Ci a ion: Guzmán, E.; Maes o, A.
So Colloidal Pa icles a Fluid
In e aces. Polyme s 2022,14, 1133.
h ps://doi.o g/10.3390/
polym14061133
Academic Edi o : Felix Plampe
Recei ed: 25 Feb ua y 2022
Accep ed: 10 Ma ch 2022
Published: 11 Ma ch 2022
Publishe ’s No e: MDPI s ays neu al
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polyme s
Re iew
So Colloidal Pa icles a Fluid In e aces
Edua do Guzmán1,2,* and A mando Maes o 3,4,*
1Depa amen o de Química Física, Facul ad de Ciencias Químicas, Uni e sidad Complu ense de Mad id,
Ciudad Uni e si a ia s/n, 28040 Mad id, Spain
2Ins i u o Plu idisciplina , Uni e sidad Complu ense de Mad id, Paseo de Juan XXIII, 28040 Mad id, Spain
3
Cen o de Física de Ma e iales (CSIC, UPV/EHU), Paseo Manuel de La dizabal 5, 20018 San Sebas ian, Spain
4IKERBASQUE—Basque Founda ion o Science, Plaza Euskadi 5, 48009 Bilbao, Spain
*Co espondence: [email p o ec ed] (E.G.); [email p o ec ed] (A.M.)
Abs ac :
The assembly o so colloidal pa icles a luid in e aces is e iewed in he p esen
pape , wi h emphasis on he pa icula case o mic ogels o med by c oss-linked polyme ne wo ks.
The dual polyme /colloid cha ac e as well as he s imulus esponsi eness o mic ogel pa icles
pose a challenge in hei expe imen al cha ac e iza ion and heo e ical desc ip ion when adso bed
o luid in e aces. This has led o a con o e sial and, in some cases, con adic o y pic u e ha
canno be a ionalized by conside ing mic ogels as simple colloids. The e o e, i is necessa y o
ake in o conside a ion he mic ogel polyme /colloid duali y o a physically eliable desc ip ion
o he beha io o he mic ogel-laden in e ace. In ac , di e en aspec s ela ed o he abo e-
men ioned duali y con ol he o ganiza ion o mic ogels a he luid in e ace, and he p ope ies and
esponsi eness o he ob ained mic ogel-laden in e aces. This wo ks p esen a c i ical e ision o
di e en physicochemical aspec s in ol ing he beha io o indi idual mic ogels con ined a luid
in e aces, as well as he collec i e beha io s eme ging in dense mic ogel assemblies.
Keywo ds: de o mabili y; in e aces; mic ogels; so ness; in e acial ension; econ igu able
1. In oduc ion
The assembly o colloidal pa icles a luid in e aces is cu en ly exploi ed in a b oad
ange o indus ial and echnological ields, anging om ood, oil, and cosme ic indus ies
o pha maceu ical o mula ions, and om he manu ac u ing o ad anced coa ings o
biomedical applica ions [
1
–
8
]. In pa icula , he use o so colloidal pa icles gained
impo ance in he abo e applica ions in ecen yea s [9,10].
So colloidal pa icles can be ab ica ed by c oss-linked polyme ne wo ks o colloidal
dimensions swollen by a good sol en , commonly denomina ed as mic ogels. They a e
cha ac e ized by a polyme /colloid duali y. This can be unde s ood by conside ing he
chemis y and mo phology o mic ogels. Mic ogels a e so pa icles cha ac e ized by a
uzzy s uc u e, which p esen s dangling chains in hei swollen s a e, whe eas a beha io
eminiscen o ha expec ed o ha d colloids eme ges upon hei sh inking. This leads o a
e y di e en o ganiza ion o he polyme ne wo k depending on he pa icle hyd a ion.
Swollen mic ogels p esen s an open s uc u e cha ac e ized by hei di use ou e bounda y,
and he s ong sol a ion o i s inne segmen s, which a o s he mobili y o sol en s
and solu e molecules wi hin a ne wo k, and hei expansion un il he limi imposed by
he chain elas ici y. This con ibu es o he abili y o mic ogels o unde go e e sible
ne wo k de o ma ions by he up ake o expulsion o la ge quan i ies o sol en (good
sol en condi ions) upon he applica ion o ex e nal s imuli, which eme ges as one o he
mos ou s anding p ope ies o his ype o pa icle [
11
–
16
]. This olume phase ansi ion
(VPT), commonly igge ed by ex e nal pe u ba ions (in e acial ension o ces o ex e nal
physical s imuli), p o ides a b oad ange o no el s uc u e/ unc ion ela ionships o
mic ogel-laden in e aces [17,18].
Polyme s 2022,14, 1133. h ps://doi.o g/10.3390/polym14061133 h ps://www.mdpi.com/jou nal/polyme s
Polyme s 2022,14, 1133 2 o 17
Sol a ed mic ogel ne wo ks can be comp essed upon adso p ion o luid in e aces,
leading o he de o ma ion o he pa icle shape, o adap hei con o ma ion o he in e ace.
These mo phological changes also con ol he dynamic beha io and in e acial packing o
mic ogels a he in e ace, which, in u n, domina e he in e acial ension and heological
p ope ies o he mic ogel-laden in e aces [
19
], playing a e y impo an ole in emulsion
and oam s abiliza ion [
20
,
21
]. In pa icula , he s e ching o so pa icles obse ed when
assembled a luid in e aces p o ides a be e s abili y o he in e ace han he adso p ion
o adi ional ha d colloids. This can be a ionalized by he highe adso p ion ene gy o
mic ogels (se e al o de s o magni ude han ha o ha d pa icles wi h simila dimen-
sions) [
22
]. Fu he mo e, he bulk p ope ies o mic ogels a o hei adso p ion when hey
a e close o luid in e aces wi hou ul illing he amphiphilici y equi emen s [23].
The mos common mic ogel s udied in he li e a u e, o da e, is he one buil using
poly(N-isop opyl ac ylamide) (PNIPAM). This polyme allows o he p epa a ion o he -
mosensi i e mic ogels p esen ing he abili y o unde go e e sible empe a u e- igge ed
swelling/sh inking ansi ions a ound physiological human empe a u es [
24
–
26
]. The
p ope ies o his ype o gel can be easily uned du ing he syn hesis p ocess by including
co-monome s, e.g., ac ylic o me hac ylic acid, which enables a pH- igge ed change o he
mic ogel ioniza ion deg ee, and hence he mic ogel swelling deg ee [
27
]. Addi ionally, he
ab ica ion o mic ogel pa icles based on p o eins, e.g., whey p o ein o
β
-lac oglobulin,
gained in e es in ecen yea s due o hei po en ial applica ion in di e en ood p od-
uc s [18,20].
