Multistimuli-Responsive Microstructured Superamphiphobic Surfaces

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Surfaces, Interfaces, and Applications

Multi-Stimuli Responsive Microstructured Superamphiphobic Surfaces with Large-Range, Reversible Switchable Wettability for Oil Hujun Wang, Zhihui Zhang, Zuankai Wang, Yunhong Liang, Zhenquan Cui, Jie Zhao, Xiujuan Li, and Luquan Ren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07941 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Multi-Stimuli Responsive Microstructured Superamphiphobic Surfaces with Large-Range, Reversible Switchable Wettability for Oil Hujun Wang,† Zhihui Zhang,*,†,§ Zuankai Wang,# Yunhong Liang,†,§ Zhenquan Cui,† Jie Zhao,† Xiujuan Li,† and Luquan Ren† †The

Key Laboratory of Bionic Engineering of Ministry of Education, Jilin University,

Changchun 130022, People’s Republic of China. §State

Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun

130022, People’s Republic of China. #Department

of Mechanical Engineering, City University of Hong Kong, Hong Kong 999077,

People’s Republic of China. *Corresponding author: Zhihui Zhang; E-mail: [email protected] KEYWORDS wetting switching, simple fabrication, multi-stimuli responsive surfaces, superamphiphobicity, transformable re-entrant microstructures

1 Environment ACS Paragon Plus

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ABSTRACT The switchable wettability is essential to widespread applications in droplet manipulation, rewritable liquid patterning, fluid carrying, etc. However, it remains difficult to achieve the multi-stimuli responsive, large-range, and reversible wetting switching especially for liquids with low surface tensions through surface topographical management. Here, we apply a simple and effective template-free self-assembly strategy to fabricate microstructured superamphiphobic surfaces that can reversibly switch the wetting performance for oil by transforming the surface morphology in response to multiple stimuli of magnetic fields and mechanical strains. Notably, the noticeably different wetting switching of oil triggered by different stimuli is demonstrated. The contact angles of hexadecane droplets on the as-prepared surfaces can be reversibly switched between 150 ± 1°and 38 ± 2° in response to mechanical strains. Furthermore, the underlying mechanism of wetting switching has been further elucidated using mathematical models. Interestingly, these switchable surfaces dramatically demonstrate the ability to transport oil droplets, without requiring lubricating liquid films. This work not only achieves the largerange and reversible wetting switching for oil but also opens new avenues for fabricating tunable superamphiphobic surfaces with transformable mushroom-like microstructures that can be easily extended to microstructure-dependent friction or adhesion control and used in other fields.


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1. INTRODUCTION Superamphiphobic surfaces that repel water, oil and other low-energy organic solvents have aroused considerable interest due to their widespread applications including droplet manipulation,1-4 anti-fouling,5,6 liquid patterning,7,8 self-cleaning,9,10 and so on.11-14 With a further understanding of the essentiality of re-entrant microstructures and low surface energy in designing superamphiphobic surfaces,15-19 various methods have been used to fabricate superamphiphobic surfaces with re-entrant microstructures, for example, lithographic processes,20,21 etching,22-24 electrospinning,25-27 thermal reaction,28 3D printing,29,30 and selfassembly.31 Despite the feasibility of these methods for fabricating excellent superamphiphobic surfaces, there are still some limitations that need to be overcome, such as template assistance, complicated fabrication processes, high costs, and special equipment. Therefore, the simple and effective fabrication strategy of superamphiphobic surfaces with re-entrant microstructures remains challenging. Over the past few years, switchable liquid-repellent surfaces that exhibit smart wetting performance are in strong demand with the development of intelligent technologies. Such surfaces usually display a wetting switching ability through the variation of microstructures and surface chemistry in response to external stimuli, such as magnetic fields,32-36 electric fields,37 light,38 pH,39 strain,40 and temperature.8,41 However, there have only been a few reports regarding switchable superamphiphobic surfaces on which the wetting switching for liquids with low surface tensions relies on the microstructure deformation.8,36 In addition, these switchable superamphiphobic surfaces with dynamically controllable microstructures are triggered by single stimulus, and simultaneously most of them involve relatively complex preparation processes, high costs and time-consuming control. The conflict between such requirements and the large-



