Unidirectional Wetting of Liquids on “Janus” Nanostructure Arrays

Feb 14, 2017 - We report the unidirectional wetting behavior of liquids (water and oil) on Janus silicon cylinder arrays (Si-CAs) under various media ...
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Unidirectional Wetting of Liquids on “Janus” Nanostructure Arrays under Various Media Peng Ge,† Shuli Wang,† Wendong Liu,† Tieqiang Wang,‡ Nianzuo Yu,† Shunsheng Ye,† Huaizhong Shen,† Yuxin Wu,† Junhu Zhang,*,† and Bai Yang† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡ Research Center for Molecular Science and Engineering, Northeastern University, Shenyang 110004, P. R. China S Supporting Information *

ABSTRACT: We report the unidirectional wetting behavior of liquids (water and oil) on Janus silicon cylinder arrays (Si-CAs) under various media (air, water, and oil). The Janus cylinders were prepared by chemical modification of nanocylinders with different molecules on two sides. Through adjusting surface energies of the modified molecules, the as-prepared surfaces could control the wetting behavior of different types of liquids under various media. We discuss the regularity systematically and propose a strategy for preparing anisotropic wetting surfaces under arbitrary media. That is, to find two surface modification molecules with different surface energies, one of the molecules is easy to be wetted by the liquid under the corresponding media, while the other one is difficult. Additionally, by introducing thermal-responsive polymer brushes onto one part of Janus Si-CAs, the surfaces show thermal-responsive anisotropic wetting property under various media. We believe that due to the excellent unidirectional wettability under various media, the Janus surfaces could be applied in water/oil transportation, oilrepellent and self-cleaning coatings, water/oil separation, microfluidics, and so on.

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INTRODUCTION Surface wettability is a significant performance in material design and application, which is controlled by the geometrical morphology and surface chemistry.1−6 Surfaces with anisotropic wettability have attracted enormous attention owing to their advantages in the design of smart devices,7−10 selfcleaning coatings,11,12 and evaporation-driven formation of patterns13 as well as theoretical research.14−16 In nature, many living creatures utilize anisotropic wetting structures to realize their survival.17−19 For example, the directional asymmetric micronanoscale structures of cacti surface facilitate efficient fog collection.17 Such surfaces with asymmetric morphology possess asymmetric energy barrier in different directions, leading to anisotropic wetting or liquid adhesion phenomena.17−22 Besides asymmetric morphology, another two kinds of anisotropic wetting structures, whether existing in nature or produced artificially, are surfaces with chemical heterogeneity23−27 and groove structures.15,28−33 In the case of chemical heterogeneous surfaces, the behavior of water droplet is dominated by the inhomogeneous surface energy, which causes anisotropic wetting. For surfaces with groove geometries, threephase line is discontinuous due to the groove structure, resulting in a difference of contact angle (CA) in different directions. Mimicking the above design thought, surfaces that control anisotropic wetting of water in air have been investigated systematically. © XXXX American Chemical Society

As for anisotropic wetting of organic liquids in air or under water, actually, challenges are still encountered in driving an organic liquid that has a totally different wetting behavior from water due to the low surface tension. For anisotropic wetting of oil in air, Kang et al. prepared an anisotropic surface that could achieve directional oil sliding through employing re-entrant microgroove arrays.34 In regard to anisotropic surface for oil under water, a well-known example is that Jiang et al. found the oriented hook-like spines arrayed on the unique skin of filefish, which shows anisotropic underwater oleophobicity, and PAAgrafted cloth corduroy with asymmetric morphology was prepared, which possesses the identical ability by mimicking the structure.35 Anisotropic wetting of water under oil media remains unexplored. These surfaces are of great importance for both theoretical research and practical applications including water/oil transportation, oil-repellent coatings, and water/oil separation. Therefore, to meet the demand of the above applications, it is necessary to prepare surfaces with unique anisotropic wettability, which could regulate the wetting behavior of different liquids under various media and propose a strategy for preparation. Received: January 5, 2017 Revised: February 5, 2017 Published: February 14, 2017 A