Gi en he abo e conside a ions, despi e he b oad in e es in he adso p ion o mic ogels
o luid/ luid in e aces, he cu en knowledge abou he e ec o he speci ic cha ac e is ics
o he mic ogels on hei in e acial o ganiza ion and mechanical p ope ies is sca ce, e en
hough i eme ges as a c i ical poin on he design o econ igu able in e aces o speci ic
applica ions [
21
]. This e iew p o ides an upda ed pe spec i e o he cu en knowledge abou
he assembly o mic ogel laye s a luid/ luid in e aces, and how di e en physico-chemical
aspec s can be exploi ed o ob ain econ igu able so in e aces o di e en echnological
pu poses, paying special in e es o hei uses o emulsion s abiliza ion.
2. Unde s anding he Adso p ion o So Pa icles- o-Fluid In e aces
The dual cha ac e o mic ogels plays a cen al ole in hei abili y o be adso bed
o he luid in e aces [
11
,
28
]. Mic ogels a e su ace-ac i e pa icles ha adso b o luid
in e aces educing he in e acial ension be ween wo immiscible luids, i.e., hey abso b
a he luid in e ace co e ing he maximum ac ion o he a ea a ailable o minimize he
con ac line be ween he wo immiscible luids, educing he in e acial ension [
29
]. This
leads o he mic ogel la ening a he in e ace in such a way ha i maximizes he occupied
a ea, which can push he mic ogel o adop a con o ma ion a he in e ace cha ac e ized
by a la ge diame e (mo e han wice he diame e ) han ha o a bulk mic ogel. This is
he esul o he compe i ion be ween he in e nal elas ici y o he polyme ic ne wo k and
he in e acial ension, which a o s he aniso opic de o ma ion o mic ogels adso bed
a he luid in e ace [
9
,
22
,
28
,
30
,
31
], leading o an ene ge ic landscape cha ac e ized by
high alues o he adso p ion ene gy and s ong in e -pa icle capilla y in e ac ions [
32
–
37
].
Howe e , in addi ion o his, he egion o he mic ogel imme sed in he aqueous phase
emains sol a ed and swollen a empe a u es below he cha ac e is ic VPT, adop ing he
so-called “ ied-egg” con o ma ion [
38
,
39
]. A ske ch o he concei able con o ma ional
change o mic ogels upon hei adso p ion o a luid in e ace is shown in Figu e 1a.
The sp eading o mic ogels a he in e ace is s ongly limi ed by he ne wo k elas ici y
o he pa icles, which is commonly de e mined by he c oss-linking densi y o he pa icles,
and he leng h o he chains be ween wo c oss-linking poin s associa ed wi h he polyme
sub-chain elas ici y. Consequen ly, he maximum de o ma ion o mic ogels adso bed a
a luid in e ace esul s om an in ica e balance be ween he dec ease in he in e acial
ene gy and he elas ocapilla y leng h o he mic ogel (L
EC
=
γS
/E), which e alua es he
a io be ween he in e acial ension o he solid γSand i s Young’s modulus E[22].
Polyme s 2022,14, 1133 3 o 17
The c oss-linking densi y has an impac on he in e acial ension dec ease obse ed
when mic ogels a e adso bed a a luid in e ace. In pa icula , he highe he c oss-linking
densi y, he la ge he in e acial ension dec ease and he weake he mic ogel la ening
a he in e ace [
40
]. Fu he mo e, he mic ogel size and i s de o mabili y play a e y
impo an ole in he equilib ium shape o he mic ogel a he luid in e ace ha , in u n,
has been obse ed o play a key ole in he s abiliza ion o he in e ace. On one hand, small
mic ogels, wi h a adius below he L
EC
, unde go a s ong de o ma ion o hei shape a he
in e ace, which leads o he o ma ion o liquid-like ilms o obla e-like mic ogel disks. On
he o he hand, la ge mic ogels (wi h a adius abo e he alue o L
EC
) unde go an almos
negligible change in hei shape upon adso p ion a he luid in e ace, which esul s in
beha io eminiscen o ha expec ed o non-de o mable colloids [40].
Polyme s 2022, 14, x FOR PEER REVIEW 3 o 17
mic ogels adso bed a a luid in e ace esul s om an in ica e balance be ween he
dec ease in he in e acial ene gy and he elas ocapilla y leng h o he mic ogel (LEC = γS/E),
which e alua es he a io be ween he in e acial ension o he solid γS and i s Young’s
modulus E [22].
The c oss-linking densi y has an impac on he in e acial ension dec ease obse ed
when mic ogels a e adso bed a a luid in e ace. In pa icula , he highe he c oss-linking
densi y, he la ge he in e acial ension dec ease and he weake he mic ogel la ening
a he in e ace [40]. Fu he mo e, he mic ogel size and i s de o mabili y play a e y
impo an ole in he equilib ium shape o he mic ogel a he luid in e ace ha , in u n,
has been obse ed o play a key ole in he s abiliza ion o he in e ace. On one hand,
small mic ogels, wi h a adius below he LEC, unde go a s ong de o ma ion o hei shape
a he in e ace, which leads o he o ma ion o liquid-like ilms o obla e-like mic ogel
disks. On he o he hand, la ge mic ogels (wi h a adius abo e he alue o LEC) unde go
an almos negligible change in hei shape upon adso p ion a he luid in e ace, which
esul s in beha io eminiscen o ha expec ed o non-de o mable colloids [40].
Figu e 1. (a) Ske ch o he mic ogel de o ma ion upon adso p ion o a luid in e ace. Adap ed om
Deshmukh e al. [28] wi h pe mission om he Royal Chemical Socie y, Copy igh (2014). (b)
Con o ma ion and dimension o mic ogel pa icles (cha ac e ized by he mic ogel adius a he
con ac leng h, , and heigh , h) a wa e /oil in e aces wi h di e en in e acial ensions (γ).
Rep in ed om Viale o e al. [41] unde he C ea i e Common A ibu ion License 4.0, 2022. (c)
Ske ch o he con o ma ion o he PNIPAM mic ogel a a wa e / apo in e ace a 25 °C and 50 °C.
Rep in ed om Ha e e al. [42] wi h pe mission om he Ame ican Chemical Socie y, Copy igh
(2019).
