Nano Re-Entrant-Coordinated

Jul 22, 2019 - Here, we designed a superamphiphobic surface possessing a re-entrant .... and heat transfer materials with optical transparency desired...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Designing Transparent Micro/Nano Re-Entrant-Coordinated Superamphiphobic Surfaces with Ultralow Solid/Liquid Adhesion Xiaomei Li, Dehui Wang, Yao Tan, Jinlong Yang,* and Xu Deng* Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China

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ABSTRACT: Superamphiphobic surfaces, capable of repelling both water and oil, have been extensively studied recently. Artificial superamphiphobic surfaces with ultralow solid/liquid adhesion and high transparency have been achieved separately. However, simultaneous demonstration of these two features remains a challenge. Here, we designed a superamphiphobic surface possessing a re-entrant curvature on both nano- and microscales uniformly, each maintaining their capabilities. The achieved micro/ nano re-entrant-coordinated superamphiphobic surface performed ultralow solid/liquid adhesion for liquids with high viscosities or low surface tension and showed excellent transparency. This rationally designed model of the superamphiphobic surface may provide useful guidelines for fabrication of superamphiphobic surfaces and enable potential applications ranging from self-cleaning materials to optical devices, such as solar panels, wind screens, and goggles. KEYWORDS: superamphiphobic surface, micro re-entrant, nano re-entrant, ultralow adhesion, transparency

1. INTRODUCTION Superamphiphobic surfaces that can repel both water and oil have exhibited extensive values in scientific research and practical applications, such as in self-cleaning,1 chemical shielding,2 antifouling,3 energy-efficient fluid transport,4 and oil−water separation.5 Particularly, superamphiphobic surfaces with both low solid/liquid adhesion and high transparency can greatly extend their applications, such as in digital screens and sunglasses,6 solar panels, and optical devices.7 A number of strategies have been utilized to construct superamphiphobic surfaces. Except for methods that lower surface energy and increase roughness, nano or micro re-entrant curvatures have been employed to guarantee superamphiphobicity.8,9 Despite the advancement in manufacturing superamphiphobic surfaces, most of these surfaces remain opaque.2,10−20 If transparency is obtained, it is at the cost of increasing solid/liquid adhesion.21−28 Simultaneous demonstration of low solid/liquid adhesion and high transparency remains a major challenge. Therefore, it is desirable to design and construct a superamphiphobic surface possessing ultralow solid/liquid adhesion with high transparency. Superamphiphobic surfaces with low solid/liquid adhesion make droplets not only bead on the surface with a high contact angle (θs > 150°) but also roll off the surface with negligible contact angle hysteresis (Δθ < 10°). To generate a large contact angle, a small liquid/solid contact fraction ( fs) should be guaranteed. Currently, two strategies, namely, micro re-entrant structure and nano re-entrant structure, have been mainly employed in the design of superamphiphobic surfaces. For these two strategies, low solid/liquid adhesion could be achieved by © XXXX American Chemical Society

reducing the touching area for the micro re-entrant structure or the touching points for the nano re-entrant structure. However, the contact area cannot be reduced infinitely, as it is limited by the robustness of the Cassie state and durability of the surface.22,29−32 Recently, an alternative rational design of superamphiphobic surfaces, which was constructed by the hierarchical structure with the combination of both the nano reentrant structure and micro re-entrant structure, has been proposed.25,28,33 According to the Cassie−Baxter equation,34 the apparent contact angle of a surface with the single-level micro structure is θs = acrcos[fs (1 + cos θY ) − 1]

(1)

where θY is the intrinsic contact angle. Taking hexadecane (θY = 73° ± 3°) as an example, we examine the role of fs by plotting eq 1 in Figure S1. As is evident, the contact angle increases with the decline of fs. If nano re-entrant particles of liquid/solid contact fraction (fsn) were coated on the microstructure, eq 1 can be rewritten as θs = acrcos[fsn fs (1 + cos θY ) − 1]

(2)

Taking nanostructures with fsn = 0.05 as an example, it can be found in Figure S1 that the contact angle of hexadecane on the micro/nano hierarchical surface has been effectively increased compared to the single micro re-entrant structure. The small Received: May 26, 2019 Accepted: July 22, 2019 Published: July 22, 2019 A