DOI: 10.1021/acs.langmuir.7b00034 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the procedure for the fabrication of Janus Si-CAs. “M1” and “M2” represent two types of molecules. colloidal crystal.37 Then, the close-packed PS microspheres were etched to non-close-packed (ncp) state under reactive-ion-etching process for 10 min by Plasmalab 80 Plus (Oxford Instrument) with O2 at 50 sccm. The inductively coupled plasma (ICP) power and radio frequency (RF) power were 0 and 100 W, respectively; the total gas pressure was 10 mTorr. Afterward, using the ncp PS microspheres as mask, silicon substrate was etched for 4 min with a gas mixture of SF6 at 4 sccm and CHF3 at 30 sccm (ICP = 100 W, RF = 50 W, and total gas pressure = 5 mTorr). Then, the substrate was sonicated in toluene for 1 min to dissolve the remained PS microspheres, followed by drying in nitrogen flow to form Si-CAs. Then, OTS- and PFS-modified Si-CAs were prepared through chemical vapor deposition, respectively. After the Si-CAs were deposited with Cr (3 nm in thickness, to improve the adhesion between Au and the substrate), followed by Au (20 nm in thickness) through vertical thermal evaporation, MHA- and HDT-modified Si-CAs were performed via immersing the substrates into 3 mM ethanol solution of MHA and HDT for 10 min, respectively. −OH-modified Si-CAs were prepared by treating the SiCAs in oxygen plasma cleaner for 5 min. Preparation of Janus PFS-MHA-, PFS-HDT-, OTS-MHA-, HDTOH-, and PNIPAM-MHA-Modified Si-CAs. To prepare Janus PFSMHA- or PFS-HDT-modified Si-CAs, the as-prepared Si-CAs were first modified by PFS through chemical vapor deposition; then, 3 nm Cr, followed by 20 nm Au, was deposited selectively on one side of the cylinders via oblique 45° thermal evaporation. After selective MHA or HDT modification onto the Au film, Janus PFS-MHA- or PFS-HDTmodified Si-CAs were prepared. Through modifying OTS and MHA onto the Si-CAs asymmetrically, Janus OTS-MHA-modified Si-CAs were fabricated. Janus HDT-OH-modified Si-CAs were performed by treating the Si-CAs in oxygen plasma cleaner for 5 min and modifying HDT selectively. To fabricate Janus PNIPAM-MHA-modified Si-CAs, a layer of PNIPAM was first polymerized onto the Si-CAs through SIATRP (see Supporting Information “Process”).38 After modifying MHA selectively, Janus PNIPAM-MHA-modified Si-CAs were obtained. Characterization. A JEOL FESEM 6700F electron microscope was employed to take scanning electron microscopy (SEM) micrographs with primary electron energy of 3 kV, and the substrate was sputter-coated with 2 nm Pt before testing. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) was applied to explore the chemical compositions of the Janus surfaces. Thickness of the PNIPAM brushes was measured by the atomic force microscopy (AFM). Dataphysics OCA20 was used to record the wetting behavior of liquid droplets on the Janus surfaces under various media and calculate the surface energy of solids. The calculation can be done in

Through modifying elliptical silicon nanopillars with hydrophobic and hydrophilic molecules asymmetrically, anisotropic wetting surfaces that drove water unidirectional wetting in air have been obtained via introducing chemical heterogeneous arrays.36 We demonstrate unidirectional wetting of liquids on Janus Si-CAs under various media by modifying two types of molecules asymmetrically onto Si-CAs. Because different group-modified Si-CAs possess different wettability, −COOH, −OH, −CH3, and −F are selected as the typical modified functional groups. The as-prepared Janus surfaces could control water unidirectional wetting in air or oil as well as oil unidirectional wetting in air or water media. On the basis of the experimental phenomena, a strategy for preparation is proposed, which is to find two surface modification molecules; one of the molecules is easily wetted by the liquid under the corresponding media while the other one is not. Additionally, through introducing PNIPAM brushes onto one part of Janus Si-CAs, thermal-responsive anisotropic wetting surface under various media is fabricated. Such surface switches between isotropic and unidirectional wetting properties under various media when the temperature is below or above the lower critical solution temperature (LCST) of PNIPAM molecule.

