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Patterned slippery surface through dynamically controlling surface structures for droplet microarray Wei-Pin Huang, Xiachao Chen, Mi Hu, Deng-Feng Hu, Jing Wang, He-Yang Li, Ke-Feng Ren, and Jian Ji Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03957 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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Chemistry of Materials

Patterned slippery surface through dynamically controlling surface structures for droplet microarray Wei-Pin Huang,† Xiachao Chen,† Mi Hu, Deng-Feng Hu, Jing Wang, He-Yang Li, Ke-Feng Ren,* and Jian Ji MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, China

ABSTRACT: Patterned slippery surface based on the technique of slippery liquid-infused porous surface (SLIPS) would have broad technological implications in such as biofouling, liquid-handling, biomolecules collection, and droplet microarray. However, methods to generate patterned SLIPS with flexibility and practical potential, remains a technical roadblock. Here we report an approach for generating patterned SLIPS, and by utilizing which, as a proof of technique application, the droplet microarrays have been presented. Our approach is based on a photo-crosslinkable polyelectrolyte poly(ethyleneimine)/azide-poly(acrylic acid) (PEI/PAA-N3) film whose surface structures can be dynamically controlled. We have prepared the surface patterned (PEI/PAA-N3) film containing microstructured and flat regions. We then demonstrate that, since much higher lubricant retaining capability of the microstructured regions, a patterned SLIPS can be simply generated through infusing lubricant onto the surface and applying a shear spinning process. This work highlights the importance of the dynamics of surface structures to the functional materials and showing broad potential applications.

INTRODUCTION A surface that repels various liquids shows broad interests in both basic science and industrial applications.1, 2 Such surfaces can be utilized for fluid handling,3-5 antifouling,6-8 self-cleaning,1, 6 antimicrobial,9 anti-icing,10-14 and drag reduction.15-18 Bioinspired from Nepenthes pitcher plant, Aizenberg and co-workers have designed and introduced a type of liquid-repel material surface called slippery liquidinfused porous surfaces (SLIPS).19, 20 In contrast to the classical superhydrophobic nano/microstructured coatings, which depend on an air-liquid interface between liquid and materials,10, 21-23 the SLIPS work relying on a thin liquid lubricant that pre-infused into the underlying substrate, namely a fluid/fluid interface between the lubricant and the liquid. The SLIPS present a state-of-the-art outstanding liquid-repellency capability.19 The SLIPS has been recently further developed to control and confine the liquid spatiotemporally, which mainly rely on the formation of the regional SLIPS,3, 4, 24 such as the patterned SLIPS. Although many kinds of uniform SLIPS have been previously reported, very few methods have been reported to create patterned SLIPS.25-29 Through manipulating the competitive affinity of a material surface with lubricant and liquid, Aizenberg et al. have firstly reported a patternable SLIPS by combining colloidal templating and conventional photolithography.25 The similar energetically favorable mechanism was also employed by Lee et al. to fabricate an omniphilic/omniphobic patterns for the micro-omnifluidic device.3 In another strategy, Yabu et al. created a hybrid

patterned SLIPS by combining the honeycomb-like porous structures and the pincushion-like structures, by which the restricted movement of the liquid droplet was achieved.28 Despite remarkable progress, these reported methods to create SLIPS patterns are complicated, and both surface chemistry and morphology need precisely processed.25-29 There remains a great need to explore new approaches to generate patterned SLIPS. The most important prerequisite of the SLIPS is a stable lubricant layer on the surface, and the microstructures of the underlying surface play an essential role in locking and retaining the lubricant through capillary force.30, 31 As compared the nano-/micro-featured surface to the flat one, Aizenberg et al. have demonstrated that the former had a significantly higher shear-tolerant capability, which means that the lubricant on a flat surface could be easily cast off at the shear action, and thus losing its liquid-repellency capability.25 We, therefore, envision that a lubricant-infused surface with microtextured and flat regions combining a simple shear-action could generate a patterned surface of liquid-repellency, namely a patterned SLIPS. We have previously developed a polyelectrolyte multilayer film through the layer-by-layer (LbL) assembly of poly(ethyleneimine) (PEI) and poly(acrylic acid) (PAA).32 The (PEI/PAA) film is very interesting in its capability of dynamics, such as self-healing,33, 34 and structural switching between solid and microporous states, 35, 36 in response to some mild triggering.37 More recently, we have reported that, during the assembly of the (PEI/PAA) film, the surface self-roughening was observed as showing the nano- and

