Article pubs.acs.org/Langmuir
Bioinspired Hand-Operated Smart-Wetting Systems Using Smooth Liquid Coatings Mizuki Tenjimbayashi, Takeshi Matsubayashi,† Takeo Moriya,† and Seimei Shiratori* Center for Material Design Science, School of Integrated Design Engineering, Keio University, 3-14-1 Hiyoshi, Yokohama, 223-8522, Japan S Supporting Information *
ABSTRACT: Manually controllable “hand-operated” smart systems have been developed in many fields, including smart wetting materials, electronic devices, molecular machines, and drug delivery systems. Because complex morphological or chemical control are generally required, versatile strategies for constructing the system are technologically important. Inspired by the natural phenomenon of raindrops rarely bouncing and usually spreading on a puddle, we introduce a droplet-impact-triggering smart-wetting system using “non-smart” smooth liquid coating materials. Changing the droplet impact energy by changing the volume or casting height causes the droplet to completely bounce or spread on the liquid surface, regardless of the miscibility between the two liquids, owing to the stability of air layer. As the bouncing of a droplet on a liquid interface is not usually observed during wetting, we first analyze how the droplet bounces, then prove that the wettability is triggered by the droplet’s impact energy, and finally introduce some applications using this system.
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INTRODUCTION Manually controllable “hand-operated” smart systems have been developed in many fields, including smart wetting materials,1 electronic devices,2 molecular machines,3 and drug delivery systems.4 Existing smart materials are designed to switch their morphology and/or chemistry if triggered by pressure,1 temperature,5 electricity,6 magnetic force,7 and light8 to produce a change in their function, such as wettability,1,5 transparency,1,5 adhesion,9 and structural color.10 There is significant scientific and technological interest in the development of smart functional materials for their potential application in microreactors,11 inkjet printing technology,2 electronic devices,12 and fluidic systems.13 However, a raindrop making impact with a puddle is a system in nature in which the contact of the droplet with the puddle is irregular without switching the characteristics of the puddle (i.e., a puddle is “non-smart”) (Figure 1A); this system is different from conventional smart-material-based switching systems. The natural phenomenon provides an innovative concept for designing smart systems that are more versatile and can change their function without the need to switch the characteristics of the materials using complex topological or reactive materials. Such extreme switching of the droplet contact with a liquid surface can be treated as smart-wetting of the surface because the liquid contact area A* is related to the wettability according to A*(θ, V) = π sin2θ[4V/(2θ − sin 2θ)]2/3, in which θ is the contact angle (CA) and V is the droplet volume; that is, the rain droplet contacts with liquid pool with a CA of ≈180° when it bounces and with a CA of 0° when it spreads. Unlike the smart wetting surfaces based on lotus and/or pitcher plant analogues,1,5,7 the rain droplet that bounces on the liquid © XXXX American Chemical Society
surface is mediated by an air layer to protect the contact of the liquids.14,15 We first developed a system that mimics this natural phenomenon by switching the contact of various kinds of liquids on a nonsmart smooth liquid surface coating (Figure 1B). Although this phenomenon was found in a droplet on a liquid pool, that on a thin liquid layer has never been reported except for two recent examples,14,15 and its mechanism and versatility has still been unrevealed, not developed as smart materials. The wettability of the system can be switched from an almost perfectly superomniphobic (CA > 178°) to a spreading state (CA = 0−110°) by using various liquids on a nonsmart liquid surface triggered by the impact energy of the droplet, as well as the raindrop phenomena. For instance, when phenyl silicone oil is smoothly spread at a thickness of 21.5 ± 1.2 μm on a glass slide (see Note S1 and Figures S1 and S2) using silane technology (for detailed information on the coating, see our previous work in refs 16 and 17), the water CA changes from ∼179.8° to 80.8° by varying the droplet impact energy (Figure 1C).