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EXPERIMENTAL SECTION

Materials. Silicon wafers (100) were cut into the size of 2.0 cm × 2.0 cm and immersed in Piranha solution (7:3 concentrated H2SO4/ 30% H2O2) for 2 h at 70 °C to create hydrophilic surface and then rinsed with ethanol and Milli-Q water (18.2 MΩ cm−1) repeatedly. Before use, the substrates were dried in nitrogen flow. Polystyrene (PS) microspheres (1 μm in diameter), trichloro(1H,1H,2H,2Hperfluorooctyl)silane (PFS), octyltricholosilane (OTS), copper(I) chloride (CuCl), 2-bromoisobutyryl bromide, 3-aminopropyltrimethoxysilane (APTMS), 1-hexadecanethiol (HDT), and 16-mercaptohexadecanoic acid (MHA) were all purchased from Aldrich. N,N,N′,N″,N″-Pentamethyldiethylenetriamine (PMDETA) was provided by TCI. N-Isopropylacrylamide (NIPAM) monomer was provided by J&K Chemical. Sulfuric acid, hydrogen peroxide, absolute ethanol, toluene, dichloromethane, 1,2-dichloroethane, methanol, hexadecane, and triethylamine were used as received. Water used in the experiments was deionized. Preparation of OTS-, PFS-, MHA-, HDT-, and −OH-modified Si-CAs. First, through interfacial self-assembly, PS microspheres were deposited on silicon substrate to form hexagonal close-packed 2D B

DOI: 10.1021/acs.langmuir.7b00034 Langmuir XXXX, XXX, XXX−XXX

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Langmuir the “SE Calculation Window”. We selected OWRK(Owens−Wendt− Rabel and Kaelble) calculation method to measure the surface energy of the several modified surfaces. The method needs two types of liquids CA on the surfaces. Here water and ethylene glycol were selected as test liquids. First, water and ethylene glycol contact angles on the modified surfaces were measured; then, the surface energy of the modified surfaces was calculated by the calculation method (OWRK).

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RESULTS AND DISCUSSION Preparation of Janus Si-CAs. Janus Si-CAs were prepared through colloidal lithography and asymmetric modification (Figure 1). Through interfacial self-assembly and reactive-ionetching process, Si-CAs were obtained. Figure 2a shows the

Figure 2. SEM images of (a) the Si-CAs (cross section), the inset image shows the top view and (b) the Janus Au-PFS Si-CAs (top view), the red arrow represents the Au deposition direction.

cross-section SEM image of the Si-CAs with diameter about 708 nm and height about 372 nm. The inset top-view image in Figure 2a clearly shows the hexagonal non-close-packed state. After the Si-CAs were modified by the first molecule, one side of the cylinders was obliquely deposited with Cr and Au via thermal evaporation. As shown in Figure 2b, the dark crescentshaped shadows indicated the Janus structure of the Si-CAs; that is, one side of the Si-CAs was Au, while almost no Au was deposited on the other side. The red arrow represents the Au deposition direction. Finally, after modifying the second molecule onto Au film selectively, Janus Si-CAs were prepared. Anisotropic Wetting of Water in Air. To obtain surfaces that control water anisotropic wetting in air, inspired by the Janus elliptical silicon nanopillars,36 hydrophilic and hydrophobic surfaces in air are required, respectively. First of all, we need to investigate the water wettability on different functionalgroup-modified Si-CAs. PFS (−F), OTS (−CH3), HDT (−CH3), MHA (−COOH), and −OH were selected as the functional molecules. Among them, PFS, OTS, and −OH are modified on silicon, while HDT and MHA are modified on Au. Figure 3a−e shows water (3 μL) wettability on PFS-, OTS-, HDT-, MHA-, and −OH-modified Si-CAs in air, respectively. We found that the water wettabilities on PFS (CA = 152.0°), OTS (144.5°), and HDT-modified Si-CAs (143.3°) were hydrophobic, while MHA- (13.4°) and −OH-modified (superhydrophilic) Si-CAs were hydrophilic. Therefore, PFS, OTS, and HDT were selected as hydrophobic parts, while MHA and −OH were selected as hydrophilic parts. The modification method of our Janus Si-CAs is to modify one molecule on silicon and then modify another one on Au. Therefore, from the five molecules, there are three kinds of combination, which are PFS-MHA-, OTS-MHA-, and HDT-OH-modified Si-CAs. PFS−OH-, OTS−OH-, and HDT-MHA-modified Si-CAs could not be prepared here because there are no Janus structures. These are attributed to PFS, OTS, and −OH and are

