Temperature-Responsive Anisotropic Slippery Surface for Smart

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Temperature Responsive Anisotropic Slippery Surface for Smart Control of Droplet Motion Lili Wang, Liping Heng, and Lei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16818 • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Applied Materials & Interfaces

Temperature-Responsive Anisotropic Slippery Surface for Smart Control of Droplet Motion By Lili Wang, Liping Heng* and Lei Jiang Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Chemistry, Beihang University, Beijing 100191, China.

KEYWORDS: self-cleaning material, droplet motion, temperature-driven, lubricating fluid, anisotropic porous film

ABSTRACT: The development of stimulus-responsive anisotropic slippery surfaces is important because of the high demand for such materials in the field of liquid directional driven systems. However, current studies in the field of slippery surface were mainly conducted to prepare isotropic slippery surfaces. Although we have developed electric-responsive anisotropic slippery surfaces that enable smart control of the droplet motion, there remain challenges for designing temperature-responsive anisotropic slippery surfaces to control the liquid droplet motion on the surface and in the tube. In this paper, temperature-responsive anisotropic slippery surfaces have been prepared by using paraffin, a thermo-responsive phase-transition material, as a lubricating fluid and directional porous polystyrene (PS) films as the substrate. The smart regulation of the droplet motion of several liquids on this surface was accomplished by tuning the substrate temperature. The uniqueness of this surface lies in the use of an anisotropic structure and temperature-responsive lubricating fluids to

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achieve temperature-driven smart control of the anisotropic motion of the droplets. Furthermore, this surface was used to design temperature-driven anisotropic microreactors and to manipulate liquid transfer in tubes. This work advances understanding of the principles underlying anisotropic slippery surfaces and provides a promising material for applications in biochip and microreactor system.


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1. INTRODUCTION Anisotropic self-cleaning surfaces1-8 have aroused enormous interest due to their widespread applications in liquid transportation,9,10 water-directional collection,11,12 drag reduction,13,14 and microfluidic devices.15 Currently, anisotropic self-cleaning surfaces inspired by butterfly wings,16 filefish skin,17 rice leaves18,19 have been extensively studied theoretically and experimentally. As an emerging class of anisotropic self-cleaning material, Nepenthes pitcher plants-inspired anisotropic slippery surface20,21 is gradually attracting people's research interests because they are expected to have the unique advantages of the slippery liquid-infused porous surfaces (SLIPS)22 such as good stability, excellent self-cleaning properties,23-25 and super-repellent surfaces for various liquids, particularly low-surface-tension fluids.26-30 However, current studies in the field of slippery surface were mainly conducted to prepare isotropic slippery surfaces including using diverse combinations of lubricating fluids and porous network structure substrates,31-39 3D cross-linked oil gel system29,30,40,41 and slippery omniphobic covalently attached liquid coatings42 and use them in anti-icing,35,43,44 anti-waxing,29 enhancing dropwise condensation,45 oil-water separation,46,47 non-stick containers,31,32 and anti-biofouling.48-52 Newly, smart control of the liquid drop motion on the isotropic slippery surfaces under mechanical stimuli,33,53 magnetic field54-56 and temperature30,57,58 has been reported. Although these studies represent significant progress, the reported slippery surfaces are not capable of anisotropic droplet motion behavior, which is a typical feature of the Nepenthes pitcher surface,32 because of their randomly distributed microstructure.



Thus new-generation slippery surfaces are expected to exhibit both anisotropic and stimulus-responsive properties to satisfy practical application requirements. The recent works have developed an anisotropic slippery surface that enables control of the electric-driven sliding of a droplet and exhibits outstanding anisotropic sliding properties for various liquid droplets;59,60 however, the development of smart anisotropic slippery surfaces is currently in its infancy. For example, the combining anisotropic structures and temperature-responsive lubricating fluids to obtain temperature-responsive anisotropic slippery surfaces and smart control of the liquid droplet motion on the surface or in the tube remains challenging.

