. This allows for the creation of patchy anisometric particles with defined location of
magnetic components (Fe
3
O
4
) by appropriately placing external magnets: (
b
) along the transversal or
(
c
) longitudinal diameter of the ellipsoid. In (
b
) and (
c
), the images on the right side present zoomed-in
excerpts; scale bars = 0.5 mm; other scale bars are 1 mm. Composed with permission from [
108
] (p. 5).
Copyright 2016 Wiley.
As already described in Section 2, Picknett and Bexon discussed the sessile droplet evaporation on
solid surfaces using Maxell’s approach [
41
,
42
]. This also explained the anisotropic evaporation of the
droplets on the V-shaped surfaces by introduction of an apparent CA, created by the upwardly-directed
surface legs. Comparing the values for both, the surface and apparent CA obtained by fitting the
CCA-model to the experimentally-obtained evaporation rate ratios (perpendicular vs. parallel to
the surface bending axis), yielded a good correlation with the measured CAs. This revealed that the
additional blockage by the upwardly-bent surface significantly reduces local evaporative vapor flux.
Hence, anisotropic particle accumulation is promoted, being more pronounced on the free droplet
faces, which in turn causes the controlled supraparticle elongation.
It might be worth noting that the formation of the anisometric FS supraparticles occurs in
a non-pinned state, i.e., CCA-mode. However, Zhou et al. discovered a method that applies controlled
pinning of the droplet contact-line using an ethanol-water mixture containing PS nanoparticles. This led
to anisometric receding of the TPCL, producing anisometric photonic crystals [
109
]. By successive
increase of the ethanol content, it was shown possible to create anisometric supraparticles within an
aspect ratio from 1.14 (2 vol % EtOH) to 1.28 (8 vol % EtOH), as shown in Figure 11 [
109
]. This is
an alternative approach to the FS supraparticle production, where droplet elongation occurs due
to shell deformation [
93
,
96
,
108
]. Moreover, in pure aqueous systems, the structure of the resulting
supraparticles could be tuned from microbeads to dimpled or microwelled shapes by varying the
colloid concentration.
Concluding, one may state that there exist different ways of symmetry breaking that lead to many
quite different shapes, which are of interest for various potential applications.
Gels 2017,3, 15 15 of 27
Gels 2017, 3, 15 14 of 26
As already described in Section 2, Picknett and Bexon discussed the sessile droplet evaporation
on solid surfaces using Maxell’s approach [41,42]. This also explained the anisotropic evaporation of
the droplets on the V-shaped surfaces by introduction of an apparent CA, created by the upwardly-
directed surface legs. Comparing the values for both, the surface and apparent CA obtained by fitting
the CCA-model to the experimentally-obtained evaporation rate ratios (perpendicular vs. parallel to
the surface bending axis), yielded a good correlation with the measured CAs. This revealed that the
additional blockage by the upwardly-bent surface significantly reduces local evaporative vapor flux.
Hence, anisotropic particle accumulation is promoted, being more pronounced on the free droplet
faces, which in turn causes the controlled supraparticle elongation.
It might be worth noting that the formation of the anisometric FS supraparticles occurs in a non-
pinned state, i.e., CCA-mode. However, Zhou et al. discovered a method that applies controlled
pinning of the droplet contact-line using an ethanol-water mixture containing PS nanoparticles. This
led to anisometric receding of the TPCL, producing anisometric photonic crystals [109]. By successive
increase of the ethanol content, it was shown possible to create anisometric supraparticles within an
aspect ratio from 1.14 (2 vol% EtOH) to 1.28 (8 vol% EtOH), as shown in Figure 11 [109]. This is an
alternative approach to the FS supraparticle production, where droplet elongation occurs due to shell
deformation [93,96,108]. Moreover, in pure aqueous systems, the structure of the resulting
supraparticles could be tuned from microbeads to dimpled or microwelled shapes by varying the
colloid concentration.
Figure 11. Optical microscopy of anisometric photonic crystals (PC) obtained by using mixtures of
water and ethanol with varying compositions: (a) 2, (b) 4, (c) 6 and (d) 8 vol% of EtOH; scale bars are
800 µm. The systematic dependency of aspect ratios, i.e., anisometry values, is given in (e). Adapted
with permission from [109] (p. 22647). Copyright 2015 American Chemical Society.
Concluding, one may state that there exist different ways of symmetry breaking that lead to
many quite different shapes, which are of interest for various potential applications.
5.2. Substructuring of Supraparticles
When using the droplet templating method on superhydrophobic surfaces, though the overall
geometry may be fixed, still internal structuring or porosity may be of potential interest. One well-
known structural type within this context is that of Janus or, more generally, patchy, hierarchical- or
surface-anisometric particles, respectively [110–113]. Accordingly, we want to address supraparticles
having different types of colloidal constituents that are inhomogeneously distributed throughout the
assembled supraparticles.
One straightforward way to achieve this kind of anisotropy in drying droplets is the utilization
of magnetic components and the application of an external magnetic field [92,94,108]. The principle
is shown in Figure 12, wherein besides the basic concept of forming single-patched supraparticles
(Figure 12a), altering the magnetic field setup also allows for more complex patchy assemblies, such
as presented by bi- and tri-patched supraparticles (Figure 12a,b).
Figure 11.
Optical microscopy of anisometric photonic crystals (PC) obtained by using mixtures of
water and ethanol with varying compositions: (
a
) 2; (
b
) 4; (
c
) 6 and (
d
) 8 vol % of EtOH; scale bars are
800
µ
m. The systematic dependency of aspect ratios, i.e., anisometry values, is given in (
e
). Adapted
with permission from [109] (p. 22647). Copyright 2015 American Chemical Society.
5.2. Substructuring of Supraparticles
When using the droplet templating method on superhydrophobic surfaces, though the overall
geometry may be fixed, still internal structuring or porosity may be of potential interest. One well-known
structural type within this context is that of Janus or, more generally, patchy, hierarchical- or
surface-anisometric particles, respectively [
110
–
113
]. Accordingly, we want to address supraparticles
having different types of colloidal constituents that are inhomogeneously distributed throughout the
assembled supraparticles.
One straightforward way to achieve this kind of anisotropy in drying droplets is the utilization
of magnetic components and the application of an external magnetic field [
92
,
94
,
108
]. The principle
is shown in Figure 12, wherein besides the basic concept of forming single-patched supraparticles
(Figure 12a), altering the magnetic field setup also allows for more complex patchy assemblies, such as
presented by bi- and tri-patched supraparticles (Figure 12a,b).
Gels 2017, 3, 15 15 of 26
Figure 12. Assembly of (multi-)patched supraparticles in drying sessile droplets on a superhydrophobic
surface: (a) single- (b) bi- and (c) tri-patched; scale bars are 500 µm. Altered with permission from [92]
(p. 193). Copyright 2010 Wiley.
Similarly, this patch formation can also be combined with anisometrically shaped supraparticles,
as already described before and presented above in Figure 10b,c [108]. The combination of
anisometric shape and controlled placing of patches on such particles in turn leads to much more
complex supraparticles.
It is not necessarily required to use magnetic colloids with subsequent magnetic field guiding to
generate hierarchically anisotropic, i.e., patchy particles. Rastogi et al. showed that by combining
latex particles with diameters larger than ~300 nm (which collect on the droplet surface) with small
gold nanoparticles (~22 nm), supraparticles, which exhibit remarkable optical features, as shown in
Figure 13, can be obtained [97].
Figure 13. Optical microscopy images of patchy supraparticles assembled by EISA using variably-
sized polystyrene (PS) latex nano-/micro-particles in suspension droplets generating highly light
diffracting “opal balls”. The gold nanoparticles are 22 nm in size. Reprinted with permission from
[97] (p. 4266). Copyright 2008 Wiley.
The surface collection of the suspended particles was found to arise from a combination of
Marangoni flow, already discussed in Section 2, and the comparably small size of the gold
nanoparticles. Due to the droplet evaporation taking place mostly at the top (surface blockage at the
bottom), the temperature gradient promotes Marangoni flux transporting particles to the top. Using
Figure 12.
Assembly of (multi-)patched supraparticles in drying sessile droplets on a superhydrophobic
surface: (
a
) single- (
b
) bi- and (
c
) tri-patched; scale bars are 500
µ
m. Altered with permission from [
92
]
(p. 193). Copyright 2010 Wiley.
Gels 2017,3, 15 16 of 27
Similarly, this patch formation can also be combined with anisometrically shaped supraparticles,
as already described before and presented above in Figure 10b,c [
108
]. The combination of
anisometric shape and controlled placing of patches on such particles in turn leads to much more
complex supraparticles.