DOI: 10.1021/acsami.9b08947 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

micro/nano re-entrant-coordinated superamphiphobic surface intrinsically achieves the cooperation of the nano re-entrant structure and micro re-entrant structure without the loss of each original role in liquid repellency and uniformity. Moreover, the micro/nano re-entrant-coordinated superamphiphobic surface possesses ultralow solid/liquid adhesion, excellent transparency, and mechanical robustness. These features were verified by measuring the static solid/liquid adhesion, dynamic liquids repellency, optical transmittance, tape-peel test, and sand abrasion test. We envision that this rational design strategy will be useful in self-cleaning, antifouling, and heat transfer materials with optical transparency desired.

liquid/solid contact area enables minute adhesion with the drop on the surface. Generally, optical absorption, reflection, and scattering of light are main reasons that lead to translucency or opacity of the surface.35,36 According to previous reports, lower roughness, porous character, and structural uniformity contribute to lower absorption, reflection, and scattering of light.22,28 Therefore, minimizing the size and concentration of nanoparticles or increasing the asperity spacing and aspect ratio of micro pillar arrays facilitate the transparency of the surface.7,25,37−40 However, these methods may lead to a low contact angle or high contact angle hysteresis. To design the superamphiphobic surface with high transparency, the hierarchical structure provides an alternative option because of its uniformity in microscale and reduced concentration in nanoscale.25,33 In this work, we proposed a model design of the hierarchical structure by combining both the nano re-entrant structure and micro re-entrant structure (Figure 1), namely, a micro/nano re-

2. RESULTS AND DISCUSSION 2.1. Superamphiphobic Surfaces. In our design, the micro/nano re-entrant-coordinated superamphiphobic surface was fabricated by combining the nano re-entrant structure and micro re-entrant structure (Figure 1). The preparation process is shown in Figure S2. The micropillar arrays (Figure S3) were first fabricated by photolithography and the fluorination process (Experimental Section). To generate the re-entrant structure and the connection media between the microstructure and the nanostructure on the pillars, we employed shape-controllable viscoelastic polydimethylsiloxane (PDMS) film and a modified dip-coating technique. In detail, a thin PDMS film was prepared on a cover glass by spin-coating. Then, the surface with micropillars was placed upside down on the PDMS film and pressed until a full contact was achieved. After the surface was peeled off and partially cured in a dryer at the temperature of 65 °C for 30 min, we achieved the sphere-like structure sticking on top of the pillar (Figures 2a, S4). This sphere-like structure serves as the re-entrant structure, which is similar to the structure reported in the literature.8,9 The nano re-entrant structure was prepared on a silicon wafer using candle soot templated silica nanoparticles (Experimental Section).7 The surface featured nano-size particles and the asperity spacing was around one micron (Figure 2b).41 Finally, we coated the top of the micro re-entrant structure with the nano re-entrant structure, using the same modified dip-coating technique aforementioned (Figure S2). To enhance the strength of the

Figure 1. Schematic illustration of the design of the micro/nano reentrant-coordinated superamphiphobic surface. The combination of the nano and micro re-entrant structure results in a smaller liquid/solid contact area than that found in the single nano re-entrant surface or the single micro re-entrant surface.

entrant-coordinated superamphiphobic surface. Comparing with the reported transparent hierarchical superamphiphobic surfaces fabricated by the spraying or etching technique,25,33 the

Figure 2. SEM images of the micro re-entrant, the nano re-entrant, and the micro/nano re-entrant-coordinated structure. (a) Micro re-entrant structure (diameter = 25 μm, spacing = 100 μm, height = 52 μm). (b) Nano re-entrant structure. (c) Micro/nano re-entrant-coordinated structure (diameter = 10 μm, spacing = 30 μm, height = 35 μm). B