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micro-featured wrinkle structures.38 Furthermore, such structures can be perfectly erased by exposing to 100% relative humidity. It was found that increasing water molecules into the film would weaken the ion pairs to impact the mobility of polyelectrolyte chains.38 Therefore, by controlling humidity environment, the film can be switched from glassy to liquid-like states. By utilizing such water plasticization, we could readily fabricate various patterned surfaces composed of structured and flat regions. Here we report on the design of a patterned SLIPS by regionally controlling the surface microstructure, which was further employed to fabricate the droplet microarrays as a proof of concept application. We synthesized a photoreactive polyanion by grafting phenyl azido groups onto PAA. Cationic PEI was LbL assembled with the azide PAA (PAA-N3) to fabricate a film with the structured rough surface. The (PEI/PAA-N3) film can be cross-linked upon UV irradiation due to the photo-reaction between phenyl azido groups and polyelectrolyte backbones (Scheme 1A). Then, a film with patterned cross-linked regions can be obtained by UV irradiation under a designed photo-mask (Scheme 1B). By utilizing the water plasticization, the microstructured regions of the film can be flattened at 100% relative humidity (RH) at room temperature; while, the cross-linked regions will retain its microstructured feature. After infusing lubricant and applying a subsequent shear action, a patterned SLIPS can be readily fabricated. Microstructured regions here have the much stronger capability to retain the lubricant than flat regions because of capillary force, leading to the regions with high slippery, low contact angle hysteresis, and weak liquid pinning force. On the basis of our patterned SLIPS, a simple approach for fabricating droplet microarrays was proposed (Scheme 1C). As further examples, the model fluorescence dye and photothermal nanoparticle microarrays were demonstrated. Scheme 1. Schematics of the patterned SLIPS. (A) Patterned photo-reaction between phenyl azido groups and polyelectrolytes under UV irradiation. (B) Fabrication of the patterned SLIPS by the (PEI/PAA-N3) film. (C) Creation of droplet microarrays by liquid sliding.