Figure 3. Water wettability on (a) PFS-, (b) OTS-, (c) HDT-, (d) MHA-, and (e) −OH-modified Si-CAs in air. (f−i), (j−m), and (n−q) are time-lapse photographs of water wettability on Janus PFS-MHA, OTS-MHA, and HDT-OH Si-CAs in air, respectively. The red dotted lines represent left positions of the water droplets when they contact the surfaces. The red arrows indicate water moving direction. The MHA-, MHA-, and −OH-modified directions were all placed at the right side.

all modified on silicon, while HDT and MHA are both modified on Au. First, Janus PFS-MHA-modified Si-CAs were taken as an example. Si-CAs were modified by PFS and MHA asymmetrically (Figure 1). XPS characterization proves the Janus structure of the surface (Supporting Information, Figure S1). When water was deposited onto this Janus surface, as indicated by the red dotted line, we found that water only moved toward the MHA-modified direction. The three-phase contact line along hydrophobic (PFS-modified) side is pinned, as shown in time-lapse photographs of water (10 μL) wettability on the Janus PFS-MHA Si-CAs in air (Figure 3f−i). In addition, another two examples of Janus OTS-MHA- and HDT-OH-modified Si-CAs were fabricated. Water CAs were measured to prove the successful OTS and −OH modification onto the Si-CAs (Figure S2), respectively. After asymmetric modification, the two Janus surfaces were prepared. As water was gradually injected onto the Janus OTS-MHA (Figure 4j− m) and HDT-OH (Figure 4n−q) Si-CAs in air, it is clear that the droplets only moved toward MHA- and −OH-modified directions. It is known that liquid tends to wet on the surface with larger surface energy in air.39 To explain the above experimental phenomena in Figure 3, flat silicon substrate modified with C