range reversible switching ability consequently hinders the large-scale fabrication and widespread applications of the switchable superamphiphobic surfaces in complex practical conditions, for instance, the multi-stimuli responsive transportation of oil droplets. Thus, it is still a challenge to construct switchable superamphiphobic surfaces that can achieve the large-range and reversible switching of wettability especially for oil by transforming their surface morphology in response to multiple stimuli. Herein, we developed a template-free self-assembly strategy to manufacture mushroom-like pillar arrays (MLPAs) with re-entrant microstructures. Notably, although a magnetic particleassisted self-assembly method was used to fabricate conical pillar arrays in previous reports,35,42,43 it was very surprising for us that our self-assembly strategy enabled the formation of re-entrant microstructures with superoleophobicity after the deposition of a fluorinated silane. The wettability for oil droplets on the as-prepared switchable superamphiphobic surfaces could be switched in a large range by solely transforming the morphology of the surface microstructures. Unlike prior work36,44, we demonstrated the noticeably different wetting switching of oil triggered by different stimuli. By applying an appropriate magnetic field, the mushroom-like pillars were reversibly deformed, resulting in a wetting switching for oil. When the substrate of the switchable superamphiphobic surface was a soft material, the wetting switching for oil could be actualized in response to the mechanical strain. It was surprisingly found that the contact angles of hexadecane droplets on the superamphiphobic surfaces could be reversibly switched between 150 ± 1°and 38 ± 2°. Furthermore, we demonstrated oil transport using the switchable superamphiphobic surfaces without the help of lubricating liquid films. 2. RESULTS AND DISCUSSION


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2.1. Fabrication and Formation Mechanism of MLPAs. Figure 1a displays the scheme of the fabrication of the surfaces with MLPAs. A mixture of polydimethylsiloxane (PDMS) and carbonyl iron particles (CIPs) was deposited on a substrate via a blade coating method.35,42,43 The thickness of the coating was approximately 102 μm (Figure S1). For the formation of MLPAs with re-entrant microstructures and uniform height, another glass slide was fixed above the substrate, while a certain distance was maintained between the two glass slides. To prevent the destruction of MLPAs when the top glass slide was removed, a polytetrafluoroethylene (PTFE) film was affixed to the bottom surface of the top glass slide. By placing a magnet with proper magnetic field intensity below the entire assembly, the MLPAs with uniform height were produced along the direction of magnetic field. Figure 1b,c shows an as-prepared pillar array, indicating the successful fabrication of MLPAs through the efficient template-free self-assembly strategy. It was found that the geometrical dimensions of the MLPAs can be tuned by the weight ratio of PDMS to CIPs (Figure 1d and Figure S2). In this study, CIP contents were fixed at 60 wt%, 65 wt%, 70 wt%, 75 wt% and 80 wt% of the PDMS weight. Low CIP concentrations led to a lower primary pillar height, which was unfavorable for the formation of mushroom-like caps. When CIP concentration was 60 wt%, some of the pillars did not possess mushroom-like caps. Hence, the interpillar distance of the MLPAs was large, reaching 172 ± 50 μm. As the CIP concentration was increased from 65 wt% to 80 wt%, the interpillar distance varied from 127 ± 28 μm to 139 ± 32 μm with the diameter of pillar caps ranging from 27 ± 8 μm to 42 ± 14 μm, respectively. An explanation was proposed for the formation mechanism of the MLPAs, as shown in Figure 2. Induced by a magnetic field, the CIPs in the compound aligned and aggregated along the magnetic field direction.43 Due to the uniform dispersion of the CIPs in PDMS, PDMS also



