Switching between Positive and Negative Movement Near an Air/water Interface through Lateral Laser Illumination

Switching between positive and negative movement near an air/water interface through lateral laser illumination Cite as: Appl. Phys. Lett. 117, 073701 (2020); doi: 10.1063/5.0015247 Submitted: 26 May 2020 .Accepted: 4 August 2020 . Published Online: 19 August 2020 Mayu Shono, 1 Satoshi Takatori, 1 Jose M. Carnerero, 2 and Kenichi Yoshikawa 1,3,a) AFFILIATIONS 1 Faculty of Life and Medical Sciences, Doshisha University, Kyotanabe, Kyoto 610-0394, Japan 2 Department of Physical Chemistry, Universidad de Sevilla, 41012 Seville, Spain 3 Center for Integrative Medicine and Physics, Institute for Advanced Study, Kyoto University, Kyoto 606-8501, Japan a) Author to whom correspondence should be addressed: [email protected]a.ac.jp ABSTRACT The directional movement of an aqueous solution containing gold nanoparticles under laser irradiation (532 nm) through the vicinity of an air/water interface is reported. It is shown that unidirectional flow is generated along the path of light-travel when a laser is irradiated parallel to the interface. On the contrary, liquid flow toward the light source is caused when the incident angle of the laser is tuned so as to make the total reflection at an air/water interface. Such switching of the flow direction, positive or negative to the light propagation, was applied to the movement of mm-sized floating object, revealing that repetitive back and forth motion of the solid object is generated. This directional switching of photo-induced movement is interpreted by taking into account the flow caused by the spatial gradient of the surface tension due to the thermal effect of the incident laser. A numerical model with a Navier–Stokes-type equation reproduces the essential aspects of the switching of the direction observed in the present study. Published under license by AIP Publishing. https://doi.org/10.1063/5.0015247 The movement of a desired object under remote control is an interesting topic of study not only for basic science but also for practical applications. For effective remote control, optical movement such as through the use of laser trapping or optical pressure with light scattering has been applied to micro-objects on the order of lm-nm. 1–10 Recently, there has been growing interest in the realization of lightdriven micro/nanomotors, where various physico-chemical effects of light irradiation have been proposed, including electrophoresis, diffusiophoresis, deformation propulsion of the chemical structure, and bubble propulsion. 11–18 To realize the effective transport of larger objects, it has been shown that a spatial gradient of interfacial tension caused by local photo-irradiation can be used to achieve the directional movement of mm-cm-sized objects. Specifically, a thermal Marangoni effect exerted by light irradiation at a specific local position can induce directional motion of droplets as well as floating solid/liquid objects. 19–35 It was reported that local convectional flow was generated by the irradiation of a single gold nanoparticle (AuNP) attached to a solid plate with a CW laser, where the driving force was discussed in relation to micro-bubble formation and a thermal Marangoni effect. 36 The generation of micro-bubbles by the irradiation of AuNPs using a 1064 nm laser was also studied, with particular focus on bubble motion in relation to a thermal Marangoni effect around the bubble walls. 37 In these past studies on using the thermal Marangoni effect by optical irradiation, the light source is usually positioned above or below the experimental system, or orthogonal to an interface, so as to irradiate a specific local position on the interface. In the present study, we show that directional liquid flow above the scale of several cm is generated through laser irradiation from the lateral side of a liquid vessel. Interestingly, the flow can be easily switched between positive and negative directions simply by tuning the incident angle slightly. We also demonstrate that a mm-sized floating object undergoes back-and- forth motion in a repetitive manner through laser irradiation from the lateral side. Hydrogen tetrachloroaurate(III)tetrahydrate was purchased from Sigma-Aldrich (St. Louis, MO, USA), and trisodium citrate was from Fujifilm Wako Pure Chemical