In this paper, we present temperature-responsive anisotropic slippery surfaces and smartly regulated the droplet motion of several liquids on these surfaces by adjusting the substrate temperature. These surfaces were composed of paraffin, a thermo-responsive phase-transition material, and directional porous polystyrene (PS) films. When the surface temperature exceeded the melting temperature (Tm) of paraffin, the droplets easily slid on the composite surface; the motion was anisotropic, because the composite surface was a slippery surface with a liquid-liquid-solid contact line. When the surface temperature decreased below the Tm of paraffin, the droplets could be pinned onto the surface because the composite surface was a solid surface covered by solidified paraffin. The uniqueness of this surface is the use of an anisotropic structure and temperature-responsive lubricating fluids to achieve the temperature-driven control of anisotropic sliding for several liquid droplets. Comparing with the droplet manipulation strategies on SLIPSs using other external


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stimuli, such as mechanical stimuli,33,53 electric field,59,60 and magnetic field,54,56 this strategy is more flexible and easier to control. Furthermore, this surface has been used to design a microreactor by examining different critical sliding angles (SAs) of liquids and manipulated the liquid transfer in the tube. This work improves understanding of the principles underlying anisotropic slippery surface and provides a promising system for manipulating liquids in biochip and microreactor devices.61,62 2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. PS and 1,2-dichlorobenzene were purchased from Sigma-Aldrich. First, 1, 2 and 3 mg mL-1 solutions were obtained by dissolving PS into 1,2-dichlorobenzene. After spin-coating the above solutions onto the glass substrates at 1000 rpm for 3 s with a spin coater (KW-4A, China), the substrates were placed on the surface of liquid nitrogen vertically, and the liquid films were thereby directionally frozen. The films were then placed in a beaker immersed in liquid nitrogen to ensure the stability of the frozen films. A freeze-drier (BEKO, Germany) was used to prepare the directional films by vacuum freeze-drying method.59,60 To prepare slippery surfaces, paraffin wax (chunks, Sigma-Alddrich, China) was melted at 80 °C; the liquefied paraffin was then spin-coated onto the PS films at 1000, 2000, 3000, 4000, 5000, 6000, 7000 and 8000 rpm for 20 s; paraffin was solidified on the film surface when it cooled to room temperature.

In the tube-based experiments, the PS film was prepared on the interior surface of a glass tube by immersing the glass tube into a PS solution (2 mg mL-1) for 2 min and vertically placing it on the surface of liquid nitrogen to freeze; the procedure was



identical to the preparation of PS films on glass substrates. Finally, the internal film of the glass tube was coated with liquefied paraffin by dipping and drawing. To remove excess paraffin, the glass tube was placed vertically for 10 min. Glycerol and DMSO were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Ionic liquid ([OMIm][BF4]) with a purity > 99% was supplied by Lanzhou Institute of Chemical Physics (LICP). Water with a resistivity of 18 MΩ cm was obtained from a Milli-Q purification system (Millipore Corp., Bedford, MA).

2.2. Characterization. ESEM images were recorded by an environmental scanning electron microscope (Quanta FEG250, FEI) with a thin gold coating. The films with solidified and liquefied paraffin were imaged with a digital camera (Canon 60D, Japan) and an ESEM. AFM images were obtained using a Bruker Dimension Icon. A DataPhysics OCA20 CA system was used to measure the CAs and SAs of liquid droplets. The average value was obtained by measuring at least five different positions on the same sample. The temperature in the CA and SA measurements was controlled using a programmable heater plate, which was fastened to the platform of the OCA 20. The surface tension of the liquid and the interfacial tension of the two liquids were measured by using the pendant droplet method (OCA 20, DataPhysics Instruments GmbH, Germany). An Anton Paar rheometer (MCR302, Austria) was used to measure the viscosity. To measure the friction forces between liquid colloid and substrate, a drop of water was deposited on a tipless atomic force microscope probe under colloid probe technology.63 The AFM test was operated in a “close loop” Bruker Dimension Icon system.