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RESULTS AND DISCUSSION In this work, the fundamental analysis of superomniphobic bouncing of droplets on a smooth liquid surface are first analyzed by comparison with droplets bouncing on a superhydrophobic surface, as the bouncing phenomenon on a liquid surface has not been studied extensively.14,15 We prepared a superhydrophobic surface that mimics the lotus Received: May 12, 2017 Revised: June 18, 2017 Published: June 19, 2017 A
DOI: 10.1021/acs.langmuir.7b01600 Langmuir XXXX, XXX, XXX−XXX
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superhydrophobic surface after it was modified with a smooth liquid surface using phenyl silicone oil, with a static water CA of 80.8° (Figure 2B). Although both surfaces form an air cushion layer between the droplet and their surfaces, the way the air layers are trapped differs; the air layers are trapped between the droplet and the hierarchical structure formed by the selfassembled nanoparticles on the superhydrophobic surface, whereas an air cushion pocket is temporally formed on the smooth liquid surface between the liquids, as shown in the schemes in Figure 2A,B. Figure 2C,D shows the water droplet motions on two different surfaces captured using a high-speed camera at 1000 fps, and Figure 2E shows the associated plots of the normalized height of the center droplet (y/ymax) and the bottom droplet (h/hmax) above the surface, and the normalized appearance of the contact width of the droplet (w/wmax) as a function of time τ normalized by the first bouncing term T. The motion capture of the water droplet revealed that there was no apparent difference between the bounce profiles on these surfaces, including the net external force Fnet = πρD3/6 (d2y(τ)/dτ2 + g), where ρ and D are the droplet density and diameter, respectively, as shown in Figure 2F. The droplet contact times on the solid/air composite and liquid/air composite and the ideal contact time (neglecting the droplet frequency) were 19.7, 15.9, and, 11.2 ms, respectively; the differences were only owing to the sway of the droplets (see Note S2, Figures S3−S5, and Videos S1).18 Here, the theoretical value of the ideal time was calculated using τ ≈ (ρD3γ−1)0.5, where γ is the droplet surface tension.19 Thus, the liquid surface does not influence the bouncing of the droplet and the contact time, which means the air cushion prevents the liquids from contacting each other. The fluid dynamics of a droplet bouncing and the transition between levitation and spread wetting, which is a crucial factor
Figure 1. Overview of the concept of a smart wetting system. (A) Photographic images of droplets bouncing and spreading on a puddle. The transition from bouncing (contact angle is nearly 180°) to spreading (contact angle is 0°) is sometimes seen in nature when raindrops fall on a puddle. (B) Schematic images of a smart wetting system using a smooth liquid coating inspired by raindrops falling on a puddle; the droplet bounces on the liquid thin films under specific conditions, whereas the droplet spreads under other conditions. The transition occurs regardless if the liquids are miscible. (C) The smart wetting transition of water droplets on a smooth silicone-oil-infused coating on the surface. The bouncing water droplet is metastable and shows a contact angle of 179.8° on the surface (in the first touch), whereas the spreading droplet is static and shows a contact angle of 80.8° on the surface owing to the hydrophobicity of silicone oil.
