Research Article www.acsami.org
Infusing Lubricant onto Erasable Microstructured Surfaces toward Guided Sliding of Liquid Droplets Xia-chao Chen, Ke-feng Ren,* Jing Wang, Wen-xi Lei, and Jian Ji* MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *
ABSTRACT: Introducing a lubricant layer onto surfaces has emerged as a novel strategy to address a wide range of interface-related challenges. Recent studies of lubricant-infused surfaces have extended beyond repelling liquids to manipulating the mobility of fluids. In this study, we report a design of slippery surfaces based on infusing lubricant onto a polyelectrolyte multilayer film whose surface microstructures can be erased rapidly under mild condition. Unlike other lubricant-infused surfaces, the liquid movements (e.g., moving resistance and direction) on such surfaces can be manipulated via programming the surface microstructures beforehand. The work reported here offers a versatile design concept of lubricant-infused surfaces and may turn on new applications of this emerging class of bioinspired materials. KEYWORDS: lubricant-infused surfaces, erasable microstructures, guided sliding, liquid movement, polyelectrolyte multilayer films
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INTRODUCTION For centuries, natural organisms have amazed scientists with their exquisite configurations and complicated components.1−6 Not only solid matters, but also liquids can serve as a constituent material and play a key role in the function realization of biological organizations.7,8 As a typical example, a surface-immobilized liquid layer makes the leaves of Nepenthes pitcher plants slippery and effective enough to cause insects to slide into pitchers.5 Inspired by this idea, introducing a lubricant layer onto surfaces has been proposed recently to repel various contaminating fluids.9−12 These materials are mainly fabricated by infusing lubricant into underlying microstructures, yielding an overlying lubricant layer with unprecedented repellency against both polar and nonpolar liquids.13,14 To maintain a stable lubricant layer, the underlying substrates should have a microstructured surface to trap the lubricant through capillary force,14,15 as well as a greater chemical affinity for the lubricant than the liquids to be repelled.9,15,16 Since put forward, this versatile concept has attracted tremendous interests and been frequently adopted to address a wide range of interface-related challenges.17−26 Recent studies of lubricant-infused surfaces have extended beyond repelling liquids to manipulating the mobility of fluids, which paves the way for novel microfluidics, analytical devices, and liquid separation, transportation, and collection technologies.27−31 One strategy is to change the interfacial energy of partial substrates, thus creating some regions the lubricant cannot be infused into.27,29,32 For example, lubricant-repellent but hydrophilic patches can work as anchoring spots for water and © XXXX American Chemical Society
provide an in-plane control over the sliding of aqueous droplets.29 Although this strategy has distinctive features and advantages, the fabrication of such “sticky” regions may still suffer from problems associated with chemical reaction and complicated operations. One alternative approach is to regulate the liquid repellency through configuring the surface topography of underlying substrates.28,30,31,33 Aizenberg and coworkers fabricated an adaptive surface by infusing a fluorinated lubricant into a nanoporous elastic substrate.30 Conversion of various low-surface-tension droplets from free sliding to pinning was enabled by applying a graded mechanical stimulus to the substrate. Jiang and co-workers proposed an anisotropic organogel surface, in which a conformal organogel film was constructed onto a surface with anisotropic periodic microgrooves.31 A unidirectional sliding of water droplets can be realized by stretching the microgrooved organogel surface asymmetrically. Nevertheless, using these methods to change the movement angle of liquid droplets has not been reported. In addition, many methods tend to preset the surface profiles with the aid of expensive instruments like lithography machines.28,32 Therefore, it is also desirable to develop a microstructured surface that can be fabricated and regulated using a more cost-effective means. Layer-by-Layer (LbL) self-assemblies, based on alternating deposition of interacting species on substrates, have inherent potentials as stimuli-responsive films due to the sensitivity of Received: November 4, 2016 Accepted: December 22, 2016 Published: December 22, 2016 A