It is not necessarily required to use magnetic colloids with subsequent magnetic field guiding
to generate hierarchically anisotropic, i.e., patchy particles. Rastogi et al. showed that by combining
latex particles with diameters larger than ~300 nm (which collect on the droplet surface) with small
gold nanoparticles (~22 nm), supraparticles, which exhibit remarkable optical features, as shown in
Figure 13, can be obtained [97].
Gels 2017, 3, 15 15 of 26
Figure 12. Assembly of (multi-)patched supraparticles in drying sessile droplets on a superhydrophobic
surface: (a) single- (b) bi- and (c) tri-patched; scale bars are 500 µm. Altered with permission from [92]
(p. 193). Copyright 2010 Wiley.
Similarly, this patch formation can also be combined with anisometrically shaped supraparticles,
as already described before and presented above in Figure 10b,c [108]. The combination of
anisometric shape and controlled placing of patches on such particles in turn leads to much more
complex supraparticles.
It is not necessarily required to use magnetic colloids with subsequent magnetic field guiding to
generate hierarchically anisotropic, i.e., patchy particles. Rastogi et al. showed that by combining
latex particles with diameters larger than ~300 nm (which collect on the droplet surface) with small
gold nanoparticles (~22 nm), supraparticles, which exhibit remarkable optical features, as shown in
Figure 13, can be obtained [97].
Figure 13. Optical microscopy images of patchy supraparticles assembled by EISA using variably-
sized polystyrene (PS) latex nano-/micro-particles in suspension droplets generating highly light
diffracting “opal balls”. The gold nanoparticles are 22 nm in size. Reprinted with permission from
[97] (p. 4266). Copyright 2008 Wiley.
The surface collection of the suspended particles was found to arise from a combination of
Marangoni flow, already discussed in Section 2, and the comparably small size of the gold
nanoparticles. Due to the droplet evaporation taking place mostly at the top (surface blockage at the
bottom), the temperature gradient promotes Marangoni flux transporting particles to the top. Using
Figure 13.
Optical microscopy images of patchy supraparticles assembled by EISA using variably-sized
polystyrene (PS) latex nano-/micro-particles in suspension droplets generating highly light diffracting
“opal balls”. The gold nanoparticles are 22 nm in size. Reprinted with permission from [
97
] (p. 4266).
Copyright 2008 Wiley.
The surface collection of the suspended particles was found to arise from a combination of
Marangoni flow, already discussed in Section 2, and the comparably small size of the gold nanoparticles.
Due to the droplet evaporation taking place mostly at the top (surface blockage at the bottom),
the temperature gradient promotes Marangoni flux transporting particles to the top. Using very low
concentrations of PS microspheres, Chang and Velev investigated this effect inside an aqueous droplet
floating in a fluorinated oil [
33
]. In these droplets, even though the particles were allowed to sediment,
they again collected at the droplet surface after restarting the evaporation (by removing the saturated
atmosphere). Thereby, the consequence of the Marangoni effect/flow was visualized. Accordingly,
if the concentrations are properly set and because the gold nanoparticles are small enough to travel
in-between the inter-particle separations of the bigger microspheres, the resulting supraparticles are
rendered patchy with the gold particles collected at the top, as shown in Figure 13 [
97
]. Of course,
by that method, it is also possible to cover a well-defined surface area of the final supraparticles.
This effect is independent of the supraparticle shape, as exemplified by the “glazed doughnuts”, already
presented in Figure 8[
92
]. In summary, here, one faces a competition between sedimentation and
induced Marangoni flow to which the dispersed colloids respond in a fashion depending on their size
and density. This then leads to their segregation within the supraparticles and the observed internally
inhomogeneous structuring, which allows for the creation of interesting and versatile supraparticles.
Gels 2017,3, 15 17 of 27
6. Applications of Supraparticles
As the preparation method leading to the final assembled supraparticles is not necessarily linked
to their potential field of application, we will now extend our original focus and refer to supraparticles
in a more general context. Nevertheless, for a start, we may first review some applications that emerged
for supraparticles of the kinds described so far.
One example of potential applications is the field of photonics. Photonic applications require the
precise control of optical properties of the materials, which can be achieved by proper nano-structuring.
Defined spacing of monodisperse particles in highly ordered lattices allows for the diffraction of
discrete wavelengths of the incident light leading to distinct coloring of the materials. Examples of
this effect of defined coloring due to microstructuring can also be found in nature. Namely, the wings
of Morpho peleides (butterfly) show an intense blue color without the presence of any dye [
114
].
Another example from nature is represented by the blue-purple fruits of an African tropical plant,
named Pollia condensata [
115
]. Practically, assembling monodisperse particles into supraparticles with
controlled inter-particle spacing allows for the fabrication of materials having tuned optical properties.
As a first example, Rastogi et al. used PS microsphere suspensions in drying droplets on
a superhydrophobic surface and varied the diameter of the PS particles between 320 and 1000 nm [
97
].
After drying, the resulting spherical “opal balls” show colored rings originating from diffracted
light from the curved supraparticle surface (Figure 14). This diffraction is described by Bragg’s law,
but rather than being caused by the colloidal crystal lattice within the bulk, it is more likely the result
of the parallel rows at the surface. Furthermore, it was shown (see above in Figure 13) that additional
gold nanoparticles of about 22 nm in size were not affecting the reflected colors, except for amplifying
their intensity. Their color patterns can be precisely controlled by inter-particle spacing, which in turn
is determined by the size of the microparticles. Of course, here, one may imagine even much more
complex optical systems that could be achieved by appropriately designing hierarchical structures and
using different colloidal components.
Gels 2017, 3, 15 17 of 26
Figure 14. Optical microscopy images of patchy supraparticles assembled by EISA using differently-
sized polystyrene (PS) latex microparticles in suspension droplets, thereby generating these highly
light-diffracting “opal balls”. Reprinted with permission from [97] (p. 4266). Copyright 2008 Wiley.
Figure 15. Colloidal photonic crystals (supraparticles) embedded in an elastomeric matrix in: (a) free;
(b) contracted; (c) stretched state. Photographs of the resulting films are shown in (d–f) with
supraparticles made of differently-sized PS particles. The reflected color is independent of the
observing angle and strain on the films. The scale bars are 1 cm. Reprinted with permission from [116]
(p. 1587). Copyright 2015 Royal Society of Chemistry.
Figure 16. Magnetic Janus supraparticles (a) switched using the different hemispheres at different
light intensities: upwards directed (b) PS hemisphere at low light intensity or (c) Fe
3
O
4
-TMPTA
hemisphere under strong light intensity; scale bars are 500 µm. Adapted with permission from [117]
(p. 9435). Copyright 2014 Royal Society of Chemistry.
Figure 14.
Optical microscopy images of patchy supraparticles assembled by EISA using differently-sized
polystyrene (PS) latex microparticles in suspension droplets, thereby generating these highly
light-diffracting “opal balls”. Reprinted with permission from [97] (p. 4266). Copyright 2008 Wiley.
Supraparticles showing optical effects can be employed to produce colored films or layers.
Such films containing buckled or spherical supraparticles show angle and strain independent
reflections due to the colloidal matrices provided by the supraparticles. This leads to observable
structural coloring [
116
]. Such films can be obtained from emulsion droplets using osmotic annealing
and defined salt-concentration to produce buckled or spherical supraparticles and entrapping them in
a silicon matrix. The resulting films are presented in Figure 15d–f.
Gels 2017,3, 15 18 of 27
Here, the angle and strain independence is simulated by free-standing (Figure 15a), contracted
(Figure 15b) and stretched (Figure 15c) films. These films show similar structural coloring arising from
the supraparticles embedded into the silicon matrix, where buckled supraparticles showed even better
color quality than spherical ones.
Gels 2017, 3, 15 17 of 26
Figure 14. Optical microscopy images of patchy supraparticles assembled by EISA using differently-
sized polystyrene (PS) latex microparticles in suspension droplets, thereby generating these highly
light-diffracting “opal balls”. Reprinted with permission from [97] (p. 4266). Copyright 2008 Wiley.
Figure 15. Colloidal photonic crystals (supraparticles) embedded in an elastomeric matrix in: (a) free;
(b) contracted; (c) stretched state. Photographs of the resulting films are shown in (d–f) with
supraparticles made of differently-sized PS particles. The reflected color is independent of the
observing angle and strain on the films. The scale bars are 1 cm. Reprinted with permission from [116]
(p. 1587). Copyright 2015 Royal Society of Chemistry.
Figure 16. Magnetic Janus supraparticles (a) switched using the different hemispheres at different
light intensities: upwards directed (b) PS hemisphere at low light intensity or (c) Fe
3
O
4
-TMPTA
hemisphere under strong light intensity; scale bars are 500 µm. Adapted with permission from [117]
(p. 9435). Copyright 2014 Royal Society of Chemistry.