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3. RESULTS AND DISCUSSION 3.1. Fabrication of Temperature-Responsive Anisotropic Slippery Surfaces To achieve temperature-driven smart control of the anisotropic sliding of liquid droplets on slippery surfaces, paraffin with a Tm of 44–46 °C was selected as the lubricating fluid; the paraffin can be reversibly transformed between the solid phase and the liquid phase by changing the temperature. The paraffin in this study was solid at room temperature and did not require a high temperature to melt, thus making the process flexible to control and avoiding the energy consumption associated with high-temperature heating. In fact, paraffin with other Tm values can be used according to specific requirements in practical applications. Anisotropic structure was provided by the directional porous PS film prepared from a freeze-drying process59,60. Accordingly, the slippery surfaces preparation and temperature-responsive processes of liquid droplet motion are illustrated in Figure 1. The selected paraffin was liquefied at 80 °C; the directional porous film was then spin-coated with liquefied paraffin to form the slippery surfaces. At T < Tm, droplets on the PS porous surface, which was covered with solidified paraffin, formed the air/liquid/solid system. Hence, the droplets in contact with this surface were in the Wenzel state30 and strongly pinned on the surface in either the parallel or the perpendicular direction. When the temperature exceeded Tm, the directional porous surface was covered by liquefied paraffin as the lubricant oil layer, thus forming an anisotropic slippery surface, and the liquid droplets were in the low-adhesion slippery state. Because the frictional resistance along the perpendicular direction (⊥) is greater than that along the parallel



direction (//), a larger tilt angle (TA) is needed for overcoming the greater resistance in the perpendicular direction.41 Thus, the SA is distinctly larger in the perpendicular direction than in the parallel direction, and the SA means the TA of film when the droplet begin to move along the surface. That is, the directional sliding of the droplets should be able to be guided by the anisotropic structures of the porous surface. Furthermore, the liquid-drop sliding and pinning on this surface were expected to be controllable by reversibly tuning the substrate temperature above or below the Tm. On the basis of the aforementioned principle, directional porous PS films were prepared using different concentrations of PS chlorobenzene solutions. SEM images (Figure 2a, b) showed that the excellent directional porous structure was formed by using 2 mg mL-1 solution. The film prepared by the lower concentration of 1 mg mL-1 PS solution (Figure S1a) only has a small amount of fibers and expose the glass substrate, it couldn’t hold enough paraffin to form SLIPSs. The anisotropic and porous structure of film prepared by the higher concentration of 3 mg mL-1 PS solution (Figure S1b) was nearly disappeared. So 2 mg mL-1 PS solution was the suitable concentration to uniformly prepare a anisotropic structure. A similar phenomenon has also been reported in our previous works.59,60,64 To further confirm the anisotropic structures of the films, four types of liquids with different surface tensions and viscosities, which were not miscible with the liquid paraffin, were selected as the model droplets; their contact angles (CAs) and SAs on the PS directional films without the lubricating fluid were investigated (Figure S2, Table S1). The results indicated that the PS films had anisotropic CAs in two directions and that


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the CAs decreased with decreasing liquid surface tension, except in the case of the ionic liquid because it had a higher viscosity coefficient than DMSO. All droplets cannot slide along the parallel direction on the surface even though the films were tilted to 90° (Figure S2c). It demonstrated that the anisotropic sliding behavior can’t be obtained without a lubricating fluid infused into the porous structures.

As shown in Figure 2c, after the porous PS films were infused with paraffin, the sample was translucent with a milky-white color at room temperature. When heated to T > Tm, as shown in Figure 2d, the film recovered the optical transparency of the PS film; this change of color was reversible during the heating-cooling process. Synchronous environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM) characterizations were also performed to further confirm the changes in paraffin on the surfaces during the heating-cooling process (Figure 2e-h). The ESEM image in Figure 2e showed that the underlayer directional porous structures were covered by solid paraffin at 25 °C and that the anisotropic structure remained faintly visible. When the sample was heated to 55 °C, the paraffin was in the liquid state and infused into the porous structure, whereas the homogeneous and continuous liquid layer was observed on the surface (Figure 2f); however, the anisotropic structure of the film could not be observed because liquid paraffin was floating on the surface under the ESEM vacuum conditions. Thus, the surface morphology could be finely tuned by the thermo-responsive phase change of the paraffin. The AFM image in Figure 2g was consistent with the ESEM image in Figure 2e, thus demonstrating that the protruding solid paraffin covered the porous



film and decreased the anisotropy of the film, as compared with the PS directional porous films without paraffin (Figure 2h). 3.2. SAs Variation with the Change of Lubricant Thickness and Liquid Droplet Volume The CAs and SAs of several liquids with different surface tensions and viscosity coefficients were measured to characterize the ability of the surface to repel liquids. The CAs and SAs of various liquids on the PS film filled with paraffin at 25 °C (T < Tm) were studied (Figure S3, Table S2). The results showed that the CAs in two directions on these surfaces were only slightly different because of the decrease in anisotropy tendency. Figure S3c exhibited the sliding behavior of droplets on the PS film. All the droplets couldn’t slide along surface even in the parallel direction after the film was tilted to 90°. These results demonstrated that the anisotropy of the porous film covered by solidified paraffin decreased and the anisotropic motion of liquid droplets could not be realized at T < Tm because the slippery surfaces are not formed with solid paraffin on the film, thus resulting in droplets being pinned on the surfaces.