DOI: 10.1021/acsami.9b08947 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

with high surface tensions in the literature.10,25,42 Conversely, when the spacing was increased, the apparent contact angle decreased (Figure 3a) and the contact angle hysteresis increased (Figure 3b) for the tetradecane droplet. The difference in the trend could be explained by the sagging deformation of the air/ liquid contact line (Figure S5). When the spacing was increased, the glycerol/air contact line basically remained straight because of the high surface tension of the liquid (Figure S5a). However, the tetradecane/air contact line was deformed by the low surface tension effect with increasing spacing, leading to an increase in the tetradecane/solid contact area (Figures 3c and S5b). As a result, droplets on the surface with an increase in the spacing showed a decreased contact angle and increased contact angle hysteresis for tetradecane. Thus, the results reported herein were obtained from the optimized spacing of 30 μm. We further investigated the enhancement of the liquidrepellency property of the micro/nano re-entrant-coordinated superamphiphobic surface compared to the single nano reentrant superamphiphobic surface and the single micro reentrant superamphiphobic surface by measuring θs and Δθ. Water and tetradecane were selected as the test liquids, representing high surface tension liquid and low surface tension liquid, respectively. The apparent contact angle of a water droplet on the three surfaces was all above 150° (Figure 3d). For the micro/nano re-entrant-coordinated superamphiphobic surface, the apparent contact angle even reached 164°. The apparent contact angles of the tetradecane droplet on the nano re-entrant superamphiphobic surface and the micro/nano reentrant-coordinated superamphiphobic surface still remained about 160°. While on the micro re-entrant superamphiphobic surface, the apparent contact angle was obviously lower than those on the other two surfaces. Δθ (Figure 3e) measured on the surface with the nano re-entrant structure for both water and tetradecane droplets indicated a small hysteresis, that is, Δθ < 10°. On the other hand, the Δθ of water and tetradecane droplets on the single micro re-entrant superamphiphobic surface were above 10° and Δθ even reached to 20° for tetradecane droplets. This indicated strong adhesion on the micro re-entrant superamphiphobic surface. Compared with the single nano re-entrant superamphiphobic surface, the contact angle hysteresis of tetradecane droplets on the micro/nano reentrant-coordinated superamphiphobic surface was found to be lower. This indicated that the gas area introduced in our designed surface could further reduce oil adhesion on the surface. Accordingly, we focus on the comparison between the nano re-entrant superamphiphobic surface and the micro/nano re-entrant-coordinated superamphiphobic surface in the following discussion. 2.3. Ultralow Solid/Liquid Adhesion. To verify the adhesion-reduction effect on the micro/nanore-entrant-coordinated superamphiphobic surface, we measured the solid/liquid adhesion force (Fa) by constructing a customized cantilever force sensor (details see Figure S6a−c) and measured the velocities (V) of fast liquid droplets shedding on the surface. We first measured Fa in response to the asperity spacing of the micro/nano re-entrant-coordinated superamphiphobic surface for the viscous droplet (glycerol) and the low surface tension droplet (tetradecane), as shown in Figure S6d. Fa versus b curve for the glycerol droplet and the tetradecane droplet exhibited an opposite trend. Fa increased for the glycerol droplet while it decreased for the tetradecane droplet with the enlargement of the spacing. This was in accordance with the variation of θs and Δθ.

structure and reduce the surface energy of the surface, the sample was cured (65 °C, 5 h) and then fluorinated. It can be found in Figure 2c that all the top surface of the micro re-entrant structure were fully and uniformly covered by the nano reentrant structure without loss of porosity and its original reentrant geometry. 2.2. Wetting Properties. We first optimized the asperity spacing (b) of the micro/nanore-entrant-coordinated superamphiphobic surface by investigating the variation of the apparent contact angle (θs) and contact angle hysteresis (Δθ) as a function of the spacing. Herein, the asperity spacing was defined as the center-to-center distance of the pillars (Figure S2). Glycerol and tetradecane were used as representatives of high-viscosity liquids and low surface tension liquids. For a glycerol droplet, within the designed spacing, the apparent contact angle increased (Figure 3a), while the contact angle hysteresis lowered (Figure 3b) with the enlargement of spacing. These tendencies are in line with the results reported for liquids

Figure 3. Wettability of the surface. (a) Apparent contact angle and (b) contact angle hysteresis of glycerol droplets and tetradecane droplets on the micro/nano re-entrant-coordinated superamphiphobic surface with different spacings. (c) Three-dimensional confocal microscopy image and its X−Z cross section of a tetradecane droplet (4 μL) dyed with Nile red dye on the micro/nano re-entrant-coordinated superamphiphobic surface (d = 10 μm, b = 50 μm, h = 50 μm). Fluorescent emissions from tetradecane, PDMS-soot hemisphere, and SU-8 pillars are shown in green, red, and blue, respectively. The reflections of the tetradecane/air and substrate/air contact line are shown in yellow. (d) Apparent contact angle and (e) Contact angle hysteresis of water droplets and tetradecane droplets on the micro re-entrant superamphiphobic surface, the nano re-entrant superamphiphobic surface, and the micro/nano re-entrant coordinated superamphiphobic surface. C