Figure 15. Colloidal photonic crystals (supraparticles) embedded in an elastomeric matrix in: (a) free;
(
b
) contracted; (
c
) stretched state. Photographs of the resulting films are shown in (
d
–
f
) with
supraparticles made of differently-sized PS particles. The reflected color is independent of the observing
angle and strain on the films. The scale bars are 1 cm. Reprinted with permission from [
116
] (p. 1587).
Copyright 2015 Royal Society of Chemistry.
Switchable photonic materials may be of potential use for displays, indicators or similar devices.
As an example for such a device, Janus supraparticles having hemispheres of different reflectance with
one of them being magnetic were produced using microfluidics [
117
]. These supraparticles then can be
oriented with an external magnetic field (Figure 16a) showing different reflected intensity for different
light conditions (Figure 16b,c), i.e., forming a magnetically-switchable display.
Gels 2017, 3, 15 17 of 26
Figure 14. Optical microscopy images of patchy supraparticles assembled by EISA using differently-
sized polystyrene (PS) latex microparticles in suspension droplets, thereby generating these highly
light-diffracting “opal balls”. Reprinted with permission from [97] (p. 4266). Copyright 2008 Wiley.
Figure 15. Colloidal photonic crystals (supraparticles) embedded in an elastomeric matrix in: (a) free;
(b) contracted; (c) stretched state. Photographs of the resulting films are shown in (d–f) with
supraparticles made of differently-sized PS particles. The reflected color is independent of the
observing angle and strain on the films. The scale bars are 1 cm. Reprinted with permission from [116]
(p. 1587). Copyright 2015 Royal Society of Chemistry.
Figure 16. Magnetic Janus supraparticles (a) switched using the different hemispheres at different
light intensities: upwards directed (b) PS hemisphere at low light intensity or (c) Fe
3
O
4
-TMPTA
hemisphere under strong light intensity; scale bars are 500 µm. Adapted with permission from [117]
(p. 9435). Copyright 2014 Royal Society of Chemistry.
Figure 16.
Magnetic Janus supraparticles (
a
) switched using the different hemispheres at different light
intensities: upwards directed (
b
) PS hemisphere at low light intensity or (
c
) Fe
3
O
4
-TMPTA hemisphere
under strong light intensity; scale bars are 500
µ
m. Adapted with permission from [
117
] (p. 9435).
Copyright 2014 Royal Society of Chemistry.
Similarly, also thermo-sensitive Janus supraparticles can be used in order to fabricate color
changing displays [118].
Another interesting application of supraparticles is the development of bio-assays for antigen
detection, which take advantage of the optical appearance of the supraparticles after drying depending
on the exposure to the antigen and its concentration. Rastogi and Velev prepared an on-chip setup
by drying droplets floating in fluorinated oil and containing PS microspheres and gold nanoparticles
that were additionally functionalized with anti-rabbit IgG antibodies. Via the optical appearance after
Gels 2017,3, 15 19 of 27
incubation, this system was able to serve as a sophisticated micro-bioassay for antigen detection [
119
].
This is a consequence of the antibody-antigen interaction, which is highly effective in terms of
strength and specificity, thereby in turn controlling the particle interaction inside the dry droplets and
consequently the resulting optical appearance. In other words, due to the antibody-functionalized gold
and PS-latex particles, the patch formation is highly influenced by the presence of the corresponding
antigen due to agglutination of the gold nanoparticles within the suspension. This effect also allowed
for quantitative interpretation by optical comparison, shown in Figure 17 [119].
Gels 2017, 3, 15 18 of 26
Switchable photonic materials may be of potential use for displays, indicators or similar devices.
As an example for such a device, Janus supraparticles having hemispheres of different reflectance
with one of them being magnetic were produced using microfluidics [117]. These supraparticles then
can be oriented with an external magnetic field (Figure 16a) showing different reflected intensity for
different light conditions (Figure 16b,c), i.e., forming a magnetically-switchable display.
Similarly, also thermo-sensitive Janus supraparticles can be used in order to fabricate color
changing displays [118].
Another interesting application of supraparticles is the development of bio-assays for antigen
detection, which take advantage of the optical appearance of the supraparticles after drying
depending on the exposure to the antigen and its concentration. Rastogi and Velev prepared an on-
chip setup by drying droplets floating in fluorinated oil and containing PS microspheres and gold
nanoparticles that were additionally functionalized with anti-rabbit IgG antibodies. Via the optical
appearance after incubation, this system was able to serve as a sophisticated micro-bioassay for
antigen detection [119]. This is a consequence of the antibody-antigen interaction, which is highly
effective in terms of strength and specificity, thereby in turn controlling the particle interaction inside
the dry droplets and consequently the resulting optical appearance. In other words, due to the
antibody-functionalized gold and PS-latex particles, the patch formation is highly influenced by the
presence of the corresponding antigen due to agglutination of the gold nanoparticles within the
suspension. This effect also allowed for quantitative interpretation by optical comparison, shown in
Figure 17 [119].
Figure 17. Resulting droplets for a 30-min incubation time and containing anti-rabbit IgG antibody
functionalized gold nanoparticles at different concentrations of antigen (rabbit IgG); left to right: 0,
1.0, 10.0, 100.0 µg/mL. Reprinted with permission from [119] (p. 6). Copyright 2007 AIP Publishing.
Not only for diagnostic, but also for therapeutic purposes, supraparticles can serve as drug
delivery systems [120–122]. This can be achieved by incorporating drugs meant to be released over
an extended period of time. It has been shown by Rastogi et al. that supraparticles are permeable for
solutes and able to continuously release contained solutes in a very controllable way [87]. Thus,
taking advantage of analogous mesoporous structures made of silica and gelatin hybrids, it was
shown that brain-derived neurotrophic factor (BDNF), a protein growth factor, and dexamethasone
(DEX), a steroidal anti-inflammatory drug, could be easily loaded into the supraparticles and
continuously be released over several days [120]. Using this concept, BDNF-loaded supraparticles
were implanted into the inner ears of guinea pigs previously deafened by frusemide and kanamycin
medication (damaging ion gradients between the auditory neuron network). The release of BDNF
thereby showed significant rescuing of primary auditory neurons, potentially preserving residual
hearing [121]. In another experiment, Park et al. immobilized a cysteine protease to porous calcium
phosphate supraparticles [122]. This protease-supraparticle hybrid, in vitro allowed for systematic
inactivation of the cytokine tumor necrosis factor-alpha (TNF-α), which is responsible for
inflammatory effects potentially causing autoimmune diseases.
A quite different functionality of supraparticles is their potential ability to move. Self-propelling
particles convert chemical energy into active, autonomous motion and have been developed and built
in many different ways [123,124]. Able to perform complex tasks, such artificial vehicles have
promising potential for applications in environmental treatment, like water remediation [125,126] or
can serve as smart drug-delivery systems [127]. Yet, an interesting approach extending the field
towards the millimeter scale has been taken using supraparticles prepared by EISA on a
Figure 17.
Resulting droplets for a 30-min incubation time and containing anti-rabbit IgG antibody
functionalized gold nanoparticles at different concentrations of antigen (rabbit IgG); left to right: 0, 1.0,
10.0, 100.0 µg/mL. Reprinted with permission from [119] (p. 6). Copyright 2007 AIP Publishing.
Not only for diagnostic, but also for therapeutic purposes, supraparticles can serve as drug
delivery systems [
120
–
122
]. This can be achieved by incorporating drugs meant to be released over
an extended period of time. It has been shown by Rastogi et al. that supraparticles are permeable
for solutes and able to continuously release contained solutes in a very controllable way [
87
]. Thus,
taking advantage of analogous mesoporous structures made of silica and gelatin hybrids, it was shown
that brain-derived neurotrophic factor (BDNF), a protein growth factor, and dexamethasone (DEX),
a steroidal anti-inflammatory drug, could be easily loaded into the supraparticles and continuously
be released over several days [
120
]. Using this concept, BDNF-loaded supraparticles were implanted
into the inner ears of guinea pigs previously deafened by frusemide and kanamycin medication
(damaging ion gradients between the auditory neuron network). The release of BDNF thereby
showed significant rescuing of primary auditory neurons, potentially preserving residual hearing [
121
].
In another experiment, Park et al. immobilized a cysteine protease to porous calcium phosphate
supraparticles [
122
]. This protease-supraparticle hybrid,
in vitro
allowed for systematic inactivation
of the cytokine tumor necrosis factor-alpha (TNF-
α
), which is responsible for inflammatory effects
potentially causing autoimmune diseases.