leaf structure by the self-assembly of hydrophobic silica nanoparticles. The surface had a static water CA of 156.5° (Figure 2A). We observed a water droplet bouncing on the
Figure 2. Comparison of the droplet bouncing on the lotus-inspired superhydrophobic surface and the puddle-inspired smooth liquid surface. (A,B) Schematic images of the droplet bouncing mechanism on the surfaces; the insets show scanning electron microscopy images of the surface structures and the photo images of the dynamic contact behavior of the water droplets. (C,D) High-speed image sequences showing a droplet bouncing on (C) a superhydrophobic surface (We = 0.0214) and (D) a smooth liquid surface (We = 0.0556; the scale bars are 1 mm). (E) Normalized droplet vertical position y (red circles, from the surface to the center of the mass), h (blue circles, from the surface to the bottom of the droplet) and contact width w (black circles, cross-sectional centroid) as a function of the bounce term τ/T for the image sequence in images C and D. (F) Net external force Fnet applied to the droplets on the superhydrophobic surface (green) and the smooth liquid surface (purple) as a function of time τ. B
DOI: 10.1021/acs.langmuir.7b01600 Langmuir XXXX, XXX, XXX−XXX
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Figure 3. Study of the bouncing behavior of the droplets and their thermodynamic stabilities. (A) The bouncing behavior of water, ethanol, ethylene glycol, hexadecane, rapeseed oil, and perfluoropolyether droplets. The scale bars are 0.5 mm. (B−D) Study of the droplet bouncing dynamics. (B) A schematic image of the inner flow in the droplet, air flow, and wave impacting the flow of the liquid layer when a droplet with a low or high Re bounces on a smooth liquid surface under conditions in which the inertia force dominates. (C) Relationship between Re and the restitution coefficient e categorized by bouncing (blue circle), coexistence (pink triangle) and spreading (red x mark). (D) A range of e values of water, ethanol, ethylene glycol, hexadecane, rapeseed oil, and perfluoropolyether droplets, which have a different Oh number. (E) Study of droplet bouncing or spreading criteria bordered by the droplet kinetic energy Eitk and the unit area droplet adhesion energy Eadhesion. (F) Temperature effect on the droplet wetting condition. The conditions were categorized by ‘bouncing is possible at a constant Eitk < Ecitk’ (blue circle), ‘coexistence (both bouncing and spreading occurs) at a constant Eitk < Ecitk’ (pink triangle), and ‘no bouncing phenomenon observed at any Eitk’ (red x mark).
must be influenced by not only air flow between the droplet and liquid surface but also the inner flow of the liquid in the droplet. Thus, although the droplet restitution coefficient is stable if Re < 10 (Laminar flow area), the restitution becomes unstable but the restitution coefficient increases on average as the Re increases above 10 (vortex flow).22 This discussion may be helpful for the design of an efficient droplet transportation device using the system. Moreover, the bouncing of the droplets was analyzed by determining the Ohnesorge number of the liquids Oh = We0.5Re−1 (Figure 3D). The results indicate that the droplet restitution is negatively proportional to Oh. The result means that the restitution loss is mainly influenced by the droplet inner flow derived from viscous dispersion loss rather than the air flow or the smooth liquid layers’ sway in Figure 3B. Although the droplet impact energy is the determining factor of whether the droplet bounces or spreads, the determining factor for the switching of the wettability is still unclear. Thus, we varied the droplet impact energy Eitk = πρD3v2/12 with droplets with different surface energy Esurf = πD2γ to determine the threshold conditions of the droplet bouncing or spreading on the surface, as shown in Figure 3E (as well as Figure S8 and Video S3). Even though the
for the creation of tunable wettable system, were analyzed. As the contact area between the water droplet and the surface consists of mostly air (the air ratio of the contact area fair > 0.999995; see Note S3), the liquid layer and droplet hardly interact with each other regardless of their surface tension and surface−liquid interaction (Figure S6).20 We prepared six kinds of droplets with γ ≈ 17.1−72.9 mN m−1 to observe the droplet bouncing behavior (Figure 3A, Table S1). Interestingly, even though the bouncing behavior can be strongly affected by the Weber number We = ρv2Dγl−1, where v is the droplet impact velocity,21 the droplet bouncing behaviors differ with droplet kinds (e.g., hexadecane bounces (We = 0.308), whereas rapeseed oil (We = 0.302) just hovers and does not bounce on the surface; see Video S2) and have weak relationship between We and the restitution coefficient e, which is the ratio of the droplet velocity before and after impact (Note S4 and Figure S7). We found that the bouncing of a droplet on a liquid infused surface is related to the Reynolds number Re = ρvDμ−1, where μ is the droplet viscosity. Figure 3B,C shows the influence of Re on the restitution efficiency. As the liquidinfused smooth surface forms a continuous air layer unlike the air/solid composite lotus surface, the bouncing of the droplets C
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Figure 4. Application of a smart wettable system controlled by the kinetic energy of a droplet. (A,B) A hand-operated high-speed in situ droplet manipulation system navigated by the settled droplets (the scale bars are 1 mm) expressed by image sequences and a scheme. (C,D) Application of an in situ droplet reactor; no contact or coalescence between the droplet and the settled drop can be controlled manually by the We number, which can be varied by the droplet volume or droplet casting height, as expressed by high-speed camera image sequences and schemes. The scale bars are 2 mm. The marked areas dispersed in the droplets are carbon black particle used as tracer material to observe the movement of the contents of the droplets throughout the droplet action.