The in insic cha ac e is ics o he luid in e ace can also con ol he equilib ium
con o ma ion o he mic ogel a he in e ace. Adso bed mic ogels p o ude in o he wo
luid phases acco ding o he sol ency o he luids o he polyme chains yielding an
asymme ic pa i ioning o he mic ogel ac oss he in e ace (commonly deno ed as 3D
o ganiza ion) [17,22,36,38,41,43–50]. In addi ion, he con o ma ion o mic ogels adso bed
a a luid in e ace can be uned by a ying he chemical na u e o he luid phases, i.e.,
Figu e 1.
(
a
) Ske ch o he mic ogel de o ma ion upon adso p ion o a luid in e ace. Adap ed om
Deshmukh e al. [
28
] wi h pe mission om he Royal Chemical Socie y, Copy igh (2014). (
b
) Con o ma ion
and dimension o mic ogel pa icles (cha ac e ized by he mic ogel adius a he con ac leng h, , and
heigh , h) a wa e /oil in e aces wi h di e en in e acial ensions (
γ
). Rep in ed om Viale o e al. [
41
]
unde he C ea i e Common A ibu ion License 4.0, 2022. (
c
) Ske ch o he con o ma ion o he PNIPAM
mic ogel a a wa e / apo in e ace a 25
◦
C and 50
◦
C. Rep in ed om Ha e e al. [
42
] wi h pe mission
om he Ame ican Chemical Socie y, Copy igh (2019).
The in insic cha ac e is ics o he luid in e ace can also con ol he equilib ium
con o ma ion o he mic ogel a he in e ace. Adso bed mic ogels p o ude in o he wo
luid phases acco ding o he sol ency o he luids o he polyme chains yielding an
asymme ic pa i ioning o he mic ogel ac oss he in e ace (commonly deno ed as 3D
o ganiza ion) [
17
,
22
,
36
,
38
,
41
,
43
–
50
]. In addi ion, he con o ma ion o mic ogels adso bed
a a luid in e ace can be uned by a ying he chemical na u e o he luid phases, i.e., he
in e acial ension. Thus, he lowe he wa e /oil in e acial ension o simila polyme
solubili y on he oil, he smalle he de o ma ion o he mic ogel co e [
41
]. Figu e 1b
displays he con o ma ion and dimensions (cha ac e ized by he mic ogel’s adius a he
con ac leng h, , and heigh , h) o he mic ogel pa icles adso bed a he wa e /oil in e ace
Polyme s 2022,14, 1133 4 o 17
wi h di e en in e acial ensions, in which he solubili y o he mic ogel pa icles emains
simila . These esul s, ob ained by a omic o ce mic oscopy, AFM, show ha he highe
he su ace wa e /oil in e acial ension, he s onge he collapse o he mic ogels a he
in e ace, i.e., mic ogel sp eading a he in e ace is smalle and hei 3D heigh is lowe .
The abo e discussion is in ag eemen wi h he esul s ob ained by Zhang and Pel-
on [
51
], who demons a ed ha he in e acial ension dec ease associa ed wi h he ad-
so p ion o mic ogels a a wa e / apo in e ace was co ela ed o hei sp eading a he
in e ace. Fo ins ance, he adso p ion o he mo esponsi e PNIPAM mic ogel pa icles
dec eases he in e acial ension o he wa e / apo in e ace down o a alue o abou
43 mN/m a 25
◦
C. This alue is only sligh ly modi ied by he inc ease in he empe -
a u e o up o 40
◦
C, o he change o he deg ee o mic ogel c oss-linking. Howe e ,
he la e pa ame e impac s s ongly on he equilib a ion o he mic ogel-laden in e -
ace. Thus, he di e en de o ma ion o he pa icles in bulk de e mines he di e ences in
hei con o ma ion upon adso p ion a he in e ace. In ac , pa icles unde going s ong
de o ma ions upon adso p ion a he in e ace (low c oss-linking densi y) inc ease he
in e acial co e age quicke han less de o med (high c oss-linking densi y) ones, achie ing
he in e acial sa u a ion in sho e ime scales. This may be in e p e ed conside ing ha
he lowe bulk elas ic moduli o mic ogels wi h a low c oss-linking densi y a o s hei
sp eading a he in e ace. Howe e , he s e ic hind ance associa ed wi h he ex e nal
co onae educes he sp eading a e o mic ogels a he in e ace as he equilib ium s a e is
app oached [
23
]. On he o he hand, he absence o signi ican changes o he in e acial
ension o mic ogel-laden in e aces upon empe a u e changes can only be unde s ood
conside ing ha he in e acial con inemen hinde s he VPT [
38
,
52
]. This is he esul o he
coun e ac i e ole o he in insic bulk elas ici y o he mic ogels and he in e acial ension
on he mic ogel con o ma ion, which al e s he ee ene gy balance go e ning he VPT [
42
].
Thus, he hea ing o PNIPAM a a wa e / apo in e ace esul s in he collapse o he co e
imme sed in o he aqueous phase, whe eas he co ona a ached o he in e ace emains
unal e ed. This makes ha mic ogels adso bed a he luid in e ace p esen a “ ied-egg”
o co e-co ona con o ma ion, independen o he subphase empe a u e. Figu e 1c displays
a ske ch showing he sub le changes o he con o ma ion o adso bed mic ogels upon he
change in he subphase empe a u e.
Mic ogel adso p ion a empe a u es below he olume phase ansi ion occu s in
a swollen con o ma ion, wi h he mic ogel ying o co e he maximum possible a ea
[
35
,
36
,
53
]. The si ua ion becomes e y di e en when mic ogels adso b a empe a u es
abo e he olume phase ansi ion. In his condi ion, mic ogels adop a collapsed con-
o ma ion, equi ing highe mic ogel concen a ions o ensu ing an e ec ing coa ing o
he luid in e ace. This esul s in a dense packing o he mic ogel-laden in e ace and
jamming phenomena [
54
–
56
]. A schema ic ep esen a ion o he di e en con o ma ions
o he mic ogels a he in e ace upon adso p ion abo e and below he olume phase
ansi ion empe a u e (VPTT) is shown in Figu e 2.