A quite different functionality of supraparticles is their potential ability to move. Self-propelling
particles convert chemical energy into active, autonomous motion and have been developed and
built in many different ways [
123
,
124
]. Able to perform complex tasks, such artificial vehicles have
promising potential for applications in environmental treatment, like water remediation [
125
,
126
] or
can serve as smart drug-delivery systems [
127
]. Yet, an interesting approach extending the field towards
the millimeter scale has been taken using supraparticles prepared by EISA on a superhydrophobic
surface [
94
]. These supraparticles were rendered patchy using Fe
3
O
4
core nanoparticles decorated with
Pt and a magnetic field during the synthesis. After additional surface hydrophobization and when
placed in a H
2
O
2
solution, these supraparticles generated adhering oxygen bubbles (for sufficient
adhesion, the hydrophobization is essential), which increased the buoyancy. Once the buoyancy is high
enough (typically one larger oxygen bubble is attached to the supraparticle), the whole supraparticle
is lifted to the liquid’s surface. There, it loses its oxygen bubble and drops down to the bottom of the
liquid again, where the formation of a new bubble starts. The whole process then is repeated, and the
supraparticle performs an oscillating, regular vertical motion, thus resembling “elevators”, and this
motion can continue for days. In addition, via the contained magnetic nanoparticles, the trajectory of
the supraparticle can be steered by the application of an external magnetic field, which is illustrated in
Figure 18.
Gels 2017,3, 15 20 of 27
Gels 2017, 3, 15 19 of 26
superhydrophobic surface [94]. These supraparticles were rendered patchy using Fe
3
O
4
core
nanoparticles decorated with Pt and a magnetic field during the synthesis. After additional surface
hydrophobization and when placed in a H
2
O
2
solution, these supraparticles generated adhering
oxygen bubbles (for sufficient adhesion, the hydrophobization is essential), which increased the
buoyancy. Once the buoyancy is high enough (typically one larger oxygen bubble is attached to the
supraparticle), the whole supraparticle is lifted to the liquid’s surface. There, it loses its oxygen bubble
and drops down to the bottom of the liquid again, where the formation of a new bubble starts. The
whole process then is repeated, and the supraparticle performs an oscillating, regular vertical motion,
thus resembling “elevators”, and this motion can continue for days. In addition, via the contained
magnetic nanoparticles, the trajectory of the supraparticle can be steered by the application of an
external magnetic field, which is illustrated in Figure 18.
Figure 18. Oscillating elevator supraparticle in a wt% aqueous H
2
O
2
solution. Starting at the top left,
the elevator releases the oxygen bubble and falls down to the left bottom. After producing an oxygen
bubble, it gets attracted by the bottom right magnet, following this attraction to the right during the
way up. Having reached the surface meniscus, the left magnet pulls the elevator supraparticle back
to its starting position, restarting the movement cycle; the scale bar is 1 cm. Reprinted with permission
from [94] (p. 6). Copyright 2016 Wiley.
Functionalizing the elevator supraparticle surface with the enzyme α-amylase, the catalytic
decomposition of starch could be performed as a model reaction, showing potential applications of
these elevator supraparticles in chemical catalysis [94]. Accordingly, this example proves the concept
of self-propelled particles, whose movement can be steered by an external magnetic field and which
are able to perform a chemical task (reaction) on their trajectory. Of course, the use of H
2
O
2
limits the
potential for applications substantially, but it might be noted that it is also possible to use more
benign, non-toxic fuels, such as alcohols, for particle propulsion [128].
Finally, supraparticles are also interesting for the general field of catalysis, as they constitute
rather versatile and small reaction containers. As an example in the field of catalysis, zinc sulfide
supraparticles were developed for chemical dechlorination of 2,2′,4,4′,5,5′-hexachlorobiphenyl, an
organic pollutant occurring in soils or groundwater [129]. Using UV irradiation, these supraparticles
reductively degraded the model pollutant up to about 70% after 12 h in a solution of isooctane. Even
more complicated, Xu et al. showed that Pt nanoparticles bound to a porous organo-polymer shell
synthesized via soap-free emulsion polymerization onto Fe
3
O
4
supraparticles can perform catalytic
enantioselective hydrogenation of ethyl pyruvate [130].
Of course, this short section is far from complete with respect to the applications that have been
explored for supraparticles, simply due to the fact that this is an ample field of research that allows
for a vast number of options via appropriate structural and functional design by using various kinds
of colloidal components that can be readily applied. Thus, many interesting developments have
already been achieved and many more are to be expected in the near future to emerge from this field.
Figure 18.
Oscillating elevator supraparticle in a wt % aqueous H
2
O
2
solution. Starting at the top left,
the elevator releases the oxygen bubble and falls down to the left bottom. After producing an oxygen
bubble, it gets attracted by the bottom right magnet, following this attraction to the right during the
way up. Having reached the surface meniscus, the left magnet pulls the elevator supraparticle back to
its starting position, restarting the movement cycle; the scale bar is 1 cm. Reprinted with permission
from [94] (p. 6). Copyright 2016 Wiley.
Functionalizing the elevator supraparticle surface with the enzyme
α
-amylase, the catalytic
decomposition of starch could be performed as a model reaction, showing potential applications of
these elevator supraparticles in chemical catalysis [
94
]. Accordingly, this example proves the concept
of self-propelled particles, whose movement can be steered by an external magnetic field and which
are able to perform a chemical task (reaction) on their trajectory. Of course, the use of H
2
O
2
limits the
potential for applications substantially, but it might be noted that it is also possible to use more benign,
non-toxic fuels, such as alcohols, for particle propulsion [128].
Finally, supraparticles are also interesting for the general field of catalysis, as they constitute
rather versatile and small reaction containers. As an example in the field of catalysis, zinc sulfide
supraparticles were developed for chemical dechlorination of 2,2
0
,4,4
0
,5,5
0
-hexachlorobiphenyl,
an organic pollutant occurring in soils or groundwater [
129
]. Using UV irradiation, these supraparticles
reductively degraded the model pollutant up to about 70% after 12 h in a solution of isooctane.
Even more complicated, Xu et al. showed that Pt nanoparticles bound to a porous organo-polymer
shell synthesized via soap-free emulsion polymerization onto Fe
3
O
4
supraparticles can perform
catalytic enantioselective hydrogenation of ethyl pyruvate [130].
Of course, this short section is far from complete with respect to the applications that have been
explored for supraparticles, simply due to the fact that this is an ample field of research that allows for
a vast number of options via appropriate structural and functional design by using various kinds of
colloidal components that can be readily applied. Thus, many interesting developments have already
been achieved and many more are to be expected in the near future to emerge from this field.
7. Conclusions
Supraparticles can be rich in terms of their size, shape and internal structure, and their properties
can be varied largely via the choice of their colloidal constituents. In our review, we focused deliberately
on supraparticles in the size range of hundreds of
µ
m or even mm. Of course, colloidal assembly is
not limited to this size range, and a much work has also been done on the assembly of nanoparticles,
including their structured assemblies at surfaces [
6
,
131
–
134
]. However, in this review, we explicitly
refrained from covering such investigations.
For the fabrication of supraparticles, a large number of methodologies has been developed,
where in particular evaporation-induced self-assembly (EISA) on superhydrophobic surfaces has
Gels 2017,3, 15 21 of 27
distinct advantages, which therefore was also the focus within this review. First, it is rather simple
to collect the pure supraparticles subsequent to their synthesis since no other solvent has to be
removed. Secondly, it has been shown that via EISA, one can achieve anisometric supraparticles
whose orientation can be controlled by appropriately structuring the superhydrophobic substrate.
This consequently allows preparing anisometric patchy supraparticles where the location of the patches
can be precisely controlled (for instance for patches containing magnetic nanoparticles by an external
magnet). It might be noted that this approach is not limited to the use of aqueous colloidal dispersions.
Instead, the utilization of superamphiphobic surfaces also allows for its extension to a large range of
organic solvents.
By combining the shape control of the supraparticles with a detailed control over the internal,
e.g., hierarchical, structure by careful choice of the colloidal components, increasingly more complex
systems with tailored functionality are available, which are interesting for a multitude of applications.
These functionalities include optical properties that can be interesting for photonic applications
and electric or catalytic properties that can be achieved by incorporating appropriately active
(nano-)particles. Another functional feature is self-propulsion, which for instance can be achieved
by incorporating nanoparticles and/or enzymes able to induce chemical reactions that lead to
a momentum on the supraparticle. The movement then can be of a vertical and/or horizontal
direction and also magnetically steered. One may also combine such a movement with other functional
properties additionally incorporated in these supraparticles, such as catalytic activity for performing
chemical reactions, thus providing “chemical microrobots”. These “multi-tasking” supraparticles can
be expected to become substantially further developed, and one may envision almost an unlimited
potential via the combined functionalities that can be imparted.