The SA variations on these anisotropic surfaces by changing the substrate temperatures were further investigated. Prior to this study, the relationships among the SAs, paraffin layer thickness and liquid droplet volume were studied to explore the anisotropic sliding optimum condition of the droplets at T > Tm. Because the thickness of the liquid paraffin layer decreased with increasing spin-coating speed, the relationship between the SAs and the thickness of paraffin layer was showed by measuring SAs on the samples with different spin-coating speed (Figure 3a-d).


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For 2 µL droplets of water, glycerol, and DMSO, the SAs increased with increasing spin-coating speed in both directions. As the spin-coating increased to 4000 rpm, the difference of SAs in both directions reached a maximum. However, the anisotropic SA difference of ionic liquid was maximal when the spin-coating speed was 5000 rpm, which was slightly larger than 4000 rpm. Samples with excessively thick or thin paraffin layers would made the anisotropic behavior disappear. Specifically, the anisotropic discrepancy disappears when the liquid paraffin layer was too thick at a spin-coating speed of 1000 rpm because the sufficient amount of lubricant that remained on these surfaces makes the SAs independent of the substrate structure.27,59,60 The SAs of water droplets were increasing to 78.3 ± 1.1° in the parallel direction and 82.0 ± 0.7° in the perpendicular direction when the spin-coating speed reached to 8000 rpm. Because the porous films were only infused with a small amount liquid paraffin, the microstructure and roughness of the solid surface are potentially important factors in this situation.40 Thus, the sliding of water in the parallel and perpendicular directions is difficult on such surfaces because of the large resistance. Similarly, when the spin-coating speed was 8000 rpm, DMSO and ionic liquid droplets were pinned in two directions even the films were tilted 90°, and the anisotropic discrepancy simultaneously disappeared. However, glycerol had much smaller SAs (26.3 ± 0.9° and 27.4 ± 0.5° in the parallel and perpendicular directions, respectively) than water at a spin-coating speed of 8000 rpm. According to the results of previous studies,65,66 the motion behavior of the liquid droplet on the slippery surfaces can be affected by the droplet viscosity: an increase in the viscosity markedly



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decreases the slipping motion and increases the contribution of rolling. Because glycerol has a much higher viscosity than the other liquids (Table S1), the glycerol droplet tends to roll instead of slipping on such a surface.66 Thus, the glycerol droplet appears to move with smaller SAs than water, DMSO and ionic liquid droplets; in addition, the anisotropic discrepancy of SAs disappears with insufficient lubricant.

To achieve a uniform and optimal experimental criterion, 4000 rpm was selected as the spin-coating speed for the subsequent studies. Furthermore, the relationship between the SAs and the volume of liquid droplets was studied. From Figure 3e-h, it can be found that the SAs of four liquid droplets decreased with the increase in volume of droplets in two directions, meanwhile, the anisotropic SA discrepancy between the parallel and the perpendicular direction decreased and ultimately disappeared. The results showed that the anisotropic slide of droplets can be achieved with a small volume of droplet, this phenomenon was consistent with findings from the previous publications.59,60 3.3. Theoretical Criteria of Anisotropic Slippery Surfaces At T > Tm, liquefied paraffin acts as a lubricating liquid, which allowed formation of slippery surfaces with porous PS films. Moreover, if a liquid drop floats on the lubricant-infused porous surface, then the combination of the test liquid drop, lubricating fluid and substrates should satisfy the following slippery surfaces criteria:22,67 ∆E1 = R(γBcosθB － γAcosθA) － γAB > 0


(1)

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∆E2 = R(γBcosθB － γAcosθA) ＋ γA － γB > 0

(2)

where γA, γB and γAB are the surface tensions of the target liquid drop (liquid A) and lubricant (liquid B) and the interfacial tension between them, respectively, and θA and θB are the CAs of the target liquid drop and lubricant on the flat PS film surface, respectively. The roughness factor is represented by R, which is the surface area ratio between the porous substrate and that of a flat surface and can be expressed by the water CA for a rough surface and a smooth surface:67,68 R=

cos θ ' cos θ

(3)

where θ′ = 107.6° in the parallel direction on the porous directional PS film, and θ = 96.2° on the flat PS film.