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oleophilic) were oleophilic, while the PFS-modified one (98.0°) was oleophobic. In the case of oil wettability in air, OTS, HDT, MHA, and −OH could be selected as oleophilic parts, while PFS was selected as oleophobic parts. Thus PFS-HDT (XPS survey spectra are shown in Figure S3) and PFS-MHA were prepared to study the wetting behavior of oil in air. As oil was gradually injected onto the two surfaces, we found that the oil droplet shows unidirectional wetting on Janus PFS-HDT SiCAs toward the HDT-modified direction (Figure 4f−i), while it shows isotropic wetting on Janus PFS-MHA Si-CAs (Figure 4j−m) in air. The phenomena shown in Figure 4 could also be explained by surface energy. Liquids with lower surface tension (like oil) are easier to wet on the same surface than water in air.39 That is why MHA- and −OH-modified Si-CAs are both hydrophilic and oleophilic and HDT- and OTS-modified Si-CAs are hydrophobic but oleophilic in air. The smallest surface energy of PFS-modified Si-CAs among the five surfaces, which is difficult for liquid wetting, resulted in oleophobic state. In air, as for Janus PFS-MHA-modified Si-CAs, the upper surface of the Janus surface is MHA with large surface energy, which shows ultrafast wetting speed for oil. Moreover, the oleophobicity of PFS-modified Si-CAs is not strong due to oil CA on it is 98°. Therefore, when oil is injected onto this Janus surface, the PFSmodified area nearly plays no effect on the wetting behavior of oil, resulting in isotropic wetting of oil on it. For Janus PFSHDT Si-CAs, HDT-modified Si-CAs are oleophilic with a relatively slow wetting speed. When oil is injected onto the PFS-HDT-modified one, PFS-modified area will play the role of oleophobic part. Because of the larger surface energy of HDTmodified area than PFS-modified one, oil shows unidirectional wetting toward HDT-modified direction. When the ultrafast wetting area is not contacted with oil directly, we measured the wetting behavior of oil on Janus HDT-OH Si-CAs in air. The upper surface of the Janus surface was HDT, which was oleophilic with a relatively slow wetting speed. Another part was −OH-modified area with an ultrafast wetting speed for oil in air. It is obvious that oil presented unidirectional wettability on HDT-OH-modified Si-CAs in air toward the −OH-modified direction (Figure 4n−q). When oil is injected onto this Janus surface, even though HDT- and −OH-modified Si-CAs are both oleophilic surfaces in air, oil shows unidirectional wetting on it toward the −OH-modified direction due to the considerable surface energy of the latter. The results could also help researchers understand oil isotropic wetting on PFS-MHA-modified Si-CAs. Anisotropic Wetting of Oil under Water. To prepare surfaces that dominate oil anisotropic wetting under water, oleophilic and oleophobic surfaces under water are required, respectively. We measured oil (3 μL of 1,2-dichloroethane) wettability on PFS-, OTS-, HDT-, MHA-, and −OH-modified Si-CAs under water (Figure 5a−e). It is found that PFS (30.2°), OTS (8.4°), and HDT-modified Si-CAs (18.6°) were oleophilic, while MHA (153.6°) and −OH-modified Si-CAs (158.2°) were oleophobic under water. Therefore, PFS, OTS, and HDT were selected as oleophilic parts, while MHA and −OH were selected as oleophobic parts. Consequently, oil wettability on Janus PFS-MHA, OTS-MHA, and HDT-OH SiCAs under water was investigated, as shown in Figure 5f−i,j− m,n−q, respectively. We observed that when oil was injected onto the three surfaces, the droplets only moved toward PFS, OTS, and HDT-modified directions.

Figure 4. Oil wettability on (a) PFS-, (b) OTS-, (c) HDT-, (d) MHA-, and (e) −OH-modified Si-CAs in air. (f−i), (j−m), and (n−q) are time-lapse photographs of oil wettability on Janus PFS-HDT, PFSMHA, and HDT-OH Si-CAs in air, respectively. The red dotted lines represent the center axis of the syringe needle (j−m) and left positions of the oil droplets when they contact the surfaces (f−i) and (n−q). The red arrows indicate oil moving directions. The HDT-, MHA-, and −OH-modified directions were all placed at the right side.

MHA, HDT, OTS, PFS, and −OH was prepared, and we measured the surface energy of the modified surfaces through OCA20, which are 76.69, 23.32, 17.02, 12.77, and 120.22 mN/ m, respectively. For the water wettability in air, −OH- and MHA-modified Si-CAs are hydrophilic due to the larger surface energy, while HDT-, OTS-, and PFS-modified ones are hydrophobic. When water is injected onto the Janus hydrophobic−hydrophilic Si-CAs, the air−liquid−solid three-phase contact line will move from the hydrophobic part (PFS, OTS, or HDT) to the hydrophilic part (MHA, MHA, or −OH) at every silicon cylinder and pin along the hydrophobic moleculemodified direction. This is attributed to the larger surface energy of the hydrophilic area, which is more easily to be wetted by water in air, resulting in unidirectional wetting of water. Because of the excellent unidirectional wetting behavior of water in air, our Janus surfaces may be used for phase separation (oil/water separation).36 Anisotropic Wetting of Oil in Air. To prepare surfaces that drive oil anisotropic wetting in air, oleophilic and oleophobic surfaces in air are needed, respectively. Figure 4a−e shows oil (3 μL of 1,2-dichloroethane) wettability on PFS-, OTS-, HDT-, MHA-, and −OH-modified Si-CAs in air, respectively. We found that OTS- (39.0°), HDT- (36.1°), MHA- (superoleophilic), and −OH-modified Si-CAs (superD

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Figure 5. Oil wettability on (a) PFS-, (b) OTS-, (c) HDT-, (d) MHA-, and (e) −OH-modified Si-CAs under water. (f−i), (j−m), and (n−q) are time-lapse photographs of oil wettability on Janus PFSMHA, OTS-MHA, and HDT-OH Si-CAs under water, respectively. The red dotted lines represent right positions of the oil droplets when they contact the surfaces. The red arrows indicate oil moving direction. The MHA-, MHA-, and −OH-modified directions were all placed at the right side.