Corporation (Chuo-ku, Osaka, Japan). Turkevich’s method was used with a slight modification to prepare citrate-capped AuNPs with an average diameter of 15nm. 38–40 HAuCl 4 solution (0.01%, 194 ml) was heated to 80 C; when this temperature was reached, 6 ml of trisodium citrate (38.8 mM) was added. Appl. Phys. Lett. 117, 073701 (2020); doi: 10.1063/5.0015247 117, 073701-1 Published under license by AIP Publishing Applied Physics Letters ARTICLE scitation.org/journal/apl 06 June 2025 14:11:58 The temperature was held constant for 10min, while the color of the mixture changed to resemble red wine. The solution was then stirred at room temperature for 24 h. Through the transmission electron microscopic (TEM) measurement, the average diameter of colloids was evaluated to be ca. 15 nm [see Fig. 1(b)]. The optical microscope image of the fresh AuNP is shown in Fig. 1(c),leftpanel.Thesolution of the thus-synthesized AuNPs was stored at room temperature (24 C) in a 250 ml borosilicate glass bottle. The colloidal dispersion of the “aged AuNP” solution as mentioned below was the sample stood still at room temperature under shading with a black sheet. In the present study, we have mainly adapted the aged AuNP solution stored for six months, because we could easily observe the flow profile with scattered light under laser irradiation in the experimental system as in Fig. 1(a). The actual size distribution observed by optical microscopy [Fig. 1(c) right panel] is given as the supplemental graph in Fig. S1. In the experiments, as in Fig. 1(a), the solution was irradiated from the lateral side with a continuous-wave laser (LS03TT-M01, OXIDE) with a wavelength of 532 nm. All of the container walls were made of 2-mm-thick clear glass, and the depth of the solution was 13mm. The 730 mW laser was irradiated 3 mm below the surface. A sample of AuNP dispersion (estimated concentration: 1.12 10 9 M) was made by mixing 5 ml of the AuNP solution with 5 ml of pure water (K7I73, Otsuka). The maximum absorption of the solution was at 522 nm (see supplementary material Fig. S2), and so the solution efficiently absorbed the 532 nm laser. The motion of the AuNP was recorded with a microscope camera (VH-Z75, KEYENCE) and then analyzed by image-processing software (ImageJ, National Institutes of Health, USA). For the experiment with total reflection, we adopted the position at x¼7mmwithh¼84as in Fig. 1(a). The temperature distribution of the AuNP solution was monitored by thermography (CPA-T420A, FLIR). For the experiment on a floating object (Fig. 4), a plastic sheet (Polypropylene, 1 mm 1 mm, thickness: 0.25 mm) was used. All of the experiments were performed at an ambient temperature of 24 C. Figures 1(d) and 1(e) show the experimental results for laserinduced fluidic flow of the aged AuNP dispersion, by using the experimental setup as schematically illustrated in Fig. 1(a).Therightpanel of Fig. 1(c) exemplifies the optical microscopic observations on the Au particles in the aged solution, indicating that there exist aggregated Au particlesontheorderofseverallm, where dispersed AuNPs with the diameter of around 15nm coexist. (As for the size distribution of the Au aggregates above 1 lm, see supplementary material Fig. S1.) The time-trace and spatiotemporal diagram in Fig. 1(d) indicate the occurrence of fluid flow in the same direction as photons (positive direction), where the laser is irradiated parallel to the air/water interface as in Fig. 1(a-i) (see supplementary material video S1). In contrast, translational motion opposite to the direction of photons (negative direction) is seen in Fig. 1(e), when the laser is directed slightly upward [h¼84as in Fig. 1(a-ii)]. In other words, when the laser follows a path of total reflection, the fluid flow exhibits directional motion toward the optical source (see supplementary material video S2). The flow velocity evaluated from the slopes on the spatiotemporal diagram indicates that the velocity is not so different between the two opposite motions for the region of x¼3–5 mm. Actually at x¼3–5 mm, it is found that the velocities are 0:55 60:14 mm/s (average velocity 6standard deviation) for the motion for the positive direction and 0:73 60:24 mm/s for the negative direction. We have examined the time-dependence of the flow velocity with continuous laser irradiation. It was confirmed that the flow velocity remained almost constant for the period of ca. 10 min (see supplementary material videos S3 and S4). We have also performed the measurement on the laser-induced fluid flow by using the newly prepared AuNP solution. In Figs. 1(f) and 1(g) are shown the spatiotemporal diagrams observed for the fresh AuNP solution, suggesting the occurrence of positive and negative flows with a similar speed to that in the aged AuNP solution; 0:51 60:11 mm/s FIG. 1. Experimental system and examples of directional flow induced by lateral irradiation of laser. (a) Schematic representation of the experiment setup. The laser beam (k¼532 nm) irradiates the AuNP solution (Stock solution of AuNPs 5 ml þpure water 5 ml) from the left side. (i) Parallel to the surface. (ii) Total reflection. (b) TEM image of dispersed fresh AuNPs, with the diameter of around 15 nm. (c) Optical microscopic image of AuNP solution adapted in the present experiments, where individual AuNPs are invisible because of their smallness compared to the wavelength of optical light. Left panel: fresh AuNP. Right panel: “aged” AuNP. (d) and (e) correspond to irradiation with parallel and total reflection, respectively. Top: snapshots from a movie taken from above with a microscope for the region of 3–7 mm from the left inner wall. Second from the top: flow profile with the time trace for 5 s. Third from the top: spatiotemporal diagram for the solution of aged AuNP; see supplementary material videos S1 and S2 for parallel and total reflection, respectively. (f) and (g) Spatiotemporal diagram for the solution of freshly the prepared AuNP. Applied Physics Letters ARTICLE scitation.org/journal/apl Appl. Phys. Lett. 117, 073701 (2020); doi: 10.1063/5.0015247 117, 073701-2 Published under license by AIP Publishing 06 June 2025 14:11:58 and 0:67 60:29 mm/s for positive and negative flows, respectively. Interestingly, the difference in flow velocities between the aged and fresh sample is negligibly small. Because of the reason that we can observe the laser induced flow profile in a visible manner, in the following, we will show and discuss the experimental observations obtained by using the aged AuNP solution. To gain insight into the mechanism of the switching of fluidic directional motion, we measured the temperature profile under irradiation by the laser in two different directions: parallel or total reflection. Figure 2 shows the spatial distribution of the temperature at 3 min after the start of laser irradiation, i.e., when the fluid flow was stable. The gradient of the temperature is opposite between the two different irradiation angles: parallel to the interface or total reflection. This observation suggests that the switching of fluid flow may be related to the change in the spatial gradient of the temperature. Since it is well known that the interfacial tension at the air/water interface decreases with an increase in temperature, it is expected that inversion of the spatial gradient of the interfacial tension causes the switching of directional fluid-flow. Thus, the underlying mechanism to cause the directional flow may involve the so-called thermal Marangoni effect. 41–43 To clarify the mechanism of the generation of translational flow together with switching of the direction of movement, we performed a numerical simulation based on the framework of fluid dynamics by incorporating the thermal Marangoni effect. We adapt the Navier–Stokes equations as follows: @~ v @tþ~ vr ðÞ ~ v¼1 qrpþl qr2~ vþ~ f;(1) where we consider that the driving force is induced by the spatial gradient of the interfacial tension: ~ f@r=@x. As for the temperaturedependence of the interfacial tension, we assumed a constant value: @r=@T¼0.16 mN/(m K). 44 For simplification, we performed the simulation in two dimensions, along the xand zaxes, and adopted a mesh size of 1 mm. Thus, the interfacial tension gradient is given as follows: ~ f¼fx;fz ðÞ ;fx¼a@r @T @T @x¼0:16a@T @x;fz¼0;(2) where ais a positive constant, which rescales the numerical simulation to properly express the reduction in force due to the interfacial gradient at the interface on the fluid motion assumed with a 1 mm mesh. Here, we show the results of the simulation by taking a¼0.01. For the parameters in Eq. (1),weusedq¼1g/cm 3 and l¼1.01 mPas. Figure 3 shows the results of the simulation under laser irradiation for the region x¼(3 mm, 9 mm) (a) parallel to the interface and (b) total reflection. In this simulation, we used the temperature gradient at the interface observed in the actual experiments as in Fig. 2. Thewholeflowprofile(0mm,100mm)isshowninFig.S4(seethe supplementary material). In Fig. 3(a), the occurrence of directional fluid flow toward the direction of light is noted, where the flow rate is enhanced near the interface and weak back-flow is generated below the optical path. Figure 3(b) shows anti-parallel flow near the interface for the region x<7mm, where x¼7 mm is the position of total reflection. For the region x>7 mm, flow parallel to the optical direction is induced. Thus, the essentials of the observed switching of directional flow were reproduced with the simulation. FIG. 2. Temperature profiles depending on the angle of incidence of the laser. Upper: thermographic images of the solution from above and the side. Lower: spatial change in temperature evaluated from the thermographic images (x¼3to 7 mm; y¼4 mm). The temperature profiles with a wider view are given in Fig. S3 (see the supplementary material). FIG. 3. Numerical simulations based on the data of the temperature profiles in Fig. 2. (a) Parallel optical path and (b) total reflection. Simulations were performed in two dimensions, i.e., for the side view. Applied Physics Letters ARTICLE scitation.org/journal/apl Appl. Phys. Lett. 117, 073701 (2020); doi: 10.1063/5.0015247 117, 073701-3 Published under license by AIP Publishing 06 June 2025 14:11:58 As mentioned above, the surface flow of the AuNP solution can be easily switched by a slight change in the angle of incidence between parallel and total reflection. Next, we tried to extend our experimental system to the remote control of a solid object floating on the solution. Figure 4 shows a spatiotemporal diagram of the repetitive switching of the direction of movement of a solid sheet, where the marks T and P correspond to laser irradiation in the parallel (h¼90) and total reflection (h¼84) conditions. Now, it has become evident that back-and-forth motion of a solid object can be generated as an application of the optical switching of flow of the AuNP solution (see supplementary material video S5). The results demonstrated that the direction of flow can be switched between forward and backward by a change in the angle of incidence of laser irradiation from the lateral side. The mechanism of this directional motion is explained as follows: as AuNPs dispersed in the solution absorb the photon energy of the laser, a local increase in temperature is generated at the air/water interface. The temperature gradient at the interface induces a spatial gradient of interfacial tension. This, in turn, causes directional flow near the interface. Interestingly, the sign of the gradient of interfacial tension is switched depending on the change in the incident angle, i.e., between an optical path parallel to the interface and total reflection. As an application of this controllable switching of the flow direction, it is revealed that a mm-sized plastic sheet floating on the air/water interface can be transported in the forward and backward directions, in a repetitive manner. It is expected that the methodology of optical control for the directional motion reported here may be applicable for the remote control of various kinds of objects above cm-sized. See the supplementary material for Fig. S1: size distribution in “aged” AuNPs deduced from optical microscopic observation;Fig. S2: absorption spectrum of the AuNP solution; Fig. S3: temperature profiles; Fig. S4: flow profiles; video S1: positive flow using the aged AuNP corresponding to Fig. 1(d) (MP4); video S2: negative flow using the aged AuNP corresponding to Fig. 1(e) (MP4); video S3: observation during 30–50 s from the start of laser irradiation on the experimental system shown in Fig. 1(a-ii) under the laser irradiation with total reflection (MP4); video S4: observation during 8 min 8min 20s from the start of laser irradiation on the experimental system shown in Fig. 1(a-ii) under the laser irradiation with total reflection (MP4); and video S5: repetitive back-and-forth motion of a plastic sheet corresponding to Fig. 4 (MP4). This work was partly supported by JSPS KAKENHI Grant Nos. JP15H02121 and JP20H01877 to KY. Dr. Jose M. 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