In summary, there are rather simple ways to produce supraparticles, and by appropriate design,
it is possible to integrate different functionalities into them, which can be independently combined and
addressed. Accordingly, they represent smart systems, able to exhibit many interesting properties and
being useful for several applications, like optical, electronic, chemical, mechanical ones, etc. However,
the state of the art in this area certainly is still in its infancy, and one may expect many interesting
developments leading to increased complexity in the structure and function of future materials.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
BNDF brain-derived neurotrophic factor
CA contact angle
CCA constant contact angle
CCR constant contact radius
cmc critical micelle concentration
CVD chemical vapor deposition
DEX dexamethasone
ECD electrochemical deposition
EISA evaporation-induced self-assembly
FS fumed silica
PEO polyethylene oxide
PMMA poly methyl methacrylate
PS polystyrene
SEM scanning electron microscopy
TMPTA trimethylolpropane triacrylate
TNF-αtumor necrosis factor-alpha
TPCL three-phase contact line
UV ultra-violet
WCA water contact angle
Gels 2017,3, 15 22 of 27
References
1.
Lu, Z.D.; Yin, Y.D. Colloidal nanoparticle clusters: Functional materials by design. Chem. Soc. Rev.
2012
,41,
6874–6887. [CrossRef] [PubMed]
2.
Xia, Y.N.; Yin, Y.D.; Lu, Y.; McLellan, J. Template-assisted self-assembly of spherical colloids into complex
and controllable structures. Adv. Funct. Mater. 2003,13, 907–918. [CrossRef]
3.
Stein, A.; Schroden, R.C. Colloidal crystal templating of three-dimensionally ordered macroporous solids:
Materials for photonics and beyond. Curr. Opin. Solid State Mater. Sci. 2001,5, 553–564. [CrossRef]
4.
Velev, O.D.; Gupta, S. Materials Fabricated by Micro- and Nanoparticle Assembly—The Challenging Path
from Science to Engineering. Adv. Mater. 2009,21, 1897–1905. [CrossRef]
5.
Li, F.; Josephson, D.P.; Stein, A. Colloidal Assembly: The Road from Particles to Colloidal Molecules and
Crystals. Angew. Chem. Int. Ed. 2011,50, 360–388. [CrossRef] [PubMed]
6.
Grzelczak, M.; Vermant, J.; Furst, E.M.; Liz-Marzan, L.M. Directed Self-Assembly of Nanoparticles. ACS Nano
2010,4, 3591–3605. [CrossRef] [PubMed]
7.
Galisteo-Lopez, J.F.; Ibisate, M.; Sapienza, R.; Froufe-Perez, L.S.; Blanco, A.; Lopez, C. Self-Assembled
Photonic Structures. Adv. Mater. 2011,23, 30–69. [CrossRef] [PubMed]
8.
Van Blaaderen, A.; Ruel, R.; Wiltzius, P. Template-directed colloidal crystallization. Nature
1997
,385, 321–324.
[CrossRef]
9. Bragg, W.L. Diffraction of X-rays by Two-Dimensional Crystal Lattice. Nature 1929,124, 125. [CrossRef]
10.
Donaldson, J.G.; Kantorovich, S.S. Directional self-assembly of permanently magnetised nanocubes in quasi
two dimensional layers. Nanoscale 2015,7, 3217–3228. [CrossRef] [PubMed]
11.
Li, R.; Bian, K.; Wang, Y.; Xu, H.; Hollingsworth, J.A.; Hanrath, T.; Fang, J.; Wang, Z. An Obtuse
Rhombohedral Superlattice Assembled by Pt Nanocubes. Nano Lett.
2015
,15, 6254–6260. [CrossRef]
[PubMed]
12.
Van der Stam, W.; Gantapara, A.P.; Akkerman, Q.A.; Soligno, G.; Meeldijk, J.D.; van Roij, R.; Dijkstra, M.;
de Mello Donega, C. Self-Assembly of Colloidal Hexagonal Bipyramid- and Bifrustum-Shaped ZnS
Nanocrystals into Two-Dimensional Superstructures. Nano Lett. 2014,14, 1032–1037. [CrossRef] [PubMed]
13.
Choi, J.J.; Bian, K.; Baumgardner, W.J.; Smilgies, D.-M.; Hanrath, T. Interface-Induced Nucleation,
Orientational Alignment and Symmetry Transformations in Nanocube Superlattices. Nano Lett.
2012
,
12, 4791–4798. [CrossRef] [PubMed]
14.
Nguyen, T.D.; Jankowski, E.; Glotzer, S.C. Self-Assembly and Reconfigurability of Shape-Shifting Particles.
ACS Nano 2011,5, 8892–8903. [CrossRef] [PubMed]
15.
Epstein, E.; Yoon, J.; Madhukar, A.; Hsia, K.J.; Braun, P.V. Colloidal Particles that Rapidly Change Shape via
Elastic Instabilities. Small 2015,11, 6051–6057. [CrossRef] [PubMed]
16.
Lu, Y.; Fan, H.; Stump, A.; Ward, T.L.; Rieker, T.; Brinker, C.J. Aerosol-assisted self-assembly of
mesostructured spherical nanoparticles. Nature 1999,398, 223–226.
17.
Brezesinski, T.; Groenewolt, M.; Gibaud, A.; Pinna, N.; Antonietti, M.; Smarsly, B. Evaporation-Induced
Self-Assembly (EISA) at Its Limit: Ultrathin, Crystalline Patterns by Templating of Micellar Monolayers.
Adv. Mater. 2006,18, 2260–2263. [CrossRef]
18.
Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Super-Hydrophobic
Surfaces: From Natural to Artificial. Adv. Mater. 2002,14, 1857–1860. [CrossRef]
19.
Sun, T.; Feng, L.; Gao, X.; Jiang, L. Bioinspired Surfaces with Special Wettability. Acc. Chem. Res.
2005
,38,
644–652. [CrossRef] [PubMed]
20.
Bhushan, B.; Jung, Y.C. Natural and biomimetic artificial surfaces for superhydrophobicity, self-cleaning,
low adhesion, and drag reduction. Prog. Mater. Sci. 2011,56, 1–108. [CrossRef]
21.
Genzer, J.; Efimenko, K. Recent developments in superhydrophobic surfaces and their relevance to marine
fouling: A review. Biofouling 2006,22, 339–360. [CrossRef] [PubMed]
22.
Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. What do we need for a superhydrophobic surface? A review on
the recent progress in the preparation of superhydrophobic surfaces. Chem. Soc. Rev.
2007
,36, 1350–1368.
[CrossRef] [PubMed]
23.
Brinker, C.J.; Lu, Y.; Sellinger, A.; Fan, H. Evaporation-Induced Self-Assembly: Nanostructures Made Easy.
Adv. Mater. 1999,11, 579–585. [CrossRef]
Gels 2017,3, 15 23 of 27
24.
Grosso, D.; Cagnol, F.; Soler, G.J.D.A.; Crepaldi, E.L.; Amenitsch, H.; Brunet-Bruneau, A.; Bourgeois, A.;
Sanchez, C. Fundamentals of Mesostructuring Through Evaporation-Induced Self-Assembly. Adv. Funct. Mater.
2004,14, 309–322. [CrossRef]
25.
Velev, O.D.; Lenhoff, A.M.; Kaler, E.W. A class of microstructured particles through colloidal crystallization.
Science 2000,287, 2240–2243. [CrossRef] [PubMed]
26.
Deegan, R.D.; Bakajin, O.; Dupont, T.F.; Huber, G.; Nagel, S.R.; Witten, T.A. Capillary flow as the cause of
ring stains from dried liquid drops. Nature 1997,389, 827–829. [CrossRef]
27.
Deegan, R.D.; Bakajin, O.; Dupont, T.F.; Huber, G.; Nagel, S.R.; Witten, T.A. Contact line deposits in
an evaporating drop. Phys. Rev. E 2000,62, 756–765. [CrossRef]
28.
Kuncicky, D.M.; Velev, O.D. Surface-guided templating of particle assemblies inside drying sessile droplets.
Langmuir 2008,24, 1371–1380. [CrossRef] [PubMed]
29.
Pauchard, L.; Parisse, F.; Allain, C. Influence of salt content on crack patterns formed through colloidal
suspension desiccation. Phys. Rev. E 1999,59, 3737–3740. [CrossRef]
30.
Brutin, D.; Sobac, B.; Loquet, B.; Sampol, J. Pattern formation in drying drops of blood. J. Fluid Mech.
2011
,
667, 85–95. [CrossRef]
31.
Sobac, B.; Brutin, D. Structural and evaporative evolutions in desiccating sessile drops of blood. Phys. Rev. E
2011,84, 011603. [CrossRef] [PubMed]
32.
Joksimovic, R.; Watanabe, S.; Riemer, S.; Gradzielski, M.; Yoshikawa, K. Self-organized patterning through
the dynamic segregation of DNA and silica nanoparticles. Sci. Rep. 2014,4, 3660. [CrossRef] [PubMed]
33.
Chang, S.T.; Velev, O.D. Evaporation-induced particle microseparations inside droplets floating on a chip.
Langmuir 2006,22, 1459–1468. [CrossRef] [PubMed]
34.