Figure 6. Water wettability on (a) PFS-, (b) OTS-, (c) HDT-, (d) MHA-, and (e) −OH-modified Si-CAs under oil. (f−i), (j−m), and (n−q) are time-lapse photographs of water wettability on Janus PFSMHA, OTS-MHA, and HDT-OH Si-CAs under oil, respectively. The red dotted lines represent left positions of the water droplets when they contact the surfaces. The red arrows indicate water moving direction. The MHA-, MHA-, and −OH-modified directions were all placed at the right side.

Similarly, from the viewpoint of surface energy, we could explain the phenomena in Figure 5. Because of the relatively large surface energy of MHA- and OH-modified Si-CAs, when we put these surfaces under water, water molecules will be trapped by the surfaces; then, the surfaces could repel the immiscible oil, resulting in oleophobic state. For PFS-, OTS-, and HDT-modified Si-CAs, when they are immersed under water, water molecules would not be captured by the surfaces, with remaining air pockets that can coalesce with the oil droplets easily. Therefore, they are oleophilic under water. The reason why oil droplets show unidirectional wetting on these three surfaces is that as oil is injected onto the three surfaces under water, oil droplets would move from oleophobic part (MHA, MHA, or −OH) to oleophilic part (PFS, OTS, or HDT) at every silicon cylinder, resulting in unidirectional wetting of oil. Anisotropic Wetting of Water under Oil. For preparing surfaces that modulate water anisotropic wetting under oil (hexadecane), hydrophilic and hydrophobic surfaces under oil media are necessary, respectively. Water (3 μL) wettability on PFS-, OTS-, HDT-, MHA-, and −OH-modified Si-CAs under oil were measured (Figure 6a−e), respectively. From Figure 6a−e, it is obvious that PFS- (160.7°), OTS- (161.1°), and

HDT-modified Si-CAs (160.5°) were hydrophobic, while MHA- (64.2°) and −OH-modified Si-CAs (17.8°) were hydrophilic under oil. Therefore, MHA and −OH were selected as hydrophilic parts, and PFS, OTS, and HDT were selected as hydrophobic parts. Time-lapse photographs of water wettability on Janus PFS-MHA, OTS-MHA, and HDT-OH SiCAs under oil are shown in Figure 6f−i,j−m,n−q, respectively. We saw water droplets only move toward MHA-, MHA-, and −OH-modified directions when injected onto the three surfaces. The wettability of smooth surfaces under liquids media could be evaluated by the equation derived from Young’s equation40 cos θ 3 =

γl1_g cos θ1 − γl 2_g cos θ 2 γl1_l 2

where θ1, θ2, and θ3 are the CAs of liquid 1 in air, liquid 2 in air, and liquid 1 in liquid 2, respectively. γl1_g, γl2_g, and γl1_l2 are the interface tensions of liquid 1/gas, liquid 2/gas, and liquid 1/liquid 2, respectively. We take water CA on −OHmodified Si-CAs under oil as an example. The interfacial tension of water/air (γl1_g), hexadecane/air (γl2_g), and water/hexadecane (γl1_l2) is 73, 27.5, and 53.3 mN/m,41 respectively. The CA of water (θ1) and hexadecane (θ2) on E

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Figure 7. Water and oil wettability on the five kinds of surfaces in air, water, and oil media, which contain the whole forms that are hydrophilic, hydrophobic, oleophilic, and oleophobic in air; oleophilic and oleophobic under water; and hydrophilic and hydrophobic under oil, respectively.