gathered along the magnetic field direction, forming the primary pillar arrays. When the top of the pillars came into contact with the upper glass slide, the upward aggregation trend of PDMS was prevented. The PDMS subsequently carried to the upper glass slide by the CIPs would spread horizontally, which resulted in the formation of mushroom-like caps with re-entrant microstructures. The aggregating direction of CIPs embedded in the mushroom-like pillar further verified the formation mechanism (Figure 2). 2.2 Wettability of the Superamphiphobic Surfaces and its Mechanism. Superamphiphobic surfaces can be created by modifying the MLPAs with a fluorinated silane. Energy dispersive spectrometer (EDS) and x-ray photoelectron spectroscopy (XPS) were used to investigate the chemical compositions of the surfaces before and after being modified by 1H,1H,2H,2Hperfluorodecyltrichlorosilane. According to EDS spectra, the surfaces were mainly comprised of carbon (C), oxygen (O), silicon (Si), and iron (Fe), which was attributed to PDMS and CIPs (Figure S3a,b). The 0.27 wt% chlorine (Cl) of the surface after modification demonstrated the existence of 1H,1H,2H,2H-perfluorodecyltrichlorosilane. As shown in Figure S3c, the peaks at 284.7 eV, 532.4 eV, 101.7 eV, and 153.0 eV in the spectrum corresponded to C 1s, O 1s, Si 2p, and Si 2s. The new outstanding XPS peak at 690.3 eV in Figure S3d indicated the existence of F 1s, further illustrating the successful deposition of the fluorinated silane. To better explain the superamphiphobic state, the superamphiphobic surfaces were assumed to be composed of quadrangular arrays of mushroom-like pillars for analysis purposes (Figure S4). The mushroomlike pillars possessed conical bases capped with a horizontal circular plate having round chamfering. The superamphiphobic surfaces supported liquid droplets in the Cassie-Baxter state because the as-prepared surfaces can repel various liquids with large apparent contact angles and


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small sliding angles. According to the Cassie-Baxter model, the apparent contact angle θw can be expressed as (see Supporting Information) cos  w  f1 cos   f 2  f1 (cos   1)  1 

 R2 S2

(cos   1)  1

(1)

where f1 is the solid-liquid area fraction, f2 is the vapor-liquid area fraction, θ is the Young’s contact angle, R is the radius of the mushroom-like caps, and S is the center-to-center spacing between two pillars (interpillar spacing). It can be calculated from Equation (1) that lower R/S ratios correspond to lower solid-liquid area fractions f1, leading to a higher apparent contact angle θw. However, the Cassie-Baxter model is applicable only when MLPAs enable a liquid droplet suspension.17 For a static liquid droplet in contact with MLPAs, the force balance between the pressure force and surface tension is described in Figure S5 and Figure S6. If the pressure difference ΔP acting downward on the liquid-vapor interface is greater than the largest breakthrough pressure (Pbreakthrough) provided by the re-entrant microstructures, the liquid droplet will not be suspended on the MLPAs and will fall into the cavity, triggering a transition from the Cassie-Baxter state to the Wenzel state. Typically, the pressure difference ΔP of a static droplet can be estimated as ΔP = 2γ/Rdroplet, where γ is the liquid surface tension and Rdroplet is the radius of the droplet. For a liquid droplet with a high surface tension (Young’s contact angle θ > 90°), the largest breakthrough pressure (Pbreakthrough) provided by the textured surface composed of MLPAs is given as (see Supporting Information)16,17 Pbreakthrough 

2 R cos(   ) S 2   R2

(2)

If the Young’s contact angle θ < 90°, Pbreakthrough is defined as (see Supporting Information) Pbreakthrough 

2 R sin  S   R2

(3)

2



It is obvious from Equations (2) and (3) that a low R/S results in a low breakthrough pressure. Therefore, if the R/S ratios are too low, droplets cannot be suspended on the surface comprised of MLPAs, leading to the Wenzel state instead of the Cassie-Baxter state. As shown in Figure 1e, for the surface composed of MLPAs containing 60 wt% CIPs, the apparent contact angle of hexadecane (γ = 27.5 mN m−1, θ = 73°) was 109 ± 1°, indicating that the hexadecane was in the Wenzel state. This was attributed to the fact that Pbreakthrough was relatively small, and ΔP was closed to Pbreakthrough (Pbreakthrough = 71 Pa, ΔP = 44 Pa, R = 12.5 μm, S = 172 μm). In this work, the average diameters of the pillar caps and average interpillar spacing were used to estimate Pbreakthrough. Accordingly, even a slight environmental disturbance and low hydrostatic pressure can induce droplets to penetrate into the surface. As shown in Figure 1e, the theoretical breakthrough pressure of hexadecane and water was obtained according to Equations (2) and (3). Because of the increase of breakthrough pressure, the surfaces composed of 65 wt% to 80 wt% CIPs can support the hexadecane droplets to a Cassie-Baxter state. As the CIP concentration increased from 65 wt% to 80 wt%, the apparent contact angles of hexadecane (from 152 ± 2° to 148 ± 1°) and water (from 157 ± 2° to 155 ± 1°) decreased due to the increase of the value of R/S (Figure 1e). When the CIP concentration was 65 wt%, the apparent contact angle of hexadecane was the largest (θw = 152 ± 2°), and the apparent contact angle of water was 157 ± 2°. To further demonstrate the superamphiphobic property of the prepared surfaces, we systematically characterized the wettability of the surfaces containing 65 wt% CIPs. The substrate of the surface was a glass slide. Figure 3a shows the photos of an as-prepared sample before and after being immersed in water dyed with methylene blue. The sample was removed from the water and remained its original color without a trace of pollution by the dyed water, illustrating that the surface had excellent water-repellent properties (Video S1). As shown in