In the above criteria, an assumption of the working condition is that the fluid layer covers the surface features.22 In this case, nanoscale features on the microscale fiber networks (Figure 2b) provided a large capillary force to hold liquefied paraffin even when the liquefied paraffin layer was in an unsaturated state;27 thus, the working condition was satisfied. The CAs and surface tension of various liquids and liquefied paraffin were measured to calculate ∆E1 and ∆E2 in the parallel direction at 55 °C (Table S3). Similarly, ∆E1 and ∆E2 in the perpendicular direction were also calculated (Table S4). The results show that ∆E1 > 0 and ∆E2 > 0 for all combinations in both directions at 55 °C, thus indicating that water, glycerol, DMSO and ionic liquid droplets floated on the liquid paraffin layer without directly contacting the solid surface.



The anisotropic friction forces of water droplet on anisotropic slippery surfaces were measured to illustrate the sliding energy barriers in the parallel and the perpendicular direction. Because the droplet was pinned on the surface covered by solidified paraffin at T < Tm, the friction forces of water droplet were tested in two directions at 55 °C (T > Tm). As can be seen in Figure S4a, the kinetic friction forces of water droplet were about 360 nN and 200 nN in the perpendicular and parallel direction, respectively. The results showed that friction force in the perpendicular direction was greater, so a larger TA was required to overcome the resistance in this direction. The force analysis of a droplet in two directions was shown in Figure S4 b and c. To the best of our knowledge, anisotropic force measurement based on slippery surfaces has not yet been reported, this innovation is essential for advancing the mechanism of the anisotropic properties on SLIPS. 3.4. SAs Transition on Anisotropic Slippery Surface Induced by Temperature Stimulus On the basis of the aforementioned experiments, the relationship between the variations in SAs and temperatures was systematically examined. As shown in Figure 4a-d, the SAs of various liquid droplets with a fixed volume of 2 µL increased with decreasing temperature, because the paraffin layer with increasing viscosity hindered the movement of droplets.30 Here, 55 °C was selected as the terminal heating temperature, because the anisotropic SA discrepancy did not change at higher temperatures and because excessively high temperatures can destroy the porous films


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and make the repelled liquid evaporate; the safe operating temperature of PS is below 70 °C.

The anisotropic CAs of various liquids on the PS film filled with liquefied paraffin at 55 °C is shown in Figure S5 and Figure 4e. The results showed that the CAs decreased and that the difference in the CAs between the two directions increased compared with that on the PS surface covered by solid paraffin, thus indicating that the paraffin underwent a phase transition. The SAs of water, glycerol, DMSO and the ionic liquid droplet on the PS films filled with liquefied paraffin in the parallel direction (Figure 4e) were 28.9 ± 3.2°, 13.6 ± 1.1°, 35.8 ± 3.1°, and 38.9 ± 3.3°, respectively, while the SAs in the other direction (Figure 4e) were 49.5 ± 2.5°, 20.1 ± 0.8°, 55.0 ± 2.6°, and 62.2 ± 3.2°. These results showed that droplets motion in the perpendicular direction is more difficult than in the parallel direction on the anisotropic slippery surfaces.

When the temperature decreased from 55 °C to 51 °C, the SAs of water changed only slightly in both directions (Figure 4a). As the temperature decreased from 51 °C to 43 °C, the SAs increased from 32.0 ± 3.4° (//) and 51.3 ± 3.0° (⊥) to 90°. It demonstrated that the droplet was pinned on the surface in two directions at 43 °C. The variation of SAs for glycerol (Figure 4b) was similar: the SAs remained nearly constant with decreasing temperature from 55 °C to 53 °C in two directions, and increased from 14.1 ± 1.3° (//), 20.7 ± 1.1° (⊥) to 90° when the temperature decreased from 53 °C to 43 °C. For the DMSO (Figure 4c), the SAs change in two directions were small when the temperature decreased from 55 °C to 49 °C. As the temperature ACS Paragon Plus Environment


decreased from 49 °C to 43 °C, the SAs increased from 40.6 ± 3.4° (//) and 57.5 ± 3.2° (⊥) to 90°. In the case of the ionic liquid (Figure 4d), the SAs remained constant when the temperature decreased from 55 °C to 51 °C in two directions, and increased from 42.7 ± 2.9° (//), 64.2 ± 2.7° (⊥) to 90° with decreasing temperature from 51 °C to 43 °C. All the droplets couldn’t slide in either direction when the temperature was lower than 43 °C.