Tam, D.; von Arnim, V.; McKinley, G.H.; Hosoi, A.E. Marangoni convection in droplets on superhydrophobic
surfaces. J. Fluid Mech. 2009,624, 101–123. [CrossRef]
35.
Hu, H.; Larson, R.G. Marangoni Effect Reverses Coffee-Ring Depositions. J. Phys. Chem. B
2006
,110,
7090–7094. [CrossRef] [PubMed]
36.
Hu, H.; Larson, R.G. Analysis of the Microfluid Flow in an Evaporating Sessile Droplet. Langmuir
2005
,21,
3963–3971. [CrossRef] [PubMed]
37.
Hu, H.; Larson, R.G. Analysis of the effects of Marangoni stresses on the microflow in an evaporating sessile
droplet. Langmuir 2005,21, 3972–3980. [CrossRef] [PubMed]
38.
Eral, H.B.; Augustine, D.M.; Duits, M.H.G.; Mugele, F. Suppressing the coffee stain effect: How to control
colloidal self-assembly in evaporating drops using electrowetting. Soft Matter
2011
,7, 4954–4958. [CrossRef]
39. Krupenkin, T.N.; Taylor, J.A.; Schneider, T.M.; Yang, S. From rolling ball to complete wetting: The dynamic
tuning of liquids on nanostructured surfaces. Langmuir 2004,20, 3824–3827. [CrossRef] [PubMed]
40.
McHale, G.; Brown, C.V.; Newton, M.I.; Wells, G.G.; Sampara, N. Dielectrowetting Driven Spreading of
Droplets. Phys. Rev. Lett. 2011,107, 186101. [CrossRef] [PubMed]
41.
Picknett, R.G.; Bexon, R. Evaporation of Sessile or Pendant Drops in Still Air. J. Colloid Interface Sci.
1977
,61,
336–350. [CrossRef]
42.
Maxwell, J.C. Diffusion. In Collected Scientific Papers: Diffusion; Encyclopedia Britannica: Cambridge,
UK, 1877.
43.
Snow, C. Potential Problems and Capacitance for a Conductor Bounded by Two Intersecting Spheres. J. Res.
Nat. Bur. Stand. 1949,43, 377–407. [CrossRef]
44.
Soulie, V.; Karpitschka, S.; Lequien, F.; Prene, P.; Zemb, T.; Moehwald, H.; Riegler, H. The evaporation
behavior of sessile droplets from aqueous saline solutions. Phys. Chem. Chem. Phys.
2015
,17, 22296–22303.
[CrossRef] [PubMed]
45.
Nguyen, T.A.H.; Nguyen, A.V. Increased Evaporation Kinetics of Sessile Droplets by Using Nanoparticles.
Langmuir 2012,28, 16725–16728. [CrossRef] [PubMed]
46.
Yu, Y.S.; Wang, Z.Q.; Zhao, Y.P. Experimental and theoretical investigations of evaporation of sessile water
droplet on hydrophobic surfaces. J. Colloid Interface Sci. 2012,365, 254–259. [CrossRef] [PubMed]
47.
Barthlott, W. Self-cleaning surfaces of objects and process for producing same. U.S. Patent 6,660,363,
9 December 2003.
48.
Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces.
Planta 1997,202, 1–8. [CrossRef]
Gels 2017,3, 15 24 of 27
49.
Koch, K.; Bhushan, B.; Jung, Y.C.; Barthlott, W. Fabrication of artificial Lotus leaves and significance of
hierarchical structure for superhydrophobicity and low adhesion. Soft Matter
2009
,5, 1386–1393. [CrossRef]
50.
Feng, X.J.; Jiang, L. Design and creation of superwetting/antiwetting surfaces. Adv. Mater.
2006
,18,
3063–3078. [CrossRef]
51.
Deng, X.; Mammen, L.; Butt, H.J.; Vollmer, D. Candle Soot as a Template for a Transparent Robust
Superamphiphobic Coating. Science 2012,335, 67–70. [CrossRef] [PubMed]
52. Chu, Z.; Seeger, S. Superamphiphobic surfaces. Chem. Soc. Rev. 2014,43, 2784–2798. [CrossRef] [PubMed]
53. Cassie, A.B.D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944,40, 546–551. [CrossRef]
54.
Wenzel, R.N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem.
1936
,28, 988–994. [CrossRef]
55.
Genzer, J.; Marmur, A. Biological and synthetic self-cleaning surfaces. Mrs Bull.
2008
,33, 742–746. [CrossRef]
56.
Papadopoulos, P.; Mammen, L.; Deng, X.; Vollmer, D.; Butt, H.-J. How superhydrophobicity breaks down.
Proc. Natl. Acad. Sci. USA 2013,110, 3254–3258. [CrossRef] [PubMed]
57.
Celia, E.; Darmanin, T.; de Givenchy, E.T.; Amigoni, S.; Guittard, F. Recent advances in designing
superhydrophobic surfaces. J. Colloid Interface Sci. 2013,402, 1–18. [CrossRef] [PubMed]
58.
Yan, Y.Y.; Gao, N.; Barthlott, W. Mimicking natural superhydrophobic surfaces and grasping the wetting
process: A review on recent progress in preparing superhydrophobic surfaces. Adv. Colloid Interface Sci.
2011
,
169, 80–105. [CrossRef] [PubMed]
59.
Gu, C.D.; Ren, H.; Tu, J.P.; Zhang, T.Y. Micro/Nanobinary Structure of Silver Films on Copper Alloys with
Stable Water-Repellent Property under Dynamic Conditions. Langmuir
2009
,25, 12299–12307. [CrossRef]
[PubMed]
60.
Sperling, M.; Papadopoulos, P.; Gradzielski, M. Understanding the Formation of Anisometric Supraparticles:
A Mechanistic Look Inside Droplets Drying on a Superhydrophobic Surface. Langmuir
2016
,32, 6902–6908.
[CrossRef] [PubMed]
61.
Tuteja, A.; Choi, W.; Ma, M.L.; Mabry, J.M.; Mazzella, S.A.; Rutledge, G.C.; McKinley, G.H.; Cohen, R.E.
Designing superoleophobic surfaces. Science 2007,318, 1618–1622. [CrossRef] [PubMed]
62.
Phillips, K.R.; England, G.T.; Sunny, S.; Shirman, E.; Shirman, T.; Vogel, N.; Aizenberg, J. A colloidoscope of
colloid-based porous materials and their uses. Chem. Soc. Rev. 2016,45, 281–322. [CrossRef] [PubMed]
63.
Il Park, J.; Nguyen, T.D.; Silveira, G.D.; Bahng, J.H.; Srivastava, S.; Zhao, G.P.; Sun, K.; Zhang, P.J.; Glotzer, S.C.;
Kotov, N.A. Terminal supraparticle assemblies from similarly charged protein molecules and nanoparticles.
Nat. Commun. 2014,5, 3593.
64.
Piccinini, E.; Pallarola, D.; Battaglini, F.; Azzaroni, O. Recognition-driven assembly of self-limiting
supramolecular protein nanoparticles displaying enzymatic activity. Chem. Commun.
2015
,51, 14754–14757.
[CrossRef] [PubMed]
65.
Xia, Y.; Tang, Z. Monodisperse Hollow Supraparticles via Selective Oxidation. Adv. Funct. Mater.
2012
,22,
2585–2593. [CrossRef]
66.
Yang, G.; Zhong, H.; Liu, R.; Li, Y.; Zou, B. In Situ Aggregation of ZnSe Nanoparticles into Supraparticles:
Shape Control and Doping Effects. Langmuir 2013,29, 1970–1976. [CrossRef] [PubMed]
67.
Yu, S.; Wan, J.; Chen, K. A facile synthesis of superparamagnetic Fe
3
O
4
supraparticles@MIL-100(Fe) core-shell
nanostructures: Preparation, characterization and biocompatibility. J. Colloid Interface Sci.
2016
,461, 173–178.
[CrossRef] [PubMed]
68.
Guo, J.; Yang, W.; Wang, C. Magnetic Colloidal Supraparticles: Design, Fabrication and Biomedical
Applications. Adv. Mater. 2013,25, 5196–5214. [CrossRef] [PubMed]
69.
Edwards, E.W.; Wang, D.; Möhwald, H. Hierarchical Organization of Colloidal Particles: From Colloidal
Crystallization to Supraparticle Chemistry. Macromol. Chem. Phys. 2007,208, 439–445. [CrossRef]
70.
Xia, Y.S.; Nguyen, T.D.; Yang, M.; Lee, B.; Santos, A.; Podsiadlo, P.; Tang, Z.Y.; Glotzer, S.C.;
Kotov, N.A. Self-assembly of self-limiting monodisperse supraparticles from polydisperse nanoparticles.
Nat. Nanotechnol. 2011,6, 580–587. [CrossRef] [PubMed]
71.