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Figure 3b, the as-prepared specimen was inserted into the rapeseed oil dyed red with Sudan III, and it was observed that the surface constituted by MLPAs was not contaminated by oil, indicating good oil repellency (Video S2). Figure 3c displays an image of spherical rapeseed oil, hexadecane and water droplets in “J”, “L” and “U” patterns, respectively, on the horizontal surfaces, indicating the omniphobic property of the as-prepared surfaces in air. Furthermore, the superamphiphobicity of a surface that repels various liquids with different surface tensions was evaluated by measuring apparent contact angles and sliding angles. The high apparent contact angles (> 150°), and the low sliding angles (< 10°) of the five liquids (hexadecane, rapeseed oil, ethylene glycol, water containing 5 vol% ethanol and distilled water) confirmed the superamphiphobicity of the as-prepared surfaces (Figure 3d). 2.3 Multi-Stimuli Responsive, Large-Range, and Reversible Wetting Switching for Oil and the Mechanism. The superamphiphobic surfaces consist of CIPs (a kind of magnetic particles) and flexible PDMS. Thus, the shape of the mushroom-like pillars was easily and reversibly transformed by applying an external magnetic field (Figure 4a). To clearly observe the morphology of the curved pillars, a permanent magnet (magnetic field intensity ≈ 0.5 T) which was placed below the as-prepared surfaces at a certain position was used to bend the mushroomlike pillars. The MLPAs were assembled on glass slides. Afterwards, the morphology of the curved pillars was fixed by fully solidifying the PDMS at 115 °C for 1.5 h. The SEM and 3D images show that the mushroom-like pillars can be reversibly deformed from an upright morphology to a curved morphology via altering the magnetic field direction (Figure 4b,c and Figure S7). The changes in the microstructures can affect the surface wettability which is simply represented by apparent contact angles and sliding angles.45,46 To demonstrate that the switchable superamphiphobic surfaces exhibited large-range and reversible wetting switching performance



for oil, related experiments were performed and the underlying mechanism was elucidated using mathematical models. Hexadecane, a typical oil with a low surface tension, was used to test the wetting switching ability of our surfaces in response to a magnetic field. When a hexadecane droplet was dropped on the switchable superamphiphobic surface with MLPAs prepared by 65 wt% CIPs, it was supported by both the MLPAs and air trapped underneath the liquid droplet in the Cassie-Baxter state. This was because ΔP < Pbreakthrough (Pbreakthrough = 132 Pa, ΔP = 44 Pa, R = 13.5 μm, and S = 132 μm) and the trapped air resisted the droplet from penetrating into the surface. As shown in Figure 4d and Video S3, the large apparent contact angle (θw = 152 ± 2°) which corresponded to the theoretical Cassie-Baxter model (θw = 163°) and the small sliding angle (α = 10 ± 1°) further affirmed that the droplet was suspended on the MLPAs in the CassieBaxter state.8 After applying a 0.5 T magnetic field, the erect MLPAs quickly bent to form a nearly flattened state (Figure S8a). In this condition, the re-entrant microstructures were invalid, and thereby the net traction acting downward on the composite solid-liquid-vapor interface promoted the wetting of hexadecane into the MLPAs, leading to a wetted Wenzel state (Figure S8b). When another droplet of hexadecane was dropped on the surface with curved MLPAs, it partially penetrated into the surfaces (θw = 107 ± 1°) and did not slip off the surface, which corroborated that the droplet was in the Wenzel state (Figure 4e). By changing the direction of the magnetic field, the curved MLPAs rapidly recovered from the flattened state to the initial upright state. When a hexadecane droplet was dropped on the recovered surface, it was in the Cassie-Baxter state with a high apparent contact angle (θw = 151 ± 1°) and a low sliding angle (α = 10 ± 1°). It was demonstrated that our superamphiphobic surfaces hold reversible wetting switching ability for oil in response to magnetic fields. The apparent contact angles and sliding angles after each switching cycle are shown in Figure 4f,g, respectively. It was observed that the