EXPERIMENTAL SECTION Materials. Poly(acrylic acid) (PAA, Mw 100,000), branched poly(ethyleneimine) (PEI, Mw 25,000), and 4azidoaniline hydrochloride were purchased from SigmaAldrich (Shanghai, China). 1-Ethyl-3-(3-(dimethylamino) propyl) carbodiimide hydrochloride (EDC), Nhydroxysulfosuccinimide sodium salt (NHS), 1H, 1H, 2H, 2H-perfluorodecyltrichlrosilane, and gold nanoparticles (carboxylic acid functionalized) were obtained from Aladdin (Shanghai, China). Methyl perfluorobutyl ether (HFE 7100), ethylene glycol, dopamine hydrochloride, and octane were purchased from J&K (Shanghai, China). Carboxylic acid-terminated perfluoropolyether (Krytox 157 FSL), lubricant (Krytox GPL 100) were obtained from DuPont (Shanghai, China). Rhodamine B was from TCI (Japan). Deionized water used in all experiments was from a Milli-Q water purification system (Millipore, Billerica, America). The pH of PEI and PAA aqueous solutions was adjusted by 1.0 M HCl or 1.0 M NaOH as needed. Synthesis of PAA-N3. The photo-reactive PAA-N3 was prepared by the amidation between carboxyl groups of PAA and amino groups of 4-azidoaniline hydrochloride. There were four steps: 1)500 mg PAA, 71.08 mg 4-azidoaniline hydrochloride, 239.61 mg EDC, and 479.23 mg NHS were dissolved in 50 mL deionized water. The pH of the solution was adjusted to 7.0 by adding 1M NaOH. 2) There was a sustained reaction for 24h at 4℃. 3) After that, the reaction mixture was dialyzed in deionized water for 72h. 4) The final solution was freeze-dried to get the end product. Synthesis of Rhodamine B labeled Krytox GPL100. The Rhodamine B labeled lubricant was prepared as previously reported.39 Briefly, 200 μL gold nanoparticles (100 µg/mL gold atoms, 20 nm diameter) and 200 μL Rhodamine B (0.5mg/mL) were mixed for 12 h. The solution (100 μL) was then added with 500 μL carboxyl terminated Krytox 157 FSL (diluted 1:5 v/v in HFE 7100) and shaking vigorously until phase separation developed and a pink color appeared in the oil phase. Ultimately, the oil phase was diluted in Krytox GPL100 (1:10 v/v) to get Rhodamine B labeled Krytox GPL100. Fabrication of Photo-Reactive PEI/PAA-N3 Film. The (PEI/PAA-N3) film was built by immersing the substrate in the PEI solution (1 mg/mL, pH 9.0) for 15 min firstly, followed by rinsing with deionized water three times and blow-dried by nitrogen (N2). The substrate was then immersed in the PAA-N3 solution (3 mg/mL, pH 3.5) for 15min, followed by the same rinsing and blow-drying procedures. These steps were repeated until the number of bilayers required was obtained. The film will be referred to as (PEI/PAA-N3)n, where n is the number of the bilayer. Fabrication of Patterned Film. The regional photocrosslinking of the (PEI/PAA-N3) films covered by photomask was initiated under UV irradiation (365 nm, 1165 µW/cm², UV lamp, Spectroline EN-180L) for 60s at room temperature. The (PEI/PAA-N3) film was then exposed to a 100% RH environment for 24 h to erase the surface structures of the non-crosslinked regions, while the structures of the crosslinked regions were preserved.


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Chemistry of Materials Basing on the above procedures, a patterned surface with both structured and flat regions was obtained. Fabrication of Patterned SLIPS. The patterned (PEI/PAA-N3) film surfaces were modified with 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane by the chemical vapor deposition (CVD) to obtain a low-surface-energy surface. In brief, the film was placed in a 100 mL sealed glassware which contained 100 μL 1H, 1H, 2H, 2Hperfluorodecyltrichlorosilane. The glassware was then heated into 70℃ for 2.5 h. After CVD, Krytox GPL 100 was added onto the modified patterned surfaces until complete infusion was achieved. The surfaces were then spun at 9000 rpm for a different time to selectively remove the lubricant from the flat regions. Creation of Droplet Microarrays. Droplet microarrays prepared on the patterned SLIPS. Liquids (ethylene glycol, water, and octane) were dragged over the patterned SLIPS tardily using a pipette or slid over the slanted surfaces spontaneously to get the droplet microarrays. For two examples, the fluorescent and the temperature microarrays were prepared by using the ethylene glycol Rhodamine B solution (100 μg/mL) and the aqueous polydopamine (PDA) nanoparticles solution (2 mg/mL), respectively. The PDA nanoparticles were prepared as previously reported.40, 41 Briefly, 1.5 mL ammonium hydroxide, 25 mL ethyl alcohol, and 45 mL deionized water were mixed and stirred at room temperature for 30 min. Then, the dopamine solution (5mL, 50 mg/mL) was added, and stirring (800 rpm) for 24 h. The produced PDA nanoparticles were collected by repeatedly water washing until the water was neutral. Characteristics. The chemical structure of PAA-N3 was analyzed by nuclear magnetic resonance hydrogen spectrum (1H-NMR, DMX-500, Bruker, Switzerland). Stylus profiler (DekTak-XT, Bruker, Germany) was used to follow the thickness changes in films. The (PEI/PAA-N3) films were measured by using an ultraviolet-visible (UV-Vis) spectrophotometer (UV-2550, Shimadzu, Japan). Atomic force microscope (AFM, Bruker Multimode 8, Germany) and scanning electron microscopy (SEM, Hitachi S4800, Japan) were performed to explore the surface morphology of films. X-ray photoelectron spectrometer (XPS, PHI 5000 Versaprobe I, Thermo Fisher Scientific Inc., America) was utilized to reflect the surface chemistry. For measurement of surface wettability, 20 μL test liquids (ethylene glycol, water, and octane) were set onto films in front of the camera of a drop shape analyzer (Kruss DSA 100, Germany). The film was tilted slowly until the droplet began to slide. The droplet was then recorded, and advancing and receding contact angle were measured.42 Fluorescent images and fluorescent intensity analysis were performed by a fluorescent microscope (Zeiss Axiovert 200M, Germany) with fluorescence labeled lubricant. The PDA nanoparticles were exposed to a laser (808 nm, 0.9 W/cm-2, LSR808H-7W, Lasever Inc., China). Thermal image was obtained from an imaging device (FLIR E60, Flir System. Int., America). RESULTS AND DISCUSSION We have previously reported that the (PEI/PAA) film can spontaneously form the nano/microstructures on the surface during the film’s assembly.38 Further, these