DOI: 10.1021/acsami.9b08947 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

compared to the nano re-entrant superamphiphobic surface, the combination of the nano re-entrant structure with the micro reentrant structure showed an improved adhesion-reduction effect for the liquids with low surface tensions. In addition, all the droplets underwent repetitive rebounding without failure for several cycles until completely jumping off the surface (Figure 4c, Video S1). This indicated the robust superamphiphobicity of the designed surface. We further measured the impact of the viscosity of the liquids on solid/liquid adhesion and moving velocity using water− glycerol mixtures with various mass ratios. The properties of the tested liquids are listed in Table S2. The adhesion force results showed a minute difference between the nano re-entrant superamphiphobic surface and micro/nano re-entrant-coordinated superamphiphobic surface when the viscosity was below 250 mPa·s (Figure 4e). A slight increase of adhesion force was found for the drop on the nano re-entrant superamphiphobic surface when the viscosity exceeded 250 mPa·s. Interestingly, we found that the droplets exhibited three different motion states on the surface with the increase in viscosity, namely, complete rebounding, partial bounding, and the sliding process (Figures 4d, S7 and Video S2). In the complete rebounding range (0.89− 11 mPa·s), the velocities of the moving droplets on both surfaces were almost identical (Figure 4f). However, during the partial rebounding or sliding phase (viscosities above 11 mPa·s), the velocity of droplets on the micro/nano re-entrant-coordinated superamphiphobic surface gradually surpassed the corresponding velocity on the nano re-entrant superamphiphobic surface. The variation trend of velocity is consistent with the trend of adhesion force. Hence, we can conclude that the micro/nano reentrant-coordinated superamphiphobic surface showed an improved adhesion-reduction effect for liquids with high viscosities. In addition, by changing the impacting dynamics of the droplet (Figure S8a) and the tilted angles of the surface (Figure S8b), these trends still occurred. This indicated that the reduction of solid/liquid adhesion on the micro/nano reentrant-coordinated superamphiphobic surface compared with that on the nano re-entrant superamphiphobic surface was universal for viscous droplets under different dynamic wetting conditions. The adhesion-reduction effect on the micro/nano re-entrantcoordinated superamphiphobic surface could be attributed to the reduced energy dissipation of the droplet.43,44 The energy dissipated (Ed) by a droplet moving on the surface at a velocity of v could be calculated by

The impact of surface tension on solid/liquid adhesion and the moving velocity of droplets was investigated using test fluids with surface tensions ranging from 72 to 26.5 mN/m. To avoid the influence of viscosity on the results, the viscosities of the selected liquids were in the range of 1−3 mPa·s (Table S1). From Figure 4a, Fa increased with the reduction of surface

Figure 4. Ultralow solid/liquid adhesion property. (a) Adhesion force of droplets with various surface tensions moving on the nano re-entrant superamphiphobic surface and micro/nano re-entrant-coordinated superamphiphobic surface. (b) Velocity of droplets with various surface tensions shedding on the nano re-entrant superamphiphobic surface and micro/nano re-entrant coordinated superamphiphobic surface. (c) Time-lapsed images of droplets with different surface tensions collision on the micro/nano re-entrant-coordinated superamphiphobic surface. (d) Time-lapsed images of droplets with different viscosities collision on the micro/nano re-entrant-coordinated superamphiphobic surface. (e) Adhesion force of droplets with different viscosities moving on the nano re-entrant superamphiphobic surface and the micro/nano reentrant-coordinated superamphiphobic surface. (f) Velocity of the droplets with various viscosities shedding on the nano re-entrant superamphiphobic surface and micro/nano re-entrant-coordinated superamphiphobic surface. In figure (b−f), the release height of droplets was 10 mm, the length and the tilted angle of the surface were 35 mm and 15°, respectively.