conditions for the droplet bouncing are decided by Eitk, the crucial droplet impact energy Ecitk increases with the surface energy (Note S4 and Figures S7−S9). Thus, the wetting conditions are determined by the Weber number and the crucial value Wec = 12Ecitk/Esurf ≈ 2.88; therefore, the droplet impact can control the dynamic wettability of the droplet bouncing (Wec > We) or spreading (Wec < We) without varying the surface characteristics. The dynamic wettability transition can be repeated at least 100 times owing to the quick self-healing properties of the liquid surface23 (see Figures S2 and S10). Furthermore, the temperature dependence of the dynamic wettability of the droplet was examined, as shown in Figure 3F. It is not clear why the droplet does not bounce when the droplet−liquid layer temperature difference ΔT is large, even though we can control Eitk. It has been reported that the lifetime of droplet levitation bouncing on a liquid pool (in which the thickness of the liquid layer ≫ droplet size) is proportional to the temperature difference.24 This phenomenon cannot be explained just by the slight change of surface tension, liquid density, or vapor pressure25,26 (see Note S5, Figure S11, and Table S2). It may be because of the difference in temperature between the droplet and the liquid, which causes Marangoni flow and destabilizes the surface24 (see Note S5 and Figure S12). However, more investigation is required to understand its thermostability. Finally, we performed two kinds of potential applications using the smart wettable system shown in Figure 4. The first one was high-speed in situ droplet manipulation. We settled some water droplets on the silicon-oil-infused smooth surface. The spread-wetting factor Sow = γwa − γwo − γoa (subscript w:
water; o: oil; a: air) was controlled to be positive, and thus the water droplets were weakly immobilized by the surrounding thin lubricant layer, as shown in Figure 4A,B.27 The settled droplet acts as a navigator for the bouncing droplet because the thin oil layer repels the water droplet, as shown in Figure 4A (and Video S4). As the motion of the droplet can easily be controlled by the position of the settled droplet, complex droplet motion is also possible (Figure 4B and Video S5). The settled drops are easily removed by tilting the surface approximately 30°. The features of this system are contamination-free droplet transport, and repeatedly programmable in situ droplet motion control, which can significantly advance smart materials and hand-operating technology.28 The second application is in situ microdroplet reactor.29 Because the settled droplet (Reagent I) is protected by oil and the air layer, the droplet (Reagent II) with low impact of We = 0.088 does not contact with the settled one, and the droplet contents does not transfer to the settled one (Figure 4C, Video S6). Conversely, the settled droplet immediately coalesced on high impact of the droplet with We = 0.141, and the contents first dispersed in Reagent I are transferred to Reagent II in ∼0.01 s, as shown in Figure 4D (and Video S6).
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CONCLUSION In summary, an extremely wettable switch system triggered by the impact of a droplet on a nonsmart smooth liquid coating material, inspired by a raindrop falling on a puddle, was designed, analyzed, and applied as an advanced liquid manipulator and microfluid reactor. The applications are typical examples of contact-area-controlled smart materials. The D
DOI: 10.1021/acs.langmuir.7b01600 Langmuir XXXX, XXX, XXX−XXX
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Langmuir systems provide a new insight into next-generation smart devices. It also creates an opportunity to progress the field of hand-operated materials through the design of smart systems using “non-smart” materials by lowering the hurdle to create smart systems.