structures can be perfectly erased to a flat surface at a 100% RH treatment because of the improved polyelectrolyte’s mobility through the water plasticization. In this study, for regional controlling mobility of the polyelectrolytes during the humidity treatment, we synthesized the photo-reactive PAA-N3 (Scheme 1A). As shown in Figure 1A, the absorption peaks of 6.8-7.8 ppm appeared in 1H-NMR spectrum, standing for the success of the grafting reaction,43 and the calculated grafting ratio of PAA-N3 was 5.3%. We then followed the LbL assembly of the PAA-N3 and PEI by thickness characterization. We found that the (PEI/PAA-N3) film was successfully fabricated. The thickness of films increased slowly in the initial 5 bilayers, and then it got faster showing a rapid linear growth (Figure 1B). Such a rapid growth is generally considered to be due to the polyelectrolyte diffusion ‘‘in’’ and “out’’ within the whole film during the assemblies.44-46 We measured the thickness of the 16-bilayer-film was 3194.67±99.35 nm, which is in the similar level as compared to the (PEI/PAA) film,47 indicating that the modification of PAA with phenyl azido groups did not influence its LbL assembly. We further verified that the as-prepared (PEI/PAA-N3) film also showed abundant nano/micro-scale structures on its surface (Figure S1); while showing different shapes compared to what we observed in the (PEI/PAA) film.38 The surface structures of the (PEI/PAA) film showed a wrinkling shape, and the (PEI/PAA-N3) film showed a kind of shriveled bubbles, especially for those at more than eight bilayers. For follow-up experiments, we selected the (PEI/PAA-N3)10 film as the research object.

Figure 1. (A) 1H-NMR spectrum and chemical structure of PAAN3. (B) The (PEI/PAA-N3) films’ thickness increases with the number of bilayers.

To test the photo-crosslinking of the (PEI/PAA-N3)10 film, we exposed the film to 365 nm UV irradiation for 60s, and measured their changes about absorbance by UV-vis spectrum. We found that the (PEI/PAA-N3)10 film showed a strong absorption peak at 269 nm because of the π-π conjugation of the phenyl azide groups.48, 49 The peak was dropped off significantly and had a slight blue-shift after the UV irradiation (Figure 2A), which would be ascribed to the reaction between azide group and polyelectrolyte backbone, which weakened the π-π conjugation.48 Additionally, the slight increases of absorbance at 240 nm and 360 nm gave two isosbestic points (Figure 2A).43 The appearance of the film became slight yellow after the UV irradiation (Figure S2). To further explore the photo-crosslinking behavior, we treated the film (with and without UV irradiation) with 100% RH. As shown in Figure 2B-C, the microstructures on the native film were fully erased after the treatment, leading to a light transmission from nontransparent to transparent