Ed = γwv(cos θr − cos θa)

(3)

Herein, θr is the apparent receding angle, θa is the apparent advancing angle, and w is the width of the apparent contact area between the moving droplet and the surface. For a droplet with a certain scale moving at a constant velocity, the lager (cos θr − cos θa) resulted in more energy dissipation. From the wetting properties shown in Tables S1 and S2, we plotted the ratio of energy dissipation between the nano re-entrant superamphiphobic surface and the micro/nano re-entrant-coordinated superamphiphobic surface (Figure S9). It can be found that the energy dissipated on the nano re-entrant superamphiphobic surface was always larger than that on the micro/nano reentrant-coordinated superamphiphobic surface for the whole measured spectrum of surface tensions and viscosities. The elevated energy dissipated in the droplet could contribute to the increase in adhesion measured on the surface. By comparing the structure of the micro/nano re-entrant-coordinated super-

tension for both superamphiphobic surfaces, followed by a sharp growth when the surface tension was below 30 mN/m. Particularly, the adhesion force for tetradecane and hexadecane droplets measured on the nano re-entrant superamphiphobic surface was nearly 1.5 times larger than the corresponding adhesion force measured on the micro/nano re-entrantcoordinated superamphiphobic surface. The moving velocities of droplets on both surfaces have a similar trend in variation as that of adhesion (Figure 4b). The results indicated that, D

DOI: 10.1021/acsami.9b08947 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

opaque2,10−20 or translucent,21−23 or transparent with intensive pinning effect.24−27 Specifically, the lowest contact angle hysteresis of our designed superamphiphobic surface was measured to be 3°, which was two times lower than that on the only reported surface with transmittance higher than 80% without the pinning effect.28 Higher transmittance and lower contact angle hysteresis of the micro/nano re-entrantcoordinated superamphiphobic surface could be attributed to the reduction of the solid area and the improvement of structural uniformity. 2.5. Mechanical Robustness. To examine the robustness of the transparent superamphiphobic surface, we performed a tape-peel test (Figure 6a, Video S3) and sand-abrasion test (Figure 6c, Video S4).

amphiphobic surface and the nano re-entrant superamphiphobic surface, it can be found that the introduction of the microstructure largely decreased the solid liquid contact area. Because total energy dissipation can be considered as the accumulation of the depinning energy along the contact circumference, the reduction of the solid fraction in the micro/nano re-entrant-coordinated structure facilitated small energy dissipation and therefore reduced adhesion. 2.4. Transparency. We first confirmed the improved optical transparency of the designed micro/nano re-entrant-coordinated superamphiphobic surface by putting a piece of paper with black check letters underneath the surface (Figure 5a). The

Figure 5. Optical transparency of the surface. (a) Photographs of droplets deposited on the nano re-entrant superamphiphobic surface and the micro/nano re-entrant-coordinated superamphiphobic surface. Pink, yellow, red, and white droplets are dyed water (γlv = 72 mN/m) droplet, glycerol (γlv = 63 mN/m) droplet, hexadecane (γlv = 27.5 mN/ m) droplet, and tetradecane (γlv = 26.5 mN/m) droplet, respectively. (b) Ultraviolet−visible transmittance spectra of a 35 μm-thick nano reentrant and micro/nano re-entrant-coordinated coating on a 170 μmthick glass compared to pristine glass. (c) Comparison of contact angle hysteresis of hexadecane (θY = 76°) as a function of transmittance (wavelength = 400 nm) among reported superamphiphobic surfaces (details see Figure S1). The red hollow star represented our designed surface.

Figure 6. Mechanical robustness of micro/nano re-entrant-coordinated superamphiphobic surface. (a) Schematic of the tape-peel tests performed using Elcometer 99 tape. The tape was applied uniformly by a 2 kg copper billet. (b) Effect of tape-peel cycles on the water repellency of the micro/nano re-entrant-coordinated superamphiphobic surface. (c) Schematic representation of a sand-abrasion setup. (d) Bright field reflection optical image of the sample after 10 cycles of sandabrasion tests. Labels “I” and “II” indicated two types of area abraded by sea sands. The details of area I and area II are shown in Figure S13.