smooth silicon surface in air is 5.3 and 4.8°, respectively. Thus θ3 can be calculated from the equation that is 31.8°. It is known that the wettability can be enhanced greatly by increasing the roughness of the surface.42 In our experiment, the water CA on −OH-modified Si-CAs under oil is 17.8°(more hydrophilic than 31.8°), which is consistent with the theory. Obviously, no matter whether in water or oil media, γl1_g, γl2_g, and γl1_l2 are all fixed values. Therefore, from the equation, θ3 is determined by θ1 and θ2. Meanwhile, θ1 and θ2 are the liquid CAs in air, which are determined by the surface energy of the surface. Therefore, the wettability under liquids is also decided by the surface energy of the surfaces essentially. As water is gradually injected onto the three surfaces, the oil−water−solid three-phase line would move from the hydrophobic part (PFS, OTS, or HDT) to the hydrophilic part (MHA, MHA, or −OH) at every silicon cylinder, resulting in the unidirectional wetting of water. Preparation Strategy for Anisotropic Wetting Surfaces under Various Media. Figure 7 summarizes the water and oil wettability on the five kinds of surfaces in air, water, and oil media, which contain the whole forms that are hydrophilic, hydrophobic, oleophilic, and oleophobic in air; oleophilic and oleophobic under water; and hydrophilic and hydrophobic under oil, respectively. On the basis of Figure 7, anisotropic wetting surfaces under various media could be prepared. For example, if anisotropic wetting of water in air is expected, then the modified two types of molecules can be chosen in hydrophilic and hydrophobic parts in air (Figure 7), respectively, such as Janus MHA-PFS- or OH-HDT-modified Si-CAs. We assume that not only do the

molecules in Figure 7 satisfy the demand, but also other molecules that possess a similar property as molecules shown in Figure 7 are allowed. Therefore, a strategy for preparing anisotropic wetting surfaces under arbitrary media can be given. That is, through modifying two types of molecules asymmetrically, which possess distinct surface energy onto the Si-CAs, one of the molecules is easy to be wetted by the liquid under the corresponding media, while the other one is not. In addition, because of the existence of Au on one side of the SiCAs, the two sides Si−Au can be modified with various molecules, resulting in various anisotropic surfaces. Finally, once Si-CAs were prepared, they would be recycled through removing Au (by Au etchant), remodifying one type of molecule, redepositing Au, and remodifying another type of molecule. This also improves the efficiency of our anisotropic surfaces and decreases the preparation cost. Stimulus-Responsive Anisotropic Wetting Surface under Various Media. Smart surfaces, which exhibit stimuli-responsive changes in wetting properties, have drawn considerable attention owing to their promising applications in sensors,43,44 separators,45 and microfluidic devices.46 If one part of the Janus Si-CAs is modified with stimulus-responsive molecule, the surface may exhibit reversible switching anisotropic wetting property. With the purpose of preparing responsive anisotropic wetting surface under various media, thermal-responsive PNIPAM molecule was selected as the responsive part. It could switch between hydrophilic and hydrophobic when the system temperature is below or above the LCST (∼32 °C) through the transformation between interand intramolecular hydrogen bonding.47 Here 20 and 50 °C F

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speed for oil, when oil was gradually injected onto this surface, oil showed isotropic wettability on it whether at 20 or 50 °C.

were chosen as the temperature below and above the LCST, respectively. First, a layer of PNIPAM brush was first polymerized onto Si-CAs through SI-ATRP (the thickness was ∼32 nm, Figure S4). After modifying MHA selectively, Janus PNIPAM-MHA Si-CAs were prepared (Figure S5, XPS survey spectrum). When the system temperature was below the LCST of PNIPAM, whether water in air (Figure 8a), oil under