(Figure S2A). The absorbed water molecules in the film increased the free volume of polymer chains and the mobility of the polyelectrolytes, leading to the liquid-like self-healing of (PEI/PAA-N3)10 film.36 In contrast, the microstructures on the UV irradiated (PEI/PAA-N3)10 film were mainly retained after the 100% RH treatment for even 12 h (Figure 2D-E, Figure S2B). Additionally, we measured the changes of the detailed surface structure by AFM at higher resolution (Figure S3). Taken together, these data indicated that the (PEI/PAA-N3) film can be crosslinked by UV irradiation, which thereby facilitates our controlling of film’s surface microstructures.

Figure 2. UV-vis spectrum of the (PEI/PAA-N3)10 films before and after photo-crosslinking (A). Top-down SEM images of the (PEI/PAA-N3)10 films (uncrosslinked (B and C) and crosslinked (D and E)) before and after 100% RH treatment (12 h).

We next tested if the (PEI/PAA-N3) film with or without surface microstructures can lock lubricant differently at certain shear-action. The films were fluorinated and then infused with Krytox GPL100. After that, the films were spun at 9000 rpm for various time. For directly observing the lubricant, the Rhodamine B labeled Krytox GPL100 was used. We verified that the microstructured surface can retain the lubricant at utmost, only 33% of fluorescence intensity decrease observed even spinning for 180s (Figure 3A,C); while, 96% of fluorescence intensity decrease was observed in the case of the flat surface (Figure 3B,C), which means that almost all infused lubricant was cast off in a short spinning time (Figure 3C). It was previously reported that a surface with microstructures and chemical

silanization can give large surface area and strong affinity to the lubricant, respectively.3, 19, 30 Therefore, the microstructured surface with the large surface area and capillary force can retain much more lubricant than the flat one under the shear action. However, it also needs to be determined that whether surface chemistry plays a role in effective lubricant retention. To test this, four silanized film surfaces (flat and microstructured surface with and without crosslinking) have been measured by XPS (Figure S4). We found that the crosslinking step did influence the silanization by increasing silanization density (Table 1). There were ~20% higher Fluorine density on the crosslinked films than non-crosslinked ones (both flat and microstructured surface), which may be ascribed to the difference in mobility of polyelectrolyte chains. To test whether such chemistry difference in silanization would influence lubricant retention, we used fluorescence-labeled lubricant again (Table 1). We found that, before spinning, the infused lubricant was at a same level in all four surfaces. After 180 s spinning, there was a significant difference between flat and microstructured surface: the microstructured surface showed the capability to retain lubricant. While, it worth to note that there was no obvious difference in lubricant retention between crosslinked and non-crosslinked surfaces (both flat and microstructured surface), which suggested that the difference in silanization didn’t influence the lubricant retention. We further investigated whether the lubricant-infused surfaces performed spinning action could influence the surface wettability. We selected ethylene glycol, water, and octane as model liquids. Before spinning action, we first tested the difference in contact angle of the surfaces with or without lubricant infusion. We found that, whether the flat or microstructured surfaces, when the lubricant was infused, the contact angles increased significantly; while, there were no obvious differences between the flat and the microstructured ones (Figure S5). We further tested sliding angles of these surfaces after spinning action. We found that the sliding angles of each of the three liquids were increased (Figure S6A-C). The longer the spinning lasted, the higher the sliding angles on both surfaces would be. However, the increasing levels of sliding angles on the flat surface showed significantly higher than that on the microstructured surface. After 90 s shear-action, the lubricant-infused microstructured surface still showed a typical liquidrepellent property with lower than ~10º sliding angle for all three liquids; while for the corresponding flat one, the sliding angles were much higher (Figure 3D), indicating the higher liquid-adhesive property. In addition, the contact angle hysteresis and pinning force data were calculated, and they showed a similar phenomenon (Figure S6D-J).50-52 Taken together, we verified our hypothesis that through controlling film’s surface morphology can directly control lubricant content on the surface, and then achieve modulating surface liquid-repellent property.