words under the micro/nano re-entrant-coordinated superamphiphobic surface showed a clearer image than that under the nano re-entrant superamphiphobic surface. In addition, the superamphiphobicity of the surface was indicated by the high contact angles of droplets for the four liquids. The transparency of the micro/nano re-entrant-coordinated superamphiphobic surface was further confirmed by placing the sample surface 10 cm above a piece of paper with black check letters (Figure S10). The good optical transparency could be attributed to the high transmittance of light in the visible light spectrum. We verified this by ultraviolet−visible light transmittance spectroscopy (Figure 5b). Both nano re-entrant superamphiphobic surface and micro/nano re-entrant-coordinated superamphiphobic surface transmitted more than 80% of transmittance for wavelengths higher than 500 nm. Particularly, the introduction of the micro re-entrant structure to the nano re-entrant structure resulted in 15% improvement in transmittance. To illustrate the superior performances of our designed surface, we benchmarked the contact angle hysteresis of hexadecane droplets and transmittance (wavelength of 400 nm) against those on reported superamphiphobic surfaces (Figures 5c and S11), most of the reported surfaces were either

Repetitive tape (Elcometer 99 tape) application and peel-off cycles were used to assess coating degradation. The results showed that ten peel-off cycles only caused a slight drop in θs, from 158° to ∼155°, and a slight increase in Δθ, from 3° to 5° (Figure 6b). Moreover, the surface maintained excellent oil repellent properties even after 50 tape peel-off cycles. Hexadecane drops (10 μL) can easily roll off the tested area at the inclination angle of less than 12° after 50 tape peel-off cycles (Video S3). The maintained oil repellent properties were also confirmed by the coating morphology (Figures S12a,b), which was virtually little changed. It was found that less than 12% of the coating was removed onto the Elcometer 99 tape after 50 tape peel-off cycles (Figure S12c). We attribute the above abrasion-resistant oil repellent properties to the small adhesion of the structures and the micro/nano re-entrant-coordinated strategy. With a moderate abrasion test (e.g., 10 times of tape peel-off cycles), the small sample-tape touching area and low surface energy showed no surface degradation or adhesive left behind on the surface E

DOI: 10.1021/acsami.9b08947 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

the soot particles using tetraethoxysilane (Sigma-Aldrich, 98%) and an aqueous ammonia solution (TCI, 28%) via CVD at ambient temperature for 24 h. Transparency was achieved by calcining the surface at 600 °C for 2 h in air. To reduce the surface energy, the surface was treated by oxygen plasma (25% oxygen and 75% argon) for 10 min and then fluorinated using trichloro(1H,1H,2H,2H-perfluorooctyl) silane via CVD. Since silicon wafer has high smoothness and low adhesion with nano re-entrant structure, we selected silicon wafer as the substrate to transfer the soot-templated nano re-entrant structure onto the micro re-entrant structure. 4.1.3. Fabrication of the Micro Re-Entrant Superamphiphobic Surface. PDMS, (the mixture of Sylgard 184 elastomer kit and curing agent) was used to create the micro re-entrant structure. A thin film of PDMS was first coated on a cover glass by spin-coating. The film was then transferred onto the top of the fluorinated pillars by the aforementioned modified dip-coating technique. Micro re-entrant coating was generated via curing the sample in a dryer at 65 °C for 5 h. To reduce the surface energy, the surface was activated by oxygen plasma (25% oxygen and 75% argon) for 10 min and then fluorinated by trichloro(1H,1H,2H,2H-perfluorooctyl) silane via CVD for 2 h at room temperature. 4.1.4. Fabrication of the Micro/Nano Re-Entrant-Coordinated Superamphiphobic Surface. Before transfer, the micro re-entrant coating was partially cured (65 °C for 30 min) to enhance the connection between the nano re-entrant structure and the micro reentrant surface. The nano re-entrant structure was transferred onto the coating in the same way as transferring the PDMS layer. Subsequently, the surface was put into a dryer at 65 °C for 5 h again for complete curing. The surface was then activated by oxygen plasma (25% oxygen and 75% argon) and fluorinated to reduce surface energy. 4.2. Microscopy. The morphologies of the samples were characterized using a scanning electron microscope (JEOL, JSM7500F). The samples with the micro structure were scanned with a tilted angle, including micro pillar surface, micro re-entrant superamphiphobic surface, and micro/nano re-entrant-coordinated superamphiphobic surface. The nano re-entrant superamphiphobic surface was scanned when the sample was horizontally positioned. 4.3. Wetting Properties. Apparent contact angles (4 μL droplets) and contact angle hysteresis (10 μL droplets) were measured by DataPhysics OCA 35 goniometer (DataPhysics Instruments) using 3− 5 individual measurements at different positions on each sample. The apparent contact angles were measured by carefully depositing the droplets on the samples. The contact angle hysteresis was measured by tilting the stage gently until the droplet rolled off from the surface. Each experiment was conducted at least three times. Results were shown by using mean ± standard deviation. The 3D confocal images and its X−Z cross section were recorded by an inverted laser scanning confocal microscope (Nikon, A1R) with 20X/0.85 dry objective. The tested tetradecane droplet was dyed with Nile red and the PDMS layer was labeled by home-made PDI-conjugated uPDMS2 (preparation method in ref 46). In addition, the air/liquid and air/substrate contact line were recorded in the reflection mode. 4.4. Adhesion Force Measurement. Adhesion force was measured using a cantilever-based force sensor, which is a micro needle with a diameter of 0.09 mm. During testing, a 4 μL liquid droplet was deposited on the testing surface with the force sensor stuck in the droplet. The surface underneath the droplet was moved at a constant velocity (v = 0.43 mm/s), which was controlled by a Micro Drive Stepper Motor Stage (ZOLIX, MAR100-90, China). During the movement of the surface, the droplet was pinned to the sensor and was dragged over the surface. The whole process was recorded by using a high-speed camera (Photron, Fastcam SA5, 5000 fps). The adhesion force was calculated from the elastic deflection of the sensor at the point when the drop moves at a constant velocity on the surface (Figure S6e,f). Each measurement was conducted at least three times. Results were shown using mean ± standard deviation. 4.5. Rolling/Sliding Velocity Measurement. The droplet with a diameter of 2.2 mm was released above a tilted surface with a length of 35 mm. The shedding process was recorded by a high-speed camera (Photron, Fastcam SA5, 5000 fps). During the process, droplets with