The in e acial co e age also plays a e y impo an ole in he con ol o he de o -
ma ion o mic ogel pa icles upon hei adso p ion a luid in e aces. A a low in e acial
densi y, pa icles adso b a he in e ace wi h a s e ched-ou con o ma ion o ensu e he
maximiza ion o he occupied a ea. This esul s in a a o able en opy con ibu ion, which
allows he ene gy penal y associa ed wi h he elas ic de o ma ion o he mic ogel pa icle
o be o e come, leading o mic ogel-laden luid in e aces whe e he pa icles a e mos ly
imme sed in he pola phase [
57
]. This con o ma ion o he pa icles d i es he o ma ion
o liquid-like monolaye s o clus e s a he in e ace assembled h ough capilla y in e ac-
ions [
47
,
58
,
59
]. The inc ease in he in e acial co e age educes he in e -pa icle dis ance,
which d i es he collapse o he pa icle a he in e ace [
11
], leading o he c ys alliza ion
o he mic ogels in hexagonal a ays cha ac e ized by close con ac be ween he co onas o
adjacen pa icles, i.e., he inc ease in he in e acial co e age d i es he o ma ion o ilms
wi h a solid-like hexagonal packing [60].
Polyme s 2022,14, 1133 5 o 17
Polyme s 2022, 14, x FOR PEER REVIEW 5 o 17
leading o he c ys alliza ion o he mic ogels in hexagonal a ays cha ac e ized by close
con ac be ween he co onas o adjacen pa icles, i.e., he inc ease in he in e acial
co e age d i es he o ma ion o ilms wi h a solid-like hexagonal packing [60].
Figu e 2. Ske ch o he con o ma ion o mic ogels adso bed a a luid in e ace om empe a u es
below (a) and abo e (b) he phase olume ansi ion (VPTT: olume phase ansi ion empe a u e).
Adap ed om Wu e al. [52] wi h pe mission om he Ame ican Chemical Socie y, Copy igh
(2019).
3. In e -Mic ogel In e ac ions a Fluid In e aces
The polyme /colloid duali y o mic ogels leads o a e y di e en ene ge ic landscape
han ha ound o ha d pa icles a luid in e aces [11]. In pa icula , he Van de Waals
in e ac ions, which p esen a cen al ole o ha d pa icles adso bed a luid in e aces,
a e almos negligible when so pa icles a e conside ed. Fu he mo e, he possibili y o
unde going shape de o ma ion in oduces new deg ees o eedom in mic ogels a luid
in e aces [35,48,57,61,62]. In pa icula , he exis ence o embedding and de o ma ion
modes in so pa icles in luences he in e -pa icle in e ac ion po en ial, which is dic a ed
by a complex in e play o in e ac ions, mainly domina ed by capilla y and s e ic ones
[63,64]. Fu he mo e, elec os a ic in e ac ions in he mic ogel-laden in e ace a e e y
di e en o ha ound o ha d pa icles, due o he complex in e play be ween
elec os a ic e ec s, swelling, and collec i e mechanical p ope ies [17]. In e es ingly,
Geisel e al. epo ed ha elec os a ic in e ac ions play an almos negligible ole in he
con ol o he mic ogel de o mabili y [59].
Las , bu no leas , s ong capilla y in e ac ions a ise be ween mic ogels adso bed a
a luid in e ace, which may be unde s ood conside ing he long con ac line o adso bed
mic ogels, as well as hei oughness and he e ogenei y [32,65]. The impo an
con ibu ion o he capilla y in e ac ions was e idenced by Cohin e al. [66], who showed
he abili y o mic ogels o clus e upon adso p ion a luid in e aces. This was a ibu ed
o he exis ence o long- ange capilla y in e ac ions be ween he pa icles. Fo ins ance,
capilla y o ces (quad upola a ac i e in e ac ions) go e n he agg ega ion o big
mic ogels a he wa e /oil in e ace o almos negligible su ace p essu e alues [49,63].
A simila beha io was also epo ed o mic ogels a wa e / apo in e aces [67]. The
co ona ha induces a s e ic s abiliza ion o he mic ogels in bulk yielded e y s ong
capilla y in e ac ions be ween he mic ogels, esul ing in hei agg ega ion e en a e y
low alues o he in e acial co e age [68]. Thus, a a low in e acial co e age, he
in e ac ions a e domina ed by he highly s e ched co ona, independen o he
Figu e 2.
Ske ch o he con o ma ion o mic ogels adso bed a a luid in e ace om empe a u es
below (
a
) and abo e (
b
) he phase olume ansi ion (VPTT: olume phase ansi ion empe a u e).
Adap ed om Wu e al. [
52
] wi h pe mission om he Ame ican Chemical Socie y, Copy igh (2019).
3. In e -Mic ogel In e ac ions a Fluid In e aces
The polyme /colloid duali y o mic ogels leads o a e y di e en ene ge ic landscape
han ha ound o ha d pa icles a luid in e aces [
11
]. In pa icula , he Van de Waals
in e ac ions, which p esen a cen al ole o ha d pa icles adso bed a luid in e aces,
a e almos negligible when so pa icles a e conside ed. Fu he mo e, he possibili y o
unde going shape de o ma ion in oduces new deg ees o eedom in mic ogels a luid
in e aces [
35
,
48
,
57
,
61
,
62
]. In pa icula , he exis ence o embedding and de o ma ion modes
in so pa icles in luences he in e -pa icle in e ac ion po en ial, which is dic a ed by a
complex in e play o in e ac ions, mainly domina ed by capilla y and s e ic ones [
63
,
64
].
Fu he mo e, elec os a ic in e ac ions in he mic ogel-laden in e ace a e e y di e en o
ha ound o ha d pa icles, due o he complex in e play be ween elec os a ic e ec s,
swelling, and collec i e mechanical p ope ies [
17
]. In e es ingly, Geisel e al. epo ed
ha elec os a ic in e ac ions play an almos negligible ole in he con ol o he mic ogel
de o mabili y [59].
Las , bu no leas , s ong capilla y in e ac ions a ise be ween mic ogels adso bed a
a luid in e ace, which may be unde s ood conside ing he long con ac line o adso bed
mic ogels, as well as hei oughness and he e ogenei y [
32
,
65
]. The impo an con ibu ion
o he capilla y in e ac ions was e idenced by Cohin e al. [
66
], who showed he abili y
o mic ogels o clus e upon adso p ion a luid in e aces. This was a ibu ed o he
exis ence o long- ange capilla y in e ac ions be ween he pa icles. Fo ins ance, capilla y
o ces (quad upola a ac i e in e ac ions) go e n he agg ega ion o big mic ogels a
he wa e /oil in e ace o almos negligible su ace p essu e alues [
49
,
63
]. A simila
beha io was also epo ed o mic ogels a wa e / apo in e aces [
67
]. The co ona
ha induces a s e ic s abiliza ion o he mic ogels in bulk yielded e y s ong capilla y
in e ac ions be ween he mic ogels, esul ing in hei agg ega ion e en a e y low alues
o he in e acial co e age [
68
]. Thus, a a low in e acial co e age, he in e ac ions a e
domina ed by he highly s e ched co ona, independen o he empe a u e. Howe e , a a
high in e acial co e age, he ole o he mic ogel po ion inside he aqueous subphase is
p e alen and con ols he in e -mic ogel in e ac ions [45].