Cho, Y.S.; Yi, G.R.; Kim, S.H.; Elsesser, M.T.; Breed, D.R.; Yang, S.M. Homogeneous and heterogeneous binary
colloidal clusters formed by evaporation-induced self-assembly inside droplets. J. Colloid Interface Sci.
2008
,
318, 124–133. [CrossRef] [PubMed]
72.
Velev, O.D.; Furusawa, K.; Nagayama, K. Assembly of latex particles by using emulsion droplets as templates.
2. Ball-like and composite aggregates. Langmuir 1996,12, 2385–2391. [CrossRef]
Gels 2017,3, 15 25 of 27
73.
Velev, O.D.; Furusawa, K.; Nagayama, K. Assembly of latex particles by using emulsion droplets as templates.
1. Microstructured hollow spheres. Langmuir 1996,12, 2374–2384. [CrossRef]
74.
Cho, Y.-S.; Kim, S.-H.; Yi, G.-R.; Yang, S.-M. Self-organization of colloidal nanospheres inside emulsion
droplets: Higher-order clusters, supraparticles, and supraballs. Colloids Surf. A
2009
,345, 237–245. [CrossRef]
75.
Maeda, K.; Onoe, H.; Takinoue, M.; Takeuchi, S. Controlled Synthesis of 3D Multi-Compartmental Particles
with Centrifuge-Based Microdroplet Formation from a Multi-Barrelled Capillary. Adv. Mater.
2012
,24,
1340–1346. [CrossRef] [PubMed]
76.
Yu, Z.Y.; Wang, C.F.; Ling, L.T.; Chen, L.; Chen, S. Triphase Microfluidic-Directed Self-Assembly: Anisotropic
Colloidal Photonic Crystal Supraparticles and Multicolor Patterns Made Easy. Angew. Chem. Int. Ed.
2012
,
51, 2375–2378. [CrossRef] [PubMed]
77.
Wang, J.T.; Wang, J.; Han, J.J. Fabrication of Advanced Particles and Particle-Based Materials Assisted by
Droplet-Based Microfluidics. Small 2011,7, 1728–1754. [CrossRef] [PubMed]
78.
Brugarolas, T.; Tu, F.; Lee, D. Directed assembly of particles using microfluidic droplets and bubbles.
Soft Matter 2013,9, 9046–9058. [CrossRef]
79.
Sowade, E.; Blaudeck, T.; Baumann, R.R. Self-Assembly of Spherical Colloidal Photonic Crystals inside
Inkjet-Printed Droplets. Cryst. Growth Des. 2016,16, 1017–1026. [CrossRef]
80.
Sowade, E.; Hammerschmidt, J.; Blaudeck, T.; Baumann, R.R. In-Flight Inkjet Self-Assembly of Spherical
Nanoparticle Aggregates. Adv. Eng. Mater. 2012,14, 98–100. [CrossRef]
81.
Chen, F.C.; Lu, J.P.; Huang, W.K. Using Ink-Jet Printing and Coffee Ring Effect to Fabricate Refractive
Microlens Arrays. IEEE Photonics Technol. Lett. 2009,21, 648–650. [CrossRef]
82.
Wang, D.; Park, M.; Park, J.; Moon, J. Optical properties of single droplet of photonic crystal assembled by
ink-jet printing. Appl. Phys. Lett. 2005,86, 241114. [CrossRef]
83.
Millman, J.R.; Bhatt, K.H.; Prevo, B.G.; Velev, O.D. Anisotropic particle synthesis in dielectrophoretically
controlled microdroplet reactors. Nat. Mater. 2005,4, 98–102. [CrossRef] [PubMed]
84.
Denkov, N.D.; Velev, O.D.; Kralchevsky, P.A.; Ivanov, I.B.; Yoshimura, H.; Nagayama, K. 2-Dimensional
Crystallization. Nature 1993,361, 26. [CrossRef]
85.
Denkov, N.D.; Velev, O.D.; Kralchevsky, P.A.; Ivanov, I.B.; Yoshimura, H.; Nagayama, K. Mechanism of
Formation of 2-Dimensional Crystals from Latex-Particles on Substrates. Langmuir
1992
,8, 3183–3190.
[CrossRef]
86.
Lee, D.-W.; Jin, M.-H.; Lee, C.-B.; Oh, D.; Ryi, S.-K.; Park, J.-S.; Bae, J.-S.; Lee, Y.-J.; Park, S.-J.; Choi, Y.-C.
Facile synthesis of mesoporous silica and titania supraparticles by a meniscus templating route on
a superhydrophobic surface and their application to adsorbents. Nanoscale
2014
,6, 3483–3487. [CrossRef]
[PubMed]
87.
Rastogi, V.; Velikov, K.P.; Velev, O.D. Microfluidic characterization of sustained solute release from porous
supraparticles. Phys. Chem. Chem. Phys. 2010,12, 11975–11983. [CrossRef] [PubMed]
88.
Manoharan, V.N. Colloidal spheres confined by liquid droplets: Geometry, physics, and physical chemistry.
Solid State Commun. 2006,139, 557–561. [CrossRef]
89.
Liu, M.; Zheng, Y.; Zhai, J.; Jiang, L. Bioinspired Super-antiwetting Interfaces with Special Liquid-Solid
Adhesion. Acc. Chem. Res. 2010,43, 368–377. [CrossRef] [PubMed]
90.
Bhushan, B.; Her, E.K. Fabrication of Superhydrophobic Surfaces with High and Low Adhesion Inspired
from Rose Petal. Langmuir 2010,26, 8207–8217. [CrossRef] [PubMed]
91.
Wooh, S.; Huesmann, H.; Tahir, M.N.; Paven, M.; Wichmann, K.; Vollmer, D.; Tremel, W.; Papadopoulos, P.;
Butt, H.-J. Synthesis of Mesoporous Supraparticles on Superamphiphobic Surfaces. Adv. Mater.
2015
,27,
7338–7343. [CrossRef] [PubMed]
92.
Rastogi, V.; Garcia, A.A.; Marquez, M.; Velev, O.D. Anisotropic Particle Synthesis Inside Droplet Templates
on Superhydrophobic Surfaces. Macromol. Rapid Commun. 2010,31, 190–195. [CrossRef] [PubMed]
93.
Sperling, M.; Velev, O.D.; Gradzielski, M. Controlling the Shape of Evaporating Droplets by Ionic Strength:
Formation of Highly Anisometric Silica Supraparticles. Angew. Chem. Int. Ed.
2014
,53, 586–590. [CrossRef]
[PubMed]
94.
Sperling, M.; Kim, H.J.; Velev, O.D.; Gradzielski, M. Active Steerable Catalytic Supraparticles Shuttling on
Preprogrammed Vertical Trajectories. Adv. Mater. Interf. 2016,3, 160095. [CrossRef]
95.
Sperling, M.; Velev, O.D.; Gradzielski, M. Formation of Anisometric Fumed Silica Supraparticles—Mechanism
and Application Potential. Z. Phys. Chem. 2015,229, 1055–1074. [CrossRef]
Gels 2017,3, 15 26 of 27
96.
Sperling, M.; Velev, O.D.; Gradzielski, M. Kontrolle der Form verdunstender Tropfen über die Ionenstärke:
Bildung anisometrischer SiO2-Suprapartikel. Angew. Chem. 2014,126, 597–601. [CrossRef]
97.
Rastogi, V.; Melle, S.; Calderon, O.G.; Garcia, A.A.; Marquez, M.; Velev, O.D. Synthesis of Light-Diffracting
Assemblies from Microspheres and Nanoparticles in Droplets on a Superhydrophobic Surface. Adv. Mater.
2008,20, 4263–4268. [CrossRef]
98. Princen, H.M. Surface and Colloid Science; Matijevic, E., Ed.; Wiley: New York, NY, USA, 1969; p. 1.
99.
Head, D.A. Modeling the elastic deformation of polymer crusts formed by sessile droplet evaporation.
Phys. Rev. E 2006,74, 021601. [CrossRef] [PubMed]
100.
Tsapis, N.; Dufresne, E.R.; Sinha, S.S.; Riera, C.S.; Hutchinson, J.W.; Mahadevan, L.; Weitz, D.A. Onset of
Buckling in Drying Droplets of Colloidal Suspensions. Phys. Rev. Lett.
2005
,94, 018302. [CrossRef] [PubMed]
101.
Kajiya, T.; Nishitani, E.; Yamaue, T.; Doi, M. Piling-to-buckling transition in the drying process of polymer
solution drop on substrate having a large contact angle. Phys. Rev. E
2006
,73, 011601. [CrossRef] [PubMed]
102.
Gu, Z.-Z.; Yu, Y.-H.; Zhang, H.; Chen, H.; Lu, Z.; Fujishima, A.; Sato, O. Self-assembly of monodisperse
spheres on substrates with different wettability. Appl. Phys. A 2005,81, 47–49. [CrossRef]
103.
Kuncicky, D.M.; Bose, K.; Costa, K.D.; Velev, O.D. Sessile Droplet Templating of Miniature Porous
Hemispheres from Colloid Crystals. Chem. Mater. 2007,19, 141–143. [CrossRef]
104.
Sen, D.; Bahadur, J.; Mazumder, S.; Santoro, G.; Yu, S.; Roth, S.V. Probing evaporation induced assembly
across a drying colloidal droplet using in situ small-angle X-ray scattering at the synchrotron source.
Soft Matter 2014,10, 1621–1627. [CrossRef] [PubMed]
105.
Kätzel, U.; Vorbau, M.; Stintz, M.; Gottschalk-Gaudig, T.; Barthel, H. Dynamic Light Scattering for the
Characterization of Polydisperse Fractal Systems: II. Relation between Structure and DLS Results. Part. Part.
Syst. Charact. 2008,25, 19–30. [CrossRef]
106.
Accardo, A.; Gentile, F.; Mecarini, F.; Angelis, F.D.; Burghammer, M.; Fabrizio, E.D.; Riekel, C. In Situ X-ray
Scattering Studies of Protein Solution Droplets Drying on Micro- and Nanopatterned Superhydrophobic
PMMA Surfaces. Langmuir 2010,26, 15057–15064. [CrossRef] [PubMed]
107.
Bahadur, J.; Sen, D.; Mazumder, S.; Paul, B.; Bhatt, H.; Singh, S.G. Control of Buckling in Colloidal Droplets
during Evaporation-Induced Assembly of Nanoparticles. Langmuir
2012
,28, 1914–1923. [CrossRef] [PubMed]
108.
Sperling, M.; Spiering, V.J.; Velev, O.D.; Gradzielski, M. Controlled Formation of Patchy Anisometric Fumed
Silica Supraparticles in Droplets on Bent Superhydrophobic Surfaces. Part. Part. Syst. Charact.
2017
,34,
1600176. [CrossRef]
109.
Zhou, J.; Yang, J.; Gu, Z.; Zhang, G.; Wei, Y.; Yao, X.; Song, Y.; Jiang, L. Controllable Fabrication of
Noniridescent Microshaped Photonic Crystal Assemblies by Dynamic Three-Phase Contact Line Behaviors
on Superhydrophobic Substrates. ACS Appl. Mater. Interfaces 2015,7, 22644–22651. [CrossRef] [PubMed]
110.
Pawar, A.B.; Kretzschmar, I. Fabrication, Assembly, and Application of Patchy Particles. Macromol. Rapid
Commun. 2010,31, 150–168. [CrossRef] [PubMed]
111.
Jiang, S.; Chen, Q.; Tripathy, M.; Luijten, E.; Schweizer, K.S.; Granick, S. Janus Particle Synthesis and
Assembly. Adv. Mater. 2010,22, 1060–1071. [CrossRef] [PubMed]
112. Walther, A.; Muller, A.H.E. Janus particles. Soft Matter 2008,4, 663–668. [CrossRef]
113.
Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lami, E.; Duguet, E. Design and synthesis of Janus micro- and
nanoparticles. J. Mater. Chem. 2005,15, 3745–3760. [CrossRef]
114.
Ding, Y.; Xu, S.; Wang, Z.L. Structural colors from Morpho peleides butterfly wing scales. J. Appl. Phys.
2009
,
106, 074702. [CrossRef]
115.
Vignolini, S.; Rudall, P.J.; Rowland, A.V.; Reed, A.; Moyroud, E.; Faden, R.B.; Baumberg, J.J.; Glover, B.J.;
Steiner, U. Pointillist structural color in Pollia fruit. Proc. Natl. Acad. Sci. USA
2012
,109, 15712. [CrossRef]
[PubMed]
116.
Yeo, S.J.; Tu, F.; Kim, S.; Yi, G.-R.; Yoo, P.J.; Lee, D. Angle- and strain-independent coloured free-standing
films incorporating non-spherical colloidal photonic crystals. Soft Matter
2015
,11, 1582–1588. [CrossRef]
[PubMed]
117.
Liu, S.-S.; Wang, C.-F.; Wang, X.-Q.; Zhang, J.; Tian, Y.; Yin, S.-N.; Chen, S. Tunable Janus colloidal photonic
crystal supraballs with dual photonic band gaps. J. Mater. Chem. C 2014,2, 9431–9438. [CrossRef]
118.
Wang, H.; Yang, S.; Yin, S.-N.; Chen, L.; Chen, S. Janus Suprabead Displays Derived from the Modified
Photonic Crystals toward Temperature Magnetism and Optics Multiple Responses. ACS Appl. Mater. Interfaces
2015,7, 8827–8833. [CrossRef] [PubMed]
Gels 2017,3, 15 27 of 27
119.
Rastogi, V.; Velev, O.D. Development and evaluation of realistic microbioassays in freely suspended droplets
on a chip. Biomicrofluidics 2007,1, 014107. [CrossRef] [PubMed]
120.
Maina, J.W.; Cui, J.; Björnmalm, M.; Wise, A.K.; Shepherd, R.K.; Caruso, F. Mold-Templated
Inorganic-Organic Hybrid Supraparticles for Codelivery of Drugs. Biomacromolecules
2014
,15, 4146–4151.
[CrossRef] [PubMed]
121.
Wang, Y.; Wise, A.K.; Tan, J.; Maina, J.W.; Shepherd, R.K.; Caruso, F. Mesoporous Silica Supraparticles for
Sustained Inner-Ear Drug Delivery. Small 2014,10, 4244–4248. [CrossRef] [PubMed]
122.
Park, W.M.; Yee, C.M.; Champion, J.A. Self-assembled hybrid supraparticles that proteolytically degrade
tumor necrosis factor-α.J. Mater. Chem. B 2016,4, 1633–1639. [CrossRef]
123.
Sanchez, S.; Soler, L.; Katuri, J. Chemically Powered Micro- and Nanomotors. Angew. Chem. Int. Ed.
2015
,54,
1414–1444. [CrossRef] [PubMed]
124. Ebbens, S.J.; Howse, J.R. In pursuit of propulsion at the nanoscale. Soft Matter 2010,6, 726–738. [CrossRef]
125.
Soler, L.; Sanchez, S. Catalytic nanomotors for environmental monitoring and water remediation. Nanoscale
2014,6, 7175–7182. [CrossRef] [PubMed]
126.
Gao, W.; Wang, J. The Environmental Impact of Micro/Nanomachines: A Review. ACS Nano
2014
,8,
3170–3180. [CrossRef] [PubMed]
127.
Patra, D.; Sengupta, S.; Duan, W.; Zhang, H.; Pavlick, R.; Sen, A. Intelligent, self-powered, drug delivery
systems. Nanoscale 2013,5, 1273–1283. [CrossRef] [PubMed]
128.
Yamamoto, D.; Takada, T.; Tachibana, M.; Iijima, Y.; Shioi, A.; Yoshikawa, K. Micromotors working in water
through artificial aerobic metabolism. Nanoscale 2015,7, 13186–13190. [CrossRef] [PubMed]
129.
He, L.; Xiong, Y.; Zhao, M.; Mao, X.; Liu, Y.; Zhao, H.; Tang, Z. Bioinspired Synthesis of ZnS Supraparticles
toward Photoinduced Dechlorination of 2,2
0
,4,4
0
,5,5
0
-Hexachlorobiphenyl. Chem. Asian J.
2013
,8, 1765–1767.
[CrossRef] [PubMed]
130.
Xu, S.; Weng, Z.; Tan, J.; Guo, J.; Wang, C. Hierarchically structured porous organic polymer microspheres
with built-in Fe
3
O
4
supraparticles: Construction of dual-level pores for Pt-catalyzed enantioselective
hydrogenation. Polym. Chem. 2015,6, 2892–2899. [CrossRef]
131.
Wang, D.; Möhwald, H. Template-directed colloidal self-assembly—The route to ‘top-down’ nanochemical
engineering. J. Mater. Chem. 2004,14, 459–468. [CrossRef]
132.
Renna, L.A.; Boyle, C.J.; Gehan, T.S.; Venkataraman, D. Polymer Nanoparticle Assemblies: A Versatile Route
to Functional Mesostructures. Macromolecules 2015,48, 6353–6368. [CrossRef]
133. Kotov, N.A. (Ed.) Nanoparticle Assemblies and Superstructures; Taylor & Francis: Abingdon, UK, 2016.
134.
Kuemin, C.; Huckstadt, K.C.; Lörtscher, E.; Rey, A.; Decker, A.; Spencer, N.D.; Wolf, H. Selective Assembly of
Sub-Micrometer Polymer Particles. Adv. Mater. 2010,22, 2804–2808. [CrossRef] [PubMed]
©
2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