(Figure S12a). The surface sustained excellent oil repellent properties. When intensive abrasion was applied on the surface (e.g., 50 times of tape peel-off cycles), the nano re-entrant particles on the top of the pillars could be partially removed by adhesive. However, the micro re-entrant structure was preserved in its entirety (Figure S12b). Though a decrease in the contact angle and an increase in the contact angle hysteresis were found after degradation by abrasion, the overall morphology of the micro/nano re-entrant-coordinated structure and therefore the superamphiphobicity was mostly sustained. We also employed the sand-abrasion test to investigate the mechanical resistance of the micro/nano re-entrant-coordinated superamphiphobic surface. Sand particles 300−800 μm in diameter impinged the surface from a height of 30 cm, corresponding to an impinging energy of 6.75 × 10−7 to 38.4 × 10−7 J per particle (Figure 6c). It was found that the surface retained superamphiphobicity after 10 sand-abrasion tests (Video S4). An optical microscope image of the tested sample showed that the majority of the structure remained unbroken after 10 sand-abrasion tests (Figure 6d). Two types of damage were identified after the test. A small portion of the PMDS hemisphere was partially deformed and a small number of the nano re-entrant particles on the top of the structure were removed (type I in Figures 6d and S13a). There also exist several micro re-entrant pillars inclined or collapsed (type II in Figures 6d and S13b). Despite the scarce cracked structure, the surface sustained in its entirety and therefore its superamphiphobicity.