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METHODS
AUTHOR INFORMATION
Corresponding Author
Base Coating for Immobilization of the Liquid Layer. The precursor solution was prepared by mixing 0.4322 g of phenyltriethoxysilane (Shin-Etsu Chemical Co., Ltd., Tokyo, Japan), 1.3791 g of tetramethyl orthosilicate (Junsei Chemical Co., Ltd., Tokyo, Japan), 5.59 mL of ethanol, 0.7566 mL of deionized water, 2.04 μL of aqueous hydrochloric acid (35−37 wt %) in this order, and stirring the mixture for 24 h. The base coating for immobilization of the liquid layer was prepared by spin-coating the precursor solution onto the hydrophilized Si wafer at a speed of 1000 rpm for 5 s and then at 2000 rpm for 10 s. Here, the substrate used included a plate of glass, polyethylene terephthalate, aluminum, copper, stainless steel, iron, polypropylene, polyvinyl chloride, polyaniline, and not porous materials such as wood, paper, and a sponge. A dip, squeegee, and spray method can also be used for the base coating. Liquid Layer Formation. A liquid layer was formed by spincoating the liquid at 1000 rpm for 20 s and then at 3000 rpm for 10 s. A 21.5 ± 1.2 μm thickness of phenyl silicone oil (KF-54, Shin-Etsu Chemical Co., Ltd.) was used as the liquid layer. The thickness of the lubricant did not influence the wettability; if a small amount of liquid is added to the liquid layer, almost the same bouncing behaviors are observed if the condition of liquid layer thickness hliquid layer ≪ D is fulfilled. The thickness of the lubricant only effected the self-healing ability of the liquid layer. Moreover, the switchable levitation/ spreading transition can be observed by using other oils (listed in Supporting Tables S3 and S4). To obtain a tunable and wettable system, the liquid layer must be smooth and dust-free, as shown in Figure S1. Thus, care must be taken during the treatment of the surface. If the surface is contaminated, a water droplet is slid on a slightly tilted surface to remove the dust (see Note S1). Superhydrophobic Coating. A lotus-leaf-inspired superhydrophobic coating was prepared by spraying a 4 wt % dispersion of hydrophobic silica nanoparticles (AEROSIL RX200, Evonik Industries AG, Essen, Germany) in ethanol onto the pristine Si wafer. Characterization. The motion of the droplets was captured using a high-speed camera (HAS-D3, Ditect Co., Tokyo, Japan). Contact angles were measured by analyzing the high-speed camera images with ImageJ software (Wayne Rasband). Surface morphologies were observed using FE-SEM (S-4700, Hitachi, Ltd., Tokyo, Japan) and low-vacuum SEM (Inspect S50, FEI, Hillsboro, USA). Droplets and substrate temperatures were adjusted using a Peltier unit and confirmed by thermography (PI400, Optris GmbH, Berlin, Germany). The humidity of the test environment was adjusted using a thermohygrostat.
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Complex liquid manipulation using the smart wettable system (AVI) Micro-droplet reaction controlled by the droplet impact energy (AVI)
*E-mail:
[email protected]. ORCID
Mizuki Tenjimbayashi: 0000-0002-8107-8285 Seimei Shiratori: 0000-0001-9807-3555 Author Contributions †
T. Matsubayashi, and T. Moriya equally contributed to the research. M.T. proposed the research, designed the experiments, devised the explanation, collected and analyzed the data, and wrote the manuscript. T. Matsubayashi, and T. Moriya collected the data. S.S. supervised the project. All authors discussed the data, and substantially contributed to the research. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are deeply grateful to Kengo Manabe, Issei Takenaka, and Ryo Togasawa whose meticulous comments were of enormous help. M. Tenjimbayashi acknowledges a predoctoral fellowship (DC1) from Japan Society of Promotion of Science (JSPS). This work was supported by JSPS KAKENHI (Grant Number JP16J06070, JP26420710).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01600. Supporting content, figures, and tables as described in the text (PDF) Droplet bouncing on a smooth liquid surface and a superhydrophobic surface (AVI) Various droplets bouncing on a smooth liquid surface (AVI) Wetting switch triggered by the droplet impact energy (AVI) Liquid manipulation using the smart wettable system (AVI) E
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