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CONCLUSIONS We present Janus nanostructures that control liquids anisotropic wetting under various media. The Janus nanostructures are prepared through modifying two types of molecules asymmetrically onto Si-CAs. The as-prepared Janus Si-CAs could control water unidirectional wetting in air or oil as well as oil unidirectional wetting in air or water media. We propose a strategy for preparing anisotropic wetting surfaces under arbitrary media (air, water, oil), which is to find two types of molecules; one of the molecules is easy to be wetted by the liquid under the corresponding media, while the other one is not. Essentially, the molecules with different surface energies modified on the Si-CAs asymmetrically cause the unidirectional wetting behavior of liquids. By introducing PNIPAM molecule onto one part of Janus Si-CAs, thermal-responsive Janus PNIPAM-MHA Si-CAs are prepared. Such surface switches between isotropic and unidirectional wetting property under various media when the temperature is below or above the LCST of PNIPAM molecule. Because of the excellent unidirectional property and intelligent switch of liquids wettability under various media, the as-prepared anisotropic wetting surfaces show a broad range of applications in future smart device systems and theoretical research.

Figure 8. Water and oil wettability on Janus PNIPAM-MHA Si-CAs. (a−d) Wettability of water in air, oil in air, oil under water, and water under oil at 20 °C, respectively. (e−h) Wettability of water in air, oil in air, oil under water, and water under oil at 50 °C, respectively. The red dotted lines represent center axis of the syringe needle (a−d,f), left (e,h), and right (g) positions of the droplets when they contact the surface. The red arrows indicate droplets moving directions. The MHA-modified direction was placed at the right side.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00034. Preparation of PNIPAM-modified Si-CAs through SIATRP; the X-ray photoelectron spectroscopy of the SiCAs, PFS-modified Si-CAs, Janus Au-PFS Si-CAs, Janus PFS-MHA Si-CAs, Janus PFS-HDT Si-CAs, PNIPAMmodified Si-CAs, Janus Au-PNIPAM Si-CAs, and Janus PNIPAM-MHA Si-CAs; water wettability on Si-CAs, OTS-modified Si-CAs, and OH-modified Si-CAs; the AFM image of PNIPAM brush polymerized onto a flat silicon substrate. (PDF)

water (Figure 8c), or water under oil (Figure 8d) exhibited isotropic wettability, and the droplets spread toward both the MHA- and PNIPAM-modified directions. We also measured the oil sliding angle on the Janus PNIPAM-MHA Si-CAs under water and shown in Figure S6. When 10 μL of oil droplets was dropped onto the Janus surface, oil droplets rolled immediately in random directions. The Janus surface under water was not strictly horizontal; in other words, the minimal inclined angle caused the rolling of the oil droplets. The high oil contact angle and low sliding angle make it suitable for self-cleaning surfaces. When raising the temperature above the LCST of PNIPAM, the as-prepared surface controlled water unidirectional wetting toward the MHA-modified direction in air or oil media (Figure 8e,h), while it controlled oil unidirectional wetting under water toward the PNIPAM-modified direction (Figure 8g). We found that whether at 20 or 50 °C, oil always showed isotropic wettability on PNIPAM-MHA-modified Si-CAs (Figure 8b,f). The surface energy of flat silicon substrate modified with PNIPAM was measured at 20 °C (70.57 mN/m) and 50 °C (47.78 mN/m), respectively. Therefore, when the temperature is below the LCST of PNIPAM, the surface energies of PNIPAM- and MHA-modified flat silicon surface (76.69 mN/ m) are roughly the same, and both sides of the Janus PNIPAMMHA Si-CAs are hydrophilic, leading to isotropic wetting property whether in air, water, or oil media. When the temperature is above the LCST of PNIPAM, the MHAmodified side remains hydrophilic, while the PNIPAMmodified area switches from hydrophilic to hydrophobic, resulting in unidirectional wetting of water in air or oil media toward the MHA-modified direction and unidirectional wetting of oil toward PNIPAM-modified direction under water. As for oil wettability in air, because the upper surface of the Janus surface is an MHA-modified area with an ultrafast wetting

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Junhu Zhang: 0000-0001-9100-6608 Bai Yang: 0000-0002-3873-075X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant no. 21474037), Doctoral Fund of Ministry of Education of China (20130061110019).

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DOI: 10.1021/acs.langmuir.7b00034 Langmuir XXXX, XXX, XXX−XXX