3. CONCLUSIONS In summary, we have designed a model superamphiphobic surface by combining the nano re-entrant structure with the micro re-entrant structure. By introducing aggregated nano particles onto the micro re-entrant structure, we achieved increased surface roughness on local structures at nanoscale and highly ordered structure at microscale. The obtained micro/ nano re-entrant-coordinated superamphiphobic surface possesses ultralow solid/liquid adhesion, excellent transparency, and mechanical robustness. The micro/nano re-entrantcoordinated superamphiphobic surface holds great promise for advancing the design and fabrication of superamphiphobic surfaces with ultralow solid/liquid adhesion, high transparency, and mechanical robustness. 4. EXPERIMENTAL SECTION 4.1. Fabrication Procedures. 4.1.1. Fabrication of Micropillar Arrays. Micropillar arrays were fabricated from negative photoresist (SU-8 2025, Microchem, USA) by standard photolithography. Detailed description of the procedure can be found elsewhere.45 Briefly, the negative photoresist was spin-coated onto the slides. The sample was then prebaked (65 °C for 3 min, 95 °C for 5 min, slowly cooling down to room temperature in 12 h) and followed by UV illumination (5 mW/ cm2, 35 s) through a chromium mask. Postbaking (65 °C for 3 min, 95 °C for 5 min, slowly cooling down to room temperature in 2 h) and developer immersion for 3 min were performed. Finally, the noncrosslinked areas were washed off by 2-isopropanol and the sample was hardbaked (150 °C for 30 min and slowly cooling down to room temperature in 12 h). To reduce the surface energy of the surface, the arrays were fluorinated by trichloro(1H,1H,2H,2H-perfluorooctyl) silane (Sigma-Aldrich, 97%) via chemical vapor deposition (CVD). 4.1.2. Fabrication of the Nano Re-Entrant Superamphiphobic Surface. Nano re-entrant superamphiphobic surface was made from silica-coated candle soot followed by fluorination as previously reported.7 Briefly, candle soot was deposited on a silicon wafer (for transfer) or cover glass (for measurement) by holding the subtract above the flame of a paraffin candle. Then the silica shell was coated on F

DOI: 10.1021/acsami.9b08947 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces *E-mail: [email protected] (X.D.).

different surface tensions and viscosities were released at different heights. The tilted angles of the surface were controlled in the range from 5° to 20°. Each measurement was conducted at least three times. Results were shown using mean ± standard deviation. 4.6. Light Transmittance. The ultraviolet−visible transmittance spectra were characterized using PerkinElmer Lambda 35. The photographs of color droplets (pink: water; red: hexadecane; yellow: glycerol; and white: tetradecane) on the surface were captured by using a digital SLR camera (NIKON). 4.7. Tape-Peel Test. A strong bonding tape (Elcometer 99 tape) was used to test the robustness of the coating. A 2 kg copper billet was applied on the tape for 5 s and then peeled off. The application and being peeled off of the tape comprised one cycle. A fresh piece of tape was used for each peel-off cycle. The process was repeated with the contact angle and the contact angle hysteresis measurements at every ten cycles. The removed area was calculated by analyzing the surface every ten cycles (Video S3). Each experiment was conducted at least three times. Results were shown using mean ± standard deviation. 4.8. Sand-Abrasion Test. The surface of 10 mm × 40 mm in area was impinged with sea sands of 300 to 800 μm in diameter from a height of 30 cm. The total mass of sand in each experiment was 30 g. The tested samples were mounted normally to the falling sand and the test was repeated 10 times.

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ORCID

Xu Deng: 0000-0002-9659-0417 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 (21603026) and Sichuan Science and Technology Program (2018RZ0115) and supported by MaxPlanck-Gesellschaft (Max Plank Partner Group UESTC-MPIP). J.Y. acknowledges the China Postdoctoral Science Foundation (2019M653368).

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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/acsami.9b08947. Relation between the apparent contact angle and solid fraction for the ideal Cassie-state droplet; the preparation processes; supplementary scanning electron microscopy (SEM) images of micropillar structures; supplementary SEM images of micro re-entrant structures; X−Z confocal cross section of the air/liquid and air/substrate contact line; effective spring constant measurement and adhesion as a function of spacing; partial rebounding process of 70% glycerol−water mixture droplets on the surfaces; sliding of glycerol droplets under different conditions; ratio of energy dissipated by the liquid droplet moving on the nano re-entrant superamphiphobic surface and the micro/nano re-entrant-coordinated superamphiphobic surface; optical transparency of the surface; comparison of contact angle hysteresis as a function of transmittance among the reported superamphiphobic surface; damage after the tape-peel test; two types of damage after the sand-abrasion test; properties of tested liquids with different surface tensions; and properties of tested liquids with different viscosities (PDF) Ethyl salicylate droplet and tetradecane droplet impact on the micro/nano re-entrant-coordinated superamphiphobic surface (AVI) Water droplet and glycerol droplet impact on the micro/ nano re-entrant-coordinated superamphiphobic surface (AVI) Tape peel test of micro/nano re-entrant-coordinated superamphiphobic surface and the wetting behavior after every 10 abrasion tests (AVI) Sand abrasion experiment of micro/nano re-entrantcoordinated superamphiphobic surface and the wetting behavior after 10 abrasion tests (AVI)

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REFERENCES

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Corresponding Authors

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