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transition performance was not weakened after 20 cycles, indicating a repeatable wetting switching ability for oil. In addition, the switchable superamphiphobic surface could retain its superoleophobicity even after 1000 cycles of switching between the upright and curved states of the MLPAs because the deformation of the MLPAs did not involve significant changes in the mushroom-like caps or intrinsic surface energy (Figure S9). The repeatable bending performance of the MLPAs composed of PDMS and CIPs was consistent with a previous report.35 The modified self-assembly strategy can be applied to fabricate MLPAs on substrates of soft materials. When MLPAs consisting of 70 wt% CIPs were assembled on a soft substrate, the switchable wettability of the obtained superamphiphobic surface for oil triggered by mechanical strains was investigated. Here, an elastic silicon rubber with a thickness of 1 mm was used as the stretchable soft substrate. Figure 5a,b displays the change in the contact angle and sliding angle with the stretching strain. As the superamphiphobic surface was gradually stretched, wetting switching would occur. When the stretching strain reached 20%, the hexadecane droplet sank between the microstructures, exhibiting the Wenzel state and a lower contact angle (θw = 66 ± 2°). In this state, the surface adhesion increased due to an increase in solid-liquid area fraction, leading to pinning of the droplet even if the sample was reversed (Figure 5b, inset). With the stretching of the substrate, the interpillar spacing increased (Figure S10). Therefore, the theoretical Pbreakthrough decreased, which resulted in the transition of the wetting states. It is worth noting that the droplet still adhered to the surface upon relaxation. Furthermore, even though the volume of the attached droplet was increased from 4 μL to 20 μL, it did not fall off from the relaxed surface, indicating strong adhesion (Figure S11). When another hexadecane droplet was dropped on the recovered surface, it adopted the Cassie-Baxter state due to the recovery of the interpillar spacing. The cyclic tests demonstrated repeatable wetting switching for oil (Figure



5c,d). Remarkably, the contact angles of hexadecane droplets on the switchable superamphiphobic surfaces can be reversibly switched between 150 ± 1° (straining = 0) and 38 ± 2° (straining = 100%). 2.4 Application of the Superamphiphobic Surfaces for Oil Transportation. Various superhydrophobic surfaces have been fabricated for controlled transport of water droplets.40,47 Nevertheless, the construction of superamphiphobic surfaces for oil transportation without using lubricating liquid films remains challenging. In this work, the transportation of oil droplets was realized on the tilted switchable superamphiphobic surfaces. A hexadecane droplet moved freely down the tilted surface with a tilted angle of ~15°, because the mushroom-like pillars far away from the magnet were not bent and the droplet was in the low-adhesion Cassie-Baxter state (Figure 6a and Video S4). The sliding hexadecane droplet could be stopped as it reached the region with curved mushroom-like pillars induced by an external magnetic field (magnetic field intensity ≈ 0.5 T) because it was in the Wenzel state (Figure 6b). In this state, the hexadecane droplet partially permeated into the gaps between curved pillars which provided the greater adhesion force, and thereby the droplet was firmly pinned. From the abovementioned results, it is believed that the fixed-point transportation of oil droplets can be achieved owing to the transformable microstructures and wetting switching performance of the as-prepared surfaces. One can transport oil droplets to the designated locations of the superamphiphobic surfaces by designing and optimizing the placement of magnets. By assembling MLPAs on a soft substrate, the obtained soft switchable superamphiphobic surface could be applied as a droplet tweezer to transfer tiny oil droplets. Figure 7 depicts the nearly lossless transfer of an oil droplet using soft switchable superamphiphobic surfaces. Two droplets were first placed on the as-prepared superamphiphobic surfaces. Subsequently, a soft