4. In-Plane O ganiza ion o Mic ogels a Fluid/Fluid In e aces
The abili y o mic ogels o be dis o ed o collapse unde comp ession leads o a mo e
complex assembly a in e aces, in compa ison o he incomp essible ha d colloidal pa i-
cles [
69
]. In pa icula , he co e-co ona mo phology o mic ogels de ines i s o ganiza ion a
luid in e aces, and hence hei phase beha io [17,45].
Bockenek e al. [
45
] de ined, o empe a u es below he olume phase ansi ion,
he exis ence o up o i e di e en egimes in he phase beha io o mic ogels adso bed
Polyme s 2022,14, 1133 6 o 17
a luid in e aces as a unc ion o he in e acial co e age, independen o hei speci ic
chemis y. These egimes a e iden i ied as (i) he dilu ed s a e, (ii) co ona- o-co ona con ac ,
(iii) isos uc u al phase ansi ion egion, (i ) co e- o-co e con ac , and ( ) monolaye ailu e.
Howe e , he co e- o-co e con ac egime disappea s a empe a u es abo e he VPTT. The
exis ence o he egime co esponding o he co ona- o-co ona con ac is su p ising and
can only be explained in e ms o he swelling he modynamics. Thus, conside ing he
size o a neu al mic ogel in bulk, i s size is de e mined by a complex in e play be ween
he ee ene gy and he empe a u e-dependen sol en –polyme mixing ee ene gy [
70
].
This amewo k needs o be ex ended when mic ogels a e adso bed a a luid in e ace o
include an addi ional e m accoun ing o he su ace ee ene gy gain, which is g ea e han
he mixing ones, and nea ly cons an wi h he empe a u e [
9
,
22
,
38
,
42
,
71
]. This leads o a
si ua ion in which he pa o he mic ogel ne wo k in con ac wi h he in e ace emains
s e ched un il he elas ic con ibu ion can coun e ac he su ace ee ene gy con ibu ion.
The he mo esponsi eness o he po ion o he mic ogels p o uding in o he aqueous
phase emains unal e ed. Figu e 3shows he change in he su ace p essu e,
Π
, as a
unc ion o he numbe o mic ogels pe a ea uni , N
AREA
, a wo di e en empe a u es,
and he co esponding AFM images ob ained o he di e en s a es o he monolaye . The
ansi ion be ween he dilu ed s a es o he egime cha ac e ized by he co ona- o-co ona
con ac s is accompanied by a sha p inc ease in he su ace p essu e. A u he inc ease
in he su ace co e age pushes he sys em o he eme gence o an isos uc u al phase
ansi ion. This is cha ac e ized by he mel ing o he ini ial la ice, and he o ma ion
o a second hexagonal phase wi h a smalle la ice pa ame e ; du ing his ansi ion, he
su ace p essu e changes e y slowly wi h he inc ease in he in e acial co e age. Once
he second hexagonal phase is comple ely o med, he beha io becomes e y di e en
depending on he empe a u e. A high empe a u es (abo e he olume phase ansi ion
empe a u e), he monolaye ails, p obably due o he buckling, o ma ion o mul ilaye s, o
deso p ion [
58
,
72
]. On he o he hand, a empe a u es below he olume phase ansi ion,
an addi ional phase cha ac e ized by a u he inc ease in he su ace p essu e eme ges
be o e he monolaye ailu e is obse ed ( egime i ). Howe e , he monolaye ailu e
occu s a simila alues o he su ace p essu e independen o he empe a u e. This can
be unde s ood conside ing ha he e is a maximum amoun o polyme segmen s ha can
be con ined a he in e ace be o e he ailu e. This limi can be eached by wo di e en
ou es: (i) he inc ease in he la e al packing o he ilm, o cing a 2D collapse, o (ii) he
inc ease in he empe a u e, o cing a 3D collapse.
The in insic s uc u e o mic ogel pa icles in luences he phase ansi ions obse ed
in in e acial ilms. The la ge co ona o mic ogels wi h a low c oss-linking densi y o e s
s ong esis ance o he phase ansi ions conce ning he si ua ion ound o mic ogels wi h
a high c oss-linking deg ee. The e o e, he speci ic molecula a chi ec u e o he mic ogel
pa icles allows con olling he o ganiza ion o he pa icles a he luid in e aces, wi h
he dimensions o he mic ogel co ona domina ing he assembly by in oducing an ene gy
ba ie , which makes he collapse di icul [
69
]. Fu he mo e, he la e al o ganiza ion o
mic ogels a luid in e aces eme ges s ongly dependen on he a e age size o he pa icles,
as demons a ed by Scheidegge e al. [
49
]. They s udied he in e acial o ganiza ion o
mic ogels wi h 2 di e en diame e s (210 nm and 1.45
µ
m) a a wa e /oil in e ace, and
ound e y di e en beha io s, wi h he di e se phases eme ging as he in e acial pa icle
densi y was inc eased. The la ges colloids p esen beha io wi h eminiscences o wha is
expec ed o co e–shell pa icles wi h a ha d co e and a de o mable ex e nal shell. This leads
o a capilla y-d i en clus e ing a a educed in e acial densi y, ollowed by a solid–solid
ansi ion when he comp ession deg ee is inc eased. The solid–solid phase ansi ion
occu s be ween wo phases wi h a hexagonal c ys alline o de , bu wi h di e ences in
hei la ice cons an s co esponding o he di e en o igin o he phases. Thus, he ini ial
phase eme ging om he shell–shell in e -pa icle con ac s is ans o med in he inal
c ys alline phase in which he o de ing is go e ned by co e–co e con ac s. This esul s
om he nuclea ion o clus e s o pa icles in co e–co e con ac s, which d i es he mel ing
Polyme s 2022,14, 1133 7 o 17
o he su ounding shell–shell c ys al un il he inal in e acial s uc u e go e ned by he
second phase is ob ained [
72
]. On he con a y, he smalles colloids do no unde go any
agg ega ion, showing a smoo h ansi ion om a hexagonal la ice o a dense diso de ed
monolaye . Geisel e al. [
73
] showed ha bo h he dec ease in he c oss-linking densi y
and he absence o he co e in mic ogel pa icles a e c i ical pa ame e s ha hinde he
o ma ion o o de ed hexagonal a ays a high alues o he in e acial co e age. This is he
esul o hei high de o mabili y, which acili a es he con ac s be ween mic ogels a he
in e ace, e en om he ela i ely low alues o he in e acial co e age.