The reversible switching of the SAs on the film above and below the Tm was also measured; the temperatures of 55 °C and 37 °C were selected for this series of experiments. As demonstrated in Figure 5, the reversible control the motion of droplets on the film was realized by alternately changing the temperature. Obviously, four types of liquid droplets slid at 55 °C with different SAs in the two directions, and all of them were pinned on the surface of the films at 37 °C in two directions. The surfaces showed enduring anisotropic property after five cycles of reversible control. 3.5. Temperature-Driven Smart Control of the Droplets Motion on Anisotropic Slippery Surface and Their Mechanism Furthermore, the temperature-responsive reversible switch of droplet motion has been realized in the parallel direction on these slippery surfaces (Figure 6). To obtain a clearer display, a volume of 4 µL was selected for the following study. When the droplet volume was 4 µL, the SAs of droplets in the parallel direction for water, glycerol, DMSO and the ionic liquid were 22.2 ± 2.1°, 12.5 ± 1.0°, 32.4 ± 3.0° and 34.6 ± 2.8°, respectively (Figure 3e-h). Figure 6 shows temperature-responsive smart control of the droplet motion. A water droplet (4 µL) was dropped onto the surface


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with a TA of 22°, which corresponded to the SA of 4 µL of water within a reasonable error range. The water droplet began to slide on the surface after a few seconds of its deposition onto the heating surface because the water droplet was colder than the film surface. After the heat source was removed, the water droplet further moved by approximately 1 mm and stopped sliding, because the time is needed for the liquid paraffin to completely solidify. The captured droplet began to move along the surface again when heating was resumed.

Other exhibitions of temperature-driven controllable droplets movement on these surfaces were shown in Figure 6b, 6c and 6d. Here, switching the temperature enabled control over the motion of glycerol, DMSO and ionic liquid in different TAs, which corresponded to the critical SAs in the parallel directions within a reasonable error range. The differences between SAs of various droplets on the same surfaces were depending on many physical parameters of the liquids, such as surface tension, viscosity, polarity and density.28,69,70 These results showed that the SAs of various droplets are different on the same slippery surface.

As mentioned in previous work, friction force theory can be used to explain the aforementioned phenomenon.59,60 For the equations F1 = ρgVsinα and f = µN, it is only when F1 ≥ f, a droplet can move along a surface, F1 is parallel component of gravity, f represents the maximum static friction force between the droplet and the interface.59,60 In this system, f is related to the physical properties of the liquid and the contact area between a droplet and a surface. For the liquids in the experiment, the order of surface tensions is water > glycerol > DMSO > ionic liquid (Table S1). The ACS Paragon Plus Environment


surface tension of the DMSO and ionic liquid are smaller than that of water, which result in the decrease of CAs, and the increase of the contact area between the droplet and the surface and f. The large f causes the deformation of DMSO and ionic liquid drops on tilted surfaces and the obvious CA hysteresis (Figure 6c and 6d). Therefore, the DMSO and ionic liquid have larger SAs than that of water. Notably, glycerol has the highest viscosity among all of the investigated liquids; in this case, the motion mode changes to rolling,66 thus making the SA of glycerol small. 3.6. Application of the Anisotropic Slippery Surface in Designing Microreactor and Manipulating Liquid Transfer in the Tube By using temperature-driven control of liquid-droplet motion on anisotropic slippery surfaces, an anisotropic microreactor was designed on the surface and the controllable liquid sliding in the glass tube could also be achieved via temperature stimulus, which would be crucial for practical applications in biochips. As shown in Figure 7a, b, a glycerol droplet (top, 4 µL) that contained a small amount of phenolphthalein and a KOH droplet (down, 4 µL) were simultaneously placed on the anisotropic slippery surface in the parallel and perpendicular directions at 25 °C; the TA of the substrate was 15°. The side view of the sheet glass on the heater plate is shown in Figure S6. The two droplets did not move in either direction at 25 °C after 20 s. At 55 °C, the KOH droplet (SA of 22.2 ± 2.1°; Figure 3e) did not move; however, the glycerol droplet (SA of 12.5 ± 1.0°; Figure 3f) slid downward in the parallel direction. Finally, the glycerol droplet coalesced with the KOH droplet, and the droplet became red after the color reaction. In the perpendicular direction, neither