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stretched surface with stronger adhesion performance captured the left droplet from the surface underneath. The surface was relaxed which brought the droplet into contact with another droplet on the right superamphiphobic surface. Due to the mixing of the two droplets, the gravity of the newly generated droplet overcame the adhesion force and then the droplet fell off. Ultimately, nearly lossless transfer and mixing of liquid droplets was achieved. The entire process is shown in Video S5. The transfer efficiency was used to evaluate the transfer ability and was calculated by the equation η = (m1 - mw) / mo × 100%, where m1, mw, and mo are the mass of the mixed droplet, water droplet, and oil droplet, respectively. Notably, the transfer efficiency for ethtylene glycol was ~98%. During the process of droplet transfer, the captured droplet was not released from the relaxed soft surface until it was in contact with another droplet, which would contribute to the precise transfer, mixing and reaction of droplets. If the soft surface was stretched throughout the droplet transfer process, the transfer efficiency will be decreased to ~92%. This behavior can be explained by the higher adhesion force between the liquid droplet and the soft surface due to the large contact areas (Figure S12). In view of these results, we envision that our soft switchable superamphiphobic surfaces will have a wide application in manipulating the transfer and reaction of minute oil-soluble and water-soluble droplets. While only the wetting switching ability of our switchable superamphiphobic surfaces for oil was discussed in this work, such switchable surfaces with transformable MLPAs are expected to be used in microstructuredependent friction or adhesion control and other fields. 3. CONCLUSION In conclusion, switchable superamphiphobic surfaces with re-entrant microstructures were fabricated through an effective template-free self-assembly strategy. We demonstrated the noticeably different wetting switching of oil triggered by different stimuli. The mushroom-like



microstructures of the surface could be reversibly deformed in response to magnetic fields, and thereby the reversible wetting switching for oil was actualized. In addition to the magnetic field, the mechanical strain could also be used as a stimulus to drive the wettability transition when the substrate of the switchable superamphiphobic surface was a soft material. The contact angles of hexadecane could be switched dramatically between 150 ± 1°and 38 ± 2°. We have further elucidated the underlying mechanism of wetting switching using mathematical models. It was demonstrated that the switchable superamphiphobic surface has a potential application in transferring oil droplets without using lubricating liquid films. Further, we envision that the efficient template-free self-assembly strategy can inspire the fabrication of more switchable superamphiphobic surfaces with transformable mushroom-like microstructures using other materials, such as liquid rubbers and epoxy resins, promoting the development and application of microstructure-dependent smart superamphiphobic surfaces. 4. EXPERIMENTAL SECTION 4.1 Materials: The 200-μm-thick copper foils, 1-mm-thick silicone rubbers, and NdFeB permanent magnets were purchased from retail stores. The PDMS pre-polymer and curing agent (Sylgard 184) were obtained from Dow Corning. 1H,1H,2H,2H-Perfluorodecyltrichlorosilane was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd, China. CIPs with diameters of 2-8 μm were obtained from Jiangsu Tianyi Ultra-fine Metal Powder Co., Ltd, China. 4.2 Preparation of switchable superamphiphobic surfaces: The switchable superamphiphobic surface with MLPAs was fabricated via an effective magnetic microparticleassisted self-assembly process. First, PDMS pre-polymer containing 5 wt% curing agent and CIPs were adequately mixed in various weight ratios. After degassing in a vacuum oven for 20