DOI: 10.1021/acsami.6b14081 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (A) Thickness of PEI/PAA films with respect to the number of bilayers. Note that the scale of error bars is less than that of symbols. (B) AFM three-dimensional images of the PEI/PAA films with different bilayers. (C) Optical transmission of a (PEI/PAA)8 film before and after water immersion. (inset) Schematic illustration of the water-induced flattening of the microstructures present on the film surfaces. (D) AFM threedimensional images of the (PEI/PAA)8 films immersed in water for various times. until the PEM films with the desired number of bilayers were obtained. In this paper, the PEM film will be referred to as (PEI/PAA)n, where n is the number of bilayers. Without any postprocessing, wormlike microstructures were created spontaneously during the film construction. Before thickness measurement, the PEI/PAA films were exposed to a 100% relative humidity environment for 6 h, enable the smoothing of the microstructured surfaces.34 The thickness of the PEM films was measured using a spectroscopic ellipsometry (Woollam M-2000DI, America). Smoothing of Microstructured Surfaces. Smoothing of the microstructured surfaces can be enabled by just immersing the PEM films into a bath of deionized water for a period of time. To fabricate a flat region on a microstructured surface, a certain amount of water was transferred locally onto an untreated (PEI/PAA)8 film, and afterward the water can vanish spontaneously via evaporation. The diameter of the flat regions can be regulated through transferring different amounts of water onto the untreated (PEI/PAA)8 films. This operation was performed by using a water-holding bamboo skewer, and the amount of the transferred water was controlled by its contact time with the untreated (PEI/PAA)8 films. In the presence of either microstructured or flat surfaces, the PEM films were thermally cross-linked at 180 °C for 2 h to preserve the surface morphology throughout subsequent processing and measurements. Surface Lubrication. Chemical vapor deposition of 1H,1H,2H,2H-perfluorodecyltrichlorosilane was first performed to obtain a low-surface-energy surface. To do this, the PEM films were put into a sealed chamber (100 mL) containing 100 μL of 1H,1H,2H,2H-perfluorodecyltrichlorosilane and then heated to 70 °C for 2.5 h. After that, perfluoropolyether lubricant was added dropwise onto the PEM films until complete infusion was achieved. Finally, the samples were placed vertically for 10 min to drain off the excess lubricant. Surface Characterization. Atomic force microscopy (AFM; Bruker Multimode 8, Germany), scanning electron microscopy (SEM; Hitachi S4800, Japan), and environmental scanning electron microscopy (ESEM; FEI Quanta FEG650, America) were performed to reveal the surface profiles of samples as needed. To investigate the shear tolerance of the lubricant layer, various spinning rates (from 500 to 8000 rpm, 60 s) were applied to the lubricant-infused PEM films on a spin coater (IMECAS KW-4A, China). Liquid repellency properties of these samples were measured through a drop shape analyzer (Kruss DSA 100, Germany) using water, ethylene glycol, and octane as test liquids. Sliding angles and contact angle hysteresis were determined using a liquid droplet on an inclinable stage, and each value was an average of five independent measurements. Visual analysis of the shear effect on the lubricant layer was performed on a fluorescent
their assembled units toward environmental stimuli. In this study, we report a design of slippery surfaces based on infusing lubricant onto a polyelectrolyte multilayer (PEM) film whose surface microstructures can be erased rapidly under mild condition. The stimuli-responsive nature of this film provides a means of physically regulating the interaction between the surface and the lubricant layer. A microstructured surface can hold the infused lubricant through capillary force, thus maintaining the liquid repellency when exposed to shear conditions. In comparison, there is no capillarity to retain the lubricant layer on a flat surface, leading to a massive lubricant loss under shear conditions and a heavy discount of its liquidrepellent performance. We further demonstrated that selectively creating flat regions on a microstrutured surface can be based on to fabricate a lubricant-infused patterned surface, which can be utilized to manipulate the sliding of liquid droplets (e.g., moving resistance and direction). This sample and versatile principle can offer a novel strategy to manipulate the properties of liquid-infused surfaces and may turn on new applications of this emerging class of bioinspired materials.
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EXPERIMENTAL SECTION