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the glycerol droplet (SA of 18.5 ± 1.1°; Figure 3f) nor the KOH droplet (SA of 39.3 ± 2.3°; Figure 3e) moved, even at 55 °C. Thus, these temperature-responsive anisotropic slippery surfaces may be used to develop a new temperature-controllable microreactor system. This temperature-control anisotropic sliding process is suitable to design other biphase or interface reactions using related liquids.

In addition, this anisotropic slippery surface was fabricated on the interior surface of glass tubes with an internal diameter of 4 mm (Figure 7c); the parallel direction of the directional film was identical to the tube orientation, and the TA of the tubes was approximately 20°. The photographs in Figure 7c show the thermo-controllable liquid motion, in which the colored water (50 µL) at the top of the tube did not move at 25 °C after 20 s. At 55 °C, the solid paraffin melted, the inner surfaces of the tube became slippery with liquefied paraffin infused into the porous film, and the colored water slid in the tube. When the heater plate was turned off and allowed to cool to 25 °C, the sliding motion stopped. The colored water moved farther for a distance during the cooling process because liquefied paraffin requires time to complete solidification when the temperature decreases. Figure 7d shows photographs of a comparative experiment with a bare, uncoated tube; the colored water in the tube does not move during the whole heating and cooling process. Similarly, the temperature-driven control of motion of other liquids in the tube also can be achieved, thus indicating that this system has potential applications in biochips.

4. CONCLUSIONS



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In summary, we fabricated temperature-responsive anisotropic slippery surfaces by combining anisotropic porous structures and a temperature-responsive lubricating fluid. By exploiting the phase-change property of paraffin, reversible sliding and pinning of several liquids was achieved by changing the substrate temperature. At T < Tm, the composite surface is in the solid state with high adhesion for all liquids; the droplets cannot slide off the surface in the parallel or perpendicular direction of the films even when the TA is 90°. At T > Tm, the solid paraffin covering on the directional porous film becomes a liquid; the composite films are anisotropic slippery surfaces, which enable droplets to move more easily in the parallel direction than that in

the

perpendicular

direction

to

the

fibers.

In

contrast

to

existing

temperature-responsive slippery surfaces, this system enables the anisotropic sliding of several liquids drops with good reversibility, an aspect critical for multistep droplet manipulation. This new surface may also be used to design microreactors and may be extended to glass tubes for smart control of liquid transfer, thus potentially promoting the application of slippery surfaces in microreactors, biochips, and biomedical systems.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website or from the author.

SEM images of PS films; anisotropic CA photos of various liquid droplets (2 µL) on the directional porous PS films; anisotropic CA photos of test droplets (2 µL)


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on the directional PS films coated with paraffin at 25 °C and 55 °C; force analysis of a droplet on the tilted surface; friction force measurement of droplet; depiction of the sample on the heater plate; anisotropic CAs of test droplets (2 µL) on the directional PS films without and with paraffin; CAs and surface tensions of various test liquid and liquid paraffin and calculated ∆E. AUTHOR INFORMATION

Corresponding Author

* Address correspondence to: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51673010, 51541301), the National Research Fund for Fundamental Key Projects (2014CB931802) and the Fundamental Research Funds for the Central Universities (YWF-16-BJ-Y-72).

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Figure 1. SLIPS preparation and temperature-responsive processes of liquid-drop motion. First, liquefied paraffin was spin-coated onto directional porous PS films. When cooled to room temperature, the paraffin solidified on the PS film; even when the substrate was tilted by 90°, the droplet was pinned on the surface in the parallel (//) or perpendicular direction (⊥) to the fiber. At T > Tm, the paraffin became a liquid, and the liquid droplet anisotropically slid on the surfaces in two directions.