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min, the mixture was coated on a pre-cleaned glass slide or commercial silicone rubber through a blade coating method. Then, two copper foils with a thickness of ~200 μm were fixed at either end of the substrate where the mixture was removed. A PTFE film was glued to another glass slide, followed by fixing the glass slide on the two copper foils placed on the substrate with proper force. The side of the glass slide with the PTFE film was faced down. Then, a permanent magnet (magnetic field intensity ≈ 0.4 T) was placed below the substrate, resulting in the formation of MLPAs along the direction of the magnetic field. The MLPAs were solidified at 65 °C for 4 h in a drying oven, followed by removal of the upper glass slide. Finally, the sample was modified using 1H,1H,2H,2H-perfluorodecyltrichlorosilane (120 μL) for 12 h under vacuum to obtain the switchable superamphiphobic surface. 4.3 Characterization: The surface morphology was characterized by SEM (Zeiss, EVO18) using an accelerating voltage (20 kV). The 3D morphology of the upright and curved MLPAs was measured using a laser scanning confocal microscope (Olympus, OLS3000). XPS (Thermo Fisher Scientific, Escalab 250Xi) and EDS (Oxford, X-MaxN 150) measurements were used to analyze elements of specimen. The average diameters of the pillar caps and average interpillar spacing were obtained using image analysis with Image-Pro Plus and Nano Measurer, respectively. The movement of the water and oil droplets (~13 μL) on the switchable superamphiphobic surfaces tilted at an angle of ~15° was recorded by a high-speed camera (Vision Research, Phantom V711) at 1000 frame s-1. With the help of an optical contact angle meter (Dataphysics, OCA20), the static contact angles and sliding angles measurements were carried our at ambient temperature. Droplets of various liquids (~8 μL) were carefully placed onto the surfaces, and the average static contact angle and sliding angle were obtained by



measuring five different positions on the same surface. The stretched as-prepared soft surface was glued to a flat plate, and then the static contact angle and sliding angle were investigated.

Figure 1. a) Schematic illustration of the fabrication of a MLPA with re-entrant microstructures. b,c) SEM images of the top view and side view of the as-prepared MLPAs, respectively. The


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inset displays the enlarged mushroom-like cap with a re-entrant microstructure. d) The relationships between the geometrical parameters of the mushroom-like pillars and CIP concentration. e) Apparent contact angles and theoretical breakthrough pressure of water and hexadecane on the superamphiphobic surfaces with different CIP concentrations, respectively.



Figure 2. Schematics showing the formation mechanism of a MLPA.


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Figure 3. Demonstration of the superamphiphobicity of the as-prepared surfaces. a) After immersing in methylene blue–dyed water, the surface remains its original color, exhibiting water-repellent properties. b) After immersing in rapeseed oil dyed red with Sudan III, the surface is not contaminated by the oil, demonstrating the oil repellency. c) Photos of various liquid droplets (rapeseed oil, hexadecane and water) on the horizontal surfaces with patterns like letters “J”, “L”, “U”. d) Apparent contact angle and sliding angle measurements of different liquids on the surfaces, illustrating the superamphiphobicity.



Figure 4. a) Schematic of the reversible shape transformation between the upright state and the nearly flattened state. b,c) SEM images of the mushroom-like pillars in the upright and curved states, respectively. d,e) Switchable wettability of the surfaces for hexadecane corresponding to b) and c), respectively. f,g) Reversible variations in the apparent contact angle and sliding angle of hexadecane through repeated bending and recovery processes, respectively.


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Figure 5. a,b) Apparent contact angles and sliding angles of of hexadecane on the as-prepared soft surfaces as a function of strain values, respectively. c,d) Reversible variations in apparent contact angles and sliding angles of hexadecane under repeated stretching (100%) and recovery processes, respectively.



Figure 6. Demonstration of the transportation of a hexadecane droplet on a tilted superamphiphobic surface. a) The hexadecane droplet slides freely on the surface region with upright MLPAs. b) The hexadecane droplet can be pinned on the surface region with curved MLPAs by placing a magnet underneath the surface. The tilted angle α is approximately 15°.


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Figure 7. Demonstration of the nearly lossless transfer of oil droplets (ethylene glycol, γ = 46.5 mN m−1). a) Schematic for the stretching and relaxation of a soft superamphiphobic surface. b) An oil droplet and a water droplet (dyed blue with methylene blue) are placed on two asprepared superamphiphobic surfaces, respectively. The oil droplet is c) captured, d) transferred, and e) released for mixing with the dyed water droplet.