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Figure 4. Photograph of triangle arrays (A) and top-down SEM images of the pattern in low (B) and high (C) resolution.

Figure 3. Fluorescent micrography of the microstructured surface (A) and the flat surface (B) which were infused with 200 μL Rhodamine B labeled Krytox GPL 100 and spun for 90 s. (C) Fluorescence intensities of the lubricant-infused microstructured and flat films before and after shear-action for various spinning time. (D) Siding angles of the test liquids (octane, ethylene glycol, and water) after shear-action in the (PEI/PAA-N3)10 film with microstructured and flat surfaces after infusing with 200 μL lubricant and spun for 90 s. Table 1. Fluorine content, fluorescence intensity before and after spinning of different (PEI/PAA-N3)10 film surfaces. Fluorine content (%)

Fluorescence intensity (×104 a.u., no spinning)

Fluorescence intensity (×104 a.u., 180 s spinining)

Flat surface (non-crosslinked)

55.7

3.58±0.22

0.17±0.03

Flat surface (crosslinked)

76.9

3.56±0.35

0.29±0.12

Microstructured surface (noncrosslinked)

57.2

3.73±0.04

2.73±0.17

Microstructured surface (crosslinked)

77.0

3.54±0.07

2.81±0.18

Sample

The photomasks with designed patterns were employed to obtain the selectively crosslinked regions on the (PEI/PAA-N3)10 films. Figure 4A shows the film with triangle shape arrays on a silicon wafer. The opaque region was the film with microstructured surface feature because of light scattering. The black regions were the surfaces without the UV irradiation and thus uncrosslinked.53 These regions were, therefore, flattened during the 100% RH treatment (Figure 4B,C), leading to light transparency. The black was the color of the underlying silicon wafer. Similarly, by employing different photomasks, the films with circular and square shape arrays were fabricated (Figure S7).

By utilizing the difference of the liquid-repellency between microstructured and flat surfaces, we tested if these patterned (PEI/PAA-N3)10 film could be used to generate patterned SLIPS. We treated the films sequentially with fluorinating, lubricant infusing, and 90 s shear spinning. After those, a liquid droplet of ethylene glycol had been dropped onto the films’ surface at 30º tile angle. It can be observed that the droplet was rapidly sliding over the surface just like on a typical SLIPS; when the droplet slid over the flat regions, a small liquid droplet was left on the surface (Figure 5A-D), which would be ascribed to the higher liquid-adhesive property of the flat regions. We then measured and calculated pinning forces of the patterned SLIPS (Figure S8B,C).51 The pinning force of ethylene glycol on the patterned SLIPS was 126±3 μN, which is higher than on the microstructured surface and lower than that on the flat surface. Additionally, when the liquid slid over the flat regions, the liquid would be impeded within limits. Threephase lines existed in the boundary of flat and rough regions,54-57 which could develop greater resistance for the liquid. The line led droplet to develop larger internal stress and made it more possible to be divided in the boundary. Consequently, a uniform droplet microarray was fabricated (Figure 5E). By using different patterned SLIPS with square, triangle, or circular, we can easily obtain various ethylene glycol droplet microarrays (Figure 5F-H). It is noted that, by using water, the droplet microarrays can also be successfully prepared (Figure S9); while, we failed to obtain octane droplet microarrays. This may be because, in the case of octane, the difference of pinning force (~20 μN) between the flat and microstructured regions was too small to capture the liquid (Figure S6I). By using different patterned SLIPS with square, triangle, or circular, we can easily obtain various droplet microarrays (Figure 5F-H). It is also conceivable that the patterns could be diversified shapes and sizes by simply controlling the photomask. Such a flexible approach to producing droplet microarrays could be applied to highthroughput research, such as the influencing of droplet shapes and sizes for cell culturing.