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Figure 2. SEM images of PS films prepared by directional freeze-dying. (a) The low-magnification image shows that the film had a directional structure; (b) the high-magnification image shows a porous network structure. The digital pictures show that porous film infused with paraffin was translucent with a milky-white color at 25 °C in (c) and fully transparent at 55 °C in (d). (e) ESEM image of solidified paraffin on the PS directional porous films at 25 °C; (f) at 55 °C, the paraffin became liquid. AFM images reveal the surface structure of (g) the PS film infused with



paraffin at 25 °C and (h) the PS directional porous film structure.


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a)

b)

Water

80

40 20

⊥ 0

c) 100

2000 4000 6000 8000 Rotational speed (rpm)

0

0

d)100

60

SA (°)

SA (°)

⊥

//


Ionic liquid

80

40

⊥

20

// 0

60


⊥

40

⊥

// 0

f)


⊥

20 SA (°)

20

60

20

//

40 SA (°)

10

DMSO

80

e)

Glycerol

//

0

0

30 20

SA (°)

SA (°)

60

//

15 10

0 0

4 8 12 The volume of water (µL)

⊥

60

h)

20

0

4 8 12 The volume of DMSO (µL)

0

4 8 12 The volume of glycerol (µL)

⊥

60

//

40

0

5

SA (°)

g) SA (°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


//

40 20 0

0 4 8 12 16 The volume of ionic liquid (µL)

Figure 3. Relationship between the liquefied paraffin spin-coating speed and SAs at 55 °C for (a) a water droplet, (b) a glycerol droplet, (c) a DMSO droplet and (d) an ionic liquid droplet. The anisotropy in the two directions first increased and subsequently decreased with increasing liquefied paraffin coating speed. Relationship between the volume and SAs for (e) a water droplet, (f) a glycerol droplet, (g) a DMSO droplet and (h) an ionic liquid droplet. The SAs decreased in both directions



with increasing liquid droplet volume.


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Figure 4. Variation of SAs in response to temperature. The liquids are (a) water, (b) glycerol, (c) DMSO, and (d) an ionic liquid. The SAs decreased in two directions with increasing temperature. The inset pictures show that various liquid droplets were pinned on the surface at 37 °C (T < Tm) and that their anisotropic sliding behavior occurred at 55 °C (T > Tm). (e) Anisotropic CAs and SAs of various liquid droplets at 55 °C (T > Tm), indicating that the droplets in the parallel direction slid more easily than those in the perpendicular direction.



Pinning

30 0

55 °C

0

0

c)

1

2 3 Cycles

4

d)

50

Pinning

0

30 0

55 °C

0

1

2 3 Cycles

4

5

Pinning

0

1

2 3 Cycles

4

⊥

Ionic liquid

//

37 °C

Pinning

5

20 10 0

SA(°)

SA(°)

Pinning

SA(°)

0

5

//

37 °C

20

55 °C

⊥

DMSO

//

37 °C

Pinning

SA(°)

50

⊥

Glycerol

SA(°)

SA(°)

//

37 °C

Pinning

b)

⊥

Water

SA(°)

a)

SA(°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50

Pinning

40 55 °C

0

0

1

2 3 Cycles

4

5

0

Figure 5. SAs reversibly switched in response to temperature from 37 °C to 55 °C: (a) water, (b) glycerol, (c) DMSO, and (d) ionic liquid.


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Figure 6. Demonstration of the thermo-controllable drop sliding motion (4 µL) in the parallel direction of the anisotropic SLIPS film. The substrate tilt angles of (a) a water



droplet, (b) a glycerol droplet, (c) a DMSO droplet and (d) an ionic liquid droplet correspond to their critical SAs.

Figure 7. (a) Time-sequence photographs of the microchemical reaction in the parallel direction; the top droplet is glycerol (4 µL; TA ≈ 15°) containing a small amount of phenolphthalein, and the other is a KOH water solution (1 M; 4 µL). At 25 °C, the two droplets could not move; at 55 °C, the glycerol droplet slid downward from the top and finally coalesced with the KOH droplet; the droplet became red after the color reaction. (b) In the perpendicular direction, two droplets could not move at 25 °C or 55 °C, and no color reaction occurred. (c) Time-sequence photographs of the thermo-controllable sliding of colored water (50 µL; TA ≈ 20°) in a SLIPS-coated glass tube with an internal diameter of 4 mm; (d) plugs of colored water in uncoated glass tubes.


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Temperature-Responsive Anisotropic Slippery Surface for Smart

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