Polyme s 2022, 14, x FOR PEER REVIEW 7 o 17
in e acial o ganiza ion o mic ogels wi h 2 di e en diame e s (210 nm and 1.45 μm) a a
wa e /oil in e ace, and ound e y di e en beha io s, wi h he di e se phases eme ging
as he in e acial pa icle densi y was inc eased. The la ges colloids p esen beha io wi h
eminiscences o wha is expec ed o co e–shell pa icles wi h a ha d co e and a
de o mable ex e nal shell. This leads o a capilla y-d i en clus e ing a a educed
in e acial densi y, ollowed by a solid–solid ansi ion when he comp ession deg ee is
inc eased. The solid–solid phase ansi ion occu s be ween wo phases wi h a hexagonal
c ys alline o de , bu wi h di e ences in hei la ice cons an s co esponding o he
di e en o igin o he phases. Thus, he ini ial phase eme ging om he shell–shell in e -
pa icle con ac s is ans o med in he inal c ys alline phase in which he o de ing is
go e ned by co e–co e con ac s. This esul s om he nuclea ion o clus e s o pa icles in
co e–co e con ac s, which d i es he mel ing o he su ounding shell–shell c ys al un il
he inal in e acial s uc u e go e ned by he second phase is ob ained [72]. On he
con a y, he smalles colloids do no unde go any agg ega ion, showing a smoo h
ansi ion om a hexagonal la ice o a dense diso de ed monolaye . Geisel e al. [73]
showed ha bo h he dec ease in he c oss-linking densi y and he absence o he co e in
mic ogel pa icles a e c i ical pa ame e s ha hinde he o ma ion o o de ed hexagonal
a ays a high alues o he in e acial co e age. This is he esul o hei high
de o mabili y, which acili a es he con ac s be ween mic ogels a he in e ace, e en om
he ela i ely low alues o he in e acial co e age.
Figu e 3. (a) Su ace p essu e (П) s. numbe o mic ogels pe a ea uni iso he ms o PNIPAM
mic ogels a a wa e /decane in e ace. The blue and ed cu es co espond o he iso he ms ob ained
a 20 °C (below he olume phase ansi ion empe a u e) and 40 °C (abo e he olume phase
ansi ion), espec i ely. The dashed lines indica e he onse o he di e en egimes, which a e
indica ed by oman nume als in he co esponding colo s. (b) A omic Fo ce Mic oscopy images o
Langmui –Blodge deposi s ob ained o monolaye s o PNIPAM mic ogels a a wa e /decane
in e ace below he olume phase ansi ion empe a u e (a 20 °C). (c) A omic Fo ce Mic oscopy
images o Langmui –Blodge deposi s ob ained o monolaye s o PNIPAM mic ogels a a
wa e /decane in e ace abo e he olume phase ansi ion empe a u e (a 40 °C). Adap ed om
Bochenek e al. [45] wi h pe mission om he Ame ican Chemical Socie y, Copy igh (2019).
Figu e 3.
(
a
) Su ace p essu e (
Π
) s. numbe o mic ogels pe a ea uni iso he ms o PNIPAM
mic ogels a a wa e /decane in e ace. The blue and ed cu es co espond o he iso he ms ob-
ained a 20
◦
C (below he olume phase ansi ion empe a u e) and 40
◦
C (abo e he olume phase
ansi ion), espec i ely. The dashed lines indica e he onse o he di e en egimes, which a e
indica ed by oman nume als in he co esponding colo s. (
b
) A omic Fo ce Mic oscopy images o
Langmui –Blodge deposi s ob ained o monolaye s o PNIPAM mic ogels a a wa e /decane in e -
ace below he olume phase ansi ion empe a u e (a 20
◦
C). (
c
) A omic Fo ce Mic oscopy images
o Langmui –Blodge deposi s ob ained o monolaye s o PNIPAM mic ogels a a wa e /decane
in e ace abo e he olume phase ansi ion empe a u e (a 40
◦
C). Adap ed om Bochenek e al. [
45
]
wi h pe mission om he Ame ican Chemical Socie y, Copy igh (2019).
Fu he insigh s in o he o ganiza ion o mic ogel pa icles adso bed a luid/ luid
in e aces can be ob ained by using compu a ional app oaches, e.g., dissipa i e pa icle
dynamic simula ions [
74
]. This p o ides in o ma ion ela ed o he impo ance o he
c oss-linking densi y o he mic ogels, and hei compa ibili y wi h he luid phases o
con olling he de o ma ion o he pa icles a he in e ace. Thus, he inc ease in he in e -
acial densi y educes he sp eading o he mic ogels a he in e ace, passing g adually
om an obla e-like con o ma ion (dilu ed sys ems) o a quasi-sphe ical one a he highes
alues o he in e acial densi y. The e o e, he inc ease in he in e acial densi y d i es he
hickening o he mic ogel monolaye , educing i s oughness. Con e sely, he sh inking o
he mic ogels adso bed a a luid in e ace as a esul o he wo sening o he sol en quali y,
Polyme s 2022,14, 1133 8 o 17
leads o he dec ease in he monolaye hickness and he up u e o he monolaye as a
esul o he inc ease in he un a o able con ac s be ween he wo luid phases. The e ec
o he c oss-linking densi y o mic ogels on hei in-plane o ganiza ion a luid in e aces
was u he ly explo ed by Pica d e al. [75]. They epo ed ha PNIPAM mic ogels wi h a
low c oss-linking densi y sel -assemble a he wa e / apo in e ace, o ming a hexagonal
phase cha ac e ized by single la ice pa ame e s unde going a ansi ion o mo e diso de ed
phases, wi h he inc ease in he in e acial co e age. This is he esul o a con o ma ional
ea angemen o he mic ogel pa icles om a highly la ened con o ma ion wi h many
polyme segmen s di ec ly adso bed o he in e ace, o an en angled la ened o ma ion
esul ing in compac monolaye s a he highes alues o he in e acial co e age. The
inc ease in he c oss-linking densi y o he mic ogel pa icles changes he pic u e, esul ing
in he eme gence o a coexis ence be ween he wo phases wi h a hexagonal o de cha ac e -
ized by wo di e en la ice pa ame e s. Figu e 4shows he di e en in-plane-o ganiza ion
o mic ogels wi h di e en c oss-linking densi ies as a unc ion o he in e acial co e age.