We have established an approach for generating patterned SLIPS based on the dynamically controlling of the film surface microstructure. Firstly, the photo-crosslinkable (PEI/PAA-N3) film was fabricated through LbL assembly. After that, through the regional UV irradiation and the humidity treatment, the patterned film with microstructured and flat regions has been achieved. A patterned SLIPS was then prepared by the lubricant infusing and consequent shear spinning. We further presented a way to prepare the droplet microarrays with such patterned SLIPS. By designing photomasks and selecting liquids, droplet microarrays with diversified shapes and functional molecules could be achieved, showing broad potential applications in such as high throughput synthesis, analysis, and diagnosis.

ASSOCIATED CONTENT Figure 5. (A)-(D) The process of an ethylene glycol droplet developing on the patterned SLIPS. The white arrow pointed to was a flat region. (E) Lateral view of circular ethylene glycol droplets microarrays. (F)-(H) Photographs of different shapes’ (square, triangle and circular) ethylene glycol droplet microarrays. Scale bars are 2 mm.

Droplet microarray could serve as a highly efficient research platform for its one-step operation,58 high density of samples,59 and miniaturized screening for highthroughput analysis.60-63 In recent years, it has attracted increased attention in the fields of chemistry, materials, and biology. We verified that our approach is simple but practical to prepare droplet microarrays. Furthermore, we prepared two kinds of droplet microarrays by using the Rhodamine B ethylene glycol solution and the aqueous PDA nanoparticles solution. Figure 6A shows a silicon wafer with the Rhodamine B ethylene glycol droplet arrays, demonstrating the precisely confine of the fluorescence dye in the patterned regions. Figure 6B shows a silicon wafer with droplet microarrays containing PDA nanoparticles (Figure S10). Because of the excellent photo-thermal transition property of PDA, we obtained a surface at temperature microarrays under NIR irradiation.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: . Photographs of aqueous microarray, SEM and AFM images, transmission spectrum, sliding angles, contact angle, contact angle hysteresis, pinning force and XPS data.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ORCID Ke-Feng Ren: 0000-0001-5456-984X Jian Ji: 0000-0001-9870-4038

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the National Key Research and Development Program of China (2017YFB0702500), the Science and Technology Program of Zhejiang Province (Grant No. 2016C54003), the National Natural Science Foundation of China (51333005, 51573162), the Fundamental Research Funds for the Central Universities (2017XZZX001-03B), and the 111 Project under Grant No. B16042.

REFERENCES Figure 6. (A) Photography of the Rhodamine B (100 μg/mL) droplet microarray. The inset is the fluorescent micrograph of the droplet region. (B) Thermal image of the PDA nanoparticles (2 mg/mL) droplet microarray under near-infrared irradiation. The white frame indicates the region irradiated by a laser spot (808 nm, 0.9 W/cm).

(1) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. Dual-Scale Roughness Produces Unusually Water-Repellent Surfaces. Adv. Mater. 2004, 16, 1929-1932. (2) Yao, X.; Song, Y.; Jiang, L. Applications of Bio-Inspired Special Wettable Surfaces. Adv. Mater. 2011, 23, 719-734. (3) You, I.; Lee, T. G.; Nam, Y. S.; Lee, H. Fabrication of a MicroOmnifluidic Device by Omniphilic/Omniphobic Patterning on Nanostructured Surfaces. Acs Nano 2014, 8, 9016-9024. (4) Manna, U.; Lynn, D. M. Fabrication of Liquid-Infused Surfaces Using Reactive Polymer Multilayers: Principles for Manipulating

CONCLUSIONS


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