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at ×××. Estimation of theoretical contact angles, estimation of theoretical breakthrough pressure, SEM image of the mixture coated on the substrate, SEM images of MLPAs, chemical compositions of surfaces, schematics of formation mechanism of mushroom-like pillar arrays, schematics of the assumed quadrilateral array of mushroom-like pillars, schematics showing the force balance, 3D confocal microscopy images of the pillars in the upright and curved states, schematics of curved pillars, the stability of a mushroom-like cap, optical images of the pillars before and after stretching, a droplet of hexadecane, an ethylene glycol droplet adhering to the relaxed surface. (PDF) Video S1. The as-prepared superamphiphobic surface is immersed into the dyed water and then taken out. (MP4) Video S2. The as-prepared superamphiphobic surface is inserted into the dyed rapeseed oil and then taken out. (MP4) Video S3. The hexadecane droplet (~8 μL) starts to slide down along the superamphiphobic surface when the surface is tilted from 0° to ~9°. (MP4) Video S4. The fixed-point transportation of a hexadecane droplet on the tilted superamphiphobic surface with a tilted angle of ~15°. (MP4) Video S5. An oil droplet is captured by a stretched surface and then transferred to another surface for the mixing with a dyed water droplet. (MP4) AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] Author Contributions H.J., Z.H., and Z.K. designed research; H.J. and Z.Q. performed research; Y.H., J., and X.J. analyzed data; H.J. wrote the paper; Z.H., Z.K., and L.Q. revised the paper. All authors commented on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. U1601203), Young and Middle-aged Science and Technology Innovation Team Project of Jilin Province (No. 20180519007JH), and Graduate Innovation Fund of Jilin University (No. 101832018C009).



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Table of Contents


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Figure 1. a) Schematic illustration of the fabrication of a MLPA with re-entrant microstructures. b,c) SEM images of the top view and side view of the as-prepared MLPAs, respectively. The inset displays the enlarged mushroom-like cap with a re-entrant microstructure. d) The relationships between the geometrical parameters of the mushroom-like pillars and CIP concentration. e) Apparent contact angles and theoretical breakthrough pressure of water and hexadecane on the superamphiphobic surfaces with different CIP concentrations, respectively. 139x179mm (300 x 300 DPI)

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Figure 2. Schematics showing the formation mechanism of a MLPA. 139x95mm (300 x 300 DPI)


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Figure 3. Demonstration of the superamphiphobicity of the as-prepared surfaces. a) After immersing in methylene blue–dyed water, the surface remains its original color, exhibiting water-repellent properties. b) After immersing in rapeseed oil dyed red with Sudan III, the surface is not contaminated by the oil, demonstrating the oil repellency. c) Photos of various liquid droplets (rapeseed oil, hexadecane and water) on the horizontal surfaces with patterns like letters “J”, “L”, “U”. d) Apparent contact angle and sliding angle measurements of different liquids on the surfaces, illustrating the superamphiphobicity. 139x112mm (300 x 300 DPI)



Figure 4. a) Schematic of the reversible shape transformation between the upright state and the nearly flattened state. b,c) SEM images of the mushroom-like pillars in the upright and curved states, respectively. d,e) Switchable wettability of the surfaces for hexadecane corresponding to b) and c), respectively. f,g) Reversible variations in the apparent contact angle and sliding angle of hexadecane through repeated bending and recovery processes, respectively. 139x160mm (300 x 300 DPI)


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Figure 5. a,b) Apparent contact angles and sliding angles of of hexadecane on the as-prepared soft surfaces as a function of strain values, respectively. c,d) Reversible variations in apparent contact angles and sliding angles of hexadecane under repeated stretching (100%) and recovery processes, respectively. 139x114mm (600 x 600 DPI)



Figure 6. Demonstration of the transportation of a hexadecane droplet on a tilted superamphiphobic surface. a) The hexadecane droplet slides freely on the surface region with upright MLPAs. b) The hexadecane droplet can be pinned on the surface region with curved MLPAs by placing a magnet underneath the surface. The tilted angle α is approximately 15°. 119x112mm (300 x 300 DPI)


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Figure 7. Demonstration of the nearly lossless transfer of oil droplets (ethylene glycol, γ = 46.5 mN m−1). a) Schematic for the stretching and relaxation of a soft superamphiphobic surface. b) An oil droplet and a water droplet (dyed blue with methylene blue) are placed on two as-prepared superamphiphobic surfaces, respectively. The oil droplet is c) captured, d) transferred, and e) released for mixing with the dyed water droplet. 139x112mm (300 x 300 DPI)


Multistimuli-Responsive Microstructured Superamphiphobic Surfaces

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