Polyme s 2022, 14, x FOR PEER REVIEW 8 o 17
Fu he insigh s in o he o ganiza ion o mic ogel pa icles adso bed a luid/ luid
in e aces can be ob ained by using compu a ional app oaches, e.g., dissipa i e pa icle
dynamic simula ions [74]. This p o ides in o ma ion ela ed o he impo ance o he
c oss-linking densi y o he mic ogels, and hei compa ibili y wi h he luid phases o
con olling he de o ma ion o he pa icles a he in e ace. Thus, he inc ease in he
in e acial densi y educes he sp eading o he mic ogels a he in e ace, passing
g adually om an obla e-like con o ma ion (dilu ed sys ems) o a quasi-sphe ical one a
he highes alues o he in e acial densi y. The e o e, he inc ease in he in e acial
densi y d i es he hickening o he mic ogel monolaye , educing i s oughness.
Con e sely, he sh inking o he mic ogels adso bed a a luid in e ace as a esul o he
wo sening o he sol en quali y, leads o he dec ease in he monolaye hickness and he
up u e o he monolaye as a esul o he inc ease in he un a o able con ac s be ween
he wo luid phases. The e ec o he c oss-linking densi y o mic ogels on hei in-plane
o ganiza ion a luid in e aces was u he ly explo ed by Pica d e al. [75]. They epo ed
ha PNIPAM mic ogels wi h a low c oss-linking densi y sel -assemble a he wa e / apo
in e ace, o ming a hexagonal phase cha ac e ized by single la ice pa ame e s
unde going a ansi ion o mo e diso de ed phases, wi h he inc ease in he in e acial
co e age. This is he esul o a con o ma ional ea angemen o he mic ogel pa icles
om a highly la ened con o ma ion wi h many polyme segmen s di ec ly adso bed o
he in e ace, o an en angled la ened o ma ion esul ing in compac monolaye s a he
highes alues o he in e acial co e age. The inc ease in he c oss-linking densi y o he
mic ogel pa icles changes he pic u e, esul ing in he eme gence o a coexis ence
be ween he wo phases wi h a hexagonal o de cha ac e ized by wo di e en la ice
pa ame e s. Figu e 4 shows he di e en in-plane-o ganiza ion o mic ogels wi h di e en
c oss-linking densi ies as a unc ion o he in e acial co e age.
Figu e 4. A omic Fo ce Mic oscopy images o Langmui –Blodge deposi s ob ained om low c oss-
linked (a–d) and high c oss-linked (e–h) PNIPAM mic ogels a he wa e / apo in e ace, ob ained
a inc easing alues o he in e acial co e age ( om le o igh ). Rep in ed om Pica d e al. [75]
wi h pe mission om he Ame ican Chemical Socie y, Copy igh (2017).
The elec os a ic in e ac ions a e also e y impo an o he con ol o he in-plane
o ganiza ion o mic ogel pa icles a luid in e aces. This can be unde s ood as a
consequence o he di e en de o mabili y o he cha ged mic ogel pa icles. Thus,
cha ged mic ogels a wa e / luid in e aces unde go a highe comp ession han
uncha ged ones a low alues o he in e acial co e age, and, consequen ly, he a ea
occupied by he pa icles is educed wi h he educ ion in he cha ge densi y. This esul s
om he impac o he in e -pa icle in e ac ions in he la e al o ganiza ion o he pa icles
a he in e ace. Howe e , he e ec o he cha ges in he mic ogel comp essibili y becomes
negligible o la ge mic ogels, and hence he la e al in e -pa icle in e ac ions do no
Figu e 4.
A omic Fo ce Mic oscopy images o Langmui –Blodge deposi s ob ained om low c oss-
linked (
a
–
d
) and high c oss-linked (
e
–
h
) PNIPAM mic ogels a he wa e / apo in e ace, ob ained a
inc easing alues o he in e acial co e age ( om le o igh ). Rep in ed om Pica d e al. [
75
] wi h
pe mission om he Ame ican Chemical Socie y, Copy igh (2017).
The elec os a ic in e ac ions a e also e y impo an o he con ol o he in-plane o -
ganiza ion o mic ogel pa icles a luid in e aces. This can be unde s ood as a consequence
o he di e en de o mabili y o he cha ged mic ogel pa icles. Thus, cha ged mic ogels a
wa e / luid in e aces unde go a highe comp ession han uncha ged ones a low alues o
he in e acial co e age, and, consequen ly, he a ea occupied by he pa icles is educed
wi h he educ ion in he cha ge densi y. This esul s om he impac o he in e -pa icle
in e ac ions in he la e al o ganiza ion o he pa icles a he in e ace. Howe e , he e ec
o he cha ges in he mic ogel comp essibili y becomes negligible o la ge mic ogels, and
hence he la e al in e -pa icle in e ac ions do no modi y he in-plane o ganiza ion o mi-
c ogel pa icles a he in e ace. This can be explained by conside ing he dominan ole o
he olume con ibu ion o he ee ene gy o e he in e acial ones. The si ua ion becomes
e y di e en , independen o he mic ogel size, when he in e acial co e age is inc eased.
In his si ua ion, he di e ences in he comp essibili y be ween cha ged and uncha ged
mic ogels lead o changes in he leng h-scale o he in e ac ions occu ing be ween he
mic ogel ac ions p o uding in o he aqueous phase o cha ged and uncha ged pa icles,
as a esul o he di e en swelling deg ees o he pa icles. The e o e, he inc ease in he
in e acial co e age in oduces he con ibu ion o he ou -o -plane in e ac ions be ween
mic ogel pa icles, which modi y he in-plane o ganiza ion o he pa icles a he in e -
ace [
76
]. The di e ences in he o ganiza ion o mic ogels wi h di e en cha ge densi ies
a he in e aces can be mo e clea ly unde s ood by compa ing he su ace p essu e–a ea
Polyme s 2022,14, 1133 9 o 17
iso he ms depic ed in Figu e 5, oge he wi h he con o ma ion expec ed o he pa icles
a he luid in e ace cha ac e ized o he diame e o he p ojec ion o he pa icle a he
in e ace (Din e ace).
Polyme s 2022, 14, x FOR PEER REVIEW 9 o 17
modi y he in-plane o ganiza ion o mic ogel pa icles a he in e ace. This can be
explained by conside ing he dominan ole o he olume con ibu ion o he ee ene gy
o e he in e acial ones. The si ua ion becomes e y di e en , independen o he
mic ogel size, when he in e acial co e age is inc eased. In his si ua ion, he di e ences
in he comp essibili y be ween cha ged and uncha ged mic ogels lead o changes in he
leng h-scale o he in e ac ions occu ing be ween he mic ogel ac ions p o uding in o
he aqueous phase o cha ged and uncha ged pa icles, as a esul o he di e en
swelling deg ees o he pa icles. The e o e, he inc ease in he in e acial co e age
in oduces he con ibu ion o he ou -o -plane in e ac ions be ween mic ogel pa icles,
which modi y he in-plane o ganiza ion o he pa icles a he in e ace [76]. The
di e ences in he o ganiza ion o mic ogels wi h di e en cha ge densi ies a he
in e aces can be mo e clea ly unde s ood by compa ing he su ace p essu e–a ea
iso he ms depic ed in Figu e 5, oge he wi h he con o ma ion expec ed o he pa icles
a he luid in e ace cha ac e ized o he diame e o he p ojec ion o he pa icle a he
in e ace (Din e ace).
Figu e 5. E ec o he size and cha ge o mic ogels adso bed a a wa e /oil in e ace on he in-plane
o ganiza ion o he mic ogel ilm as a unc ion o he diame e o he p ojec ion o he pa icle a he
in e ace (Din e ace). Rep oduced om Schmid e al. [76] wi h pe mission om he Royal Socie y o
Chemis y, Copy igh (2020).
The phase beha io o mic ogels a luid in e aces can be modula ed by he
modi ica ion o some physico-chemical pa ame e s a ec ing he mic ogel bulk
con o ma ion, such as he pH, empe a u e, o ionic s eng h. Thus, conside ing
he mo esponsi e mic ogels, such as PNIPAM mic ogels, he phase beha io eme ges as
a he independen o he empe a u e o low in e acial densi ies, which may be
explained by conside ing he dominance o he highly s e ched co ona on mic ogel
o ganiza ion. This independence o he empe a u e disappea s wi h he inc ease in he
in e acial densi y, wi h he segmen s o he mic ogels imme sed in he aqueous subphase
s a ing o be ele an , go e ning he in e -pa icle in e ac ions. These po ions can
unde go collapse, al e ing he isos uc u al phase ansi ion. The abo e discussion poin s
ou he impo an ole o he bulk s imuli- esponsi eness o mic ogels o con ol hei
in e acial comp essibili y [69]. The e o e, e en hough he olume phase ansi ion o
PNIPAM mic ogels is commonly iden i ied as an in insic molecula p ope y, i is
s ongly modi ied upon in e acial con inemen [42].
5. Response o Mic ogel Laye s o Mechanical Pe u ba ions
The esponse o so pa icles adso bed a luid in e aces o mechanical pe u ba ions
eme ges as s ongly dependen on hei de o mabili y [20]. In pa icula , he abili y o he
Figu e 5.
E ec o he size and cha ge o mic ogels adso bed a a wa e /oil in e ace on he in-plane
o ganiza ion o he mic ogel ilm as a unc ion o he diame e o he p ojec ion o he pa icle a he
in e ace (D
in e ace
). Rep oduced om Schmid e al. [
76
] wi h pe mission om he Royal Socie y o
Chemis y, Copy igh (2020).
The phase beha io o mic ogels a luid in e aces can be modula ed by he modi-
ica ion o some physico-chemical pa ame e s a ec ing he mic ogel bulk con o ma ion,
such as he pH, empe a u e, o ionic s eng h. Thus, conside ing he mo esponsi e mi-
c ogels, such as PNIPAM mic ogels, he phase beha io eme ges as a he independen o
he empe a u e o low in e acial densi ies, which may be explained by conside ing he
dominance o he highly s e ched co ona on mic ogel o ganiza ion. This independence o
he empe a u e disappea s wi h he inc ease in he in e acial densi y, wi h he segmen s
o he mic ogels imme sed in he aqueous subphase s a ing o be ele an , go e ning he
in e -pa icle in e ac ions. These po ions can unde go collapse, al e ing he isos uc u al
phase ansi ion. The abo e discussion poin s ou he impo an ole o he bulk s imuli-
esponsi eness o mic ogels o con ol hei in e acial comp essibili y [
69
]. The e o e, e en
hough he olume phase ansi ion o PNIPAM mic ogels is commonly iden i ied as an
in insic molecula p ope y, i is s ongly modi ied upon in e acial con inemen [42].
5. Response o Mic ogel Laye s o Mechanical Pe u ba ions
The esponse o so pa icles adso bed a luid in e aces o mechanical pe u ba ions
eme ges as s ongly dependen on hei de o mabili y [
20
]. In pa icula , he abili y o
he ex e nal co ona o unde going ea angemen s plays an essen ial ole in con olling
he elaxa ion mechanisms o mic ogels agains mechanical s esses [
34
]. These depend
on bo h Young’s modulus and he we ing o he mic ogels a he in e ace [
9
]. Fu he -
mo e, he heological esponse o mic ogels adso bed a a luid in e ace depends on he
pa icle concen a ion, bu also on he in e -pa icle dis ance and la e al o ganiza ion o
he mic ogels a he in e ace. In ac , i is possible o iden i y up o ou di e en egimes
o he heological esponse o mic ogel-laden luid in e aces agains dila ion, depending
on he in e acial co e age. A a e y low in e acial co e age, he mic ogel laye mainly
beha es as an elas ic ilm o so disks in e connec ed o ne wo ks as a esul o he capilla y
in e ac ions. The inc ease in he la e al packing abo e he in e acial co e age co espond-
ing o a dense andom packing o he mic ogel shells leads o a linea inc ease in he
elas ic modulus, which, in u n, can be conside ed as a eminiscence o he comp ession
o so disks. The onse o he hi d egime is cha ac e ized by he dec ease in he s o age
Polyme s 2022,14, 1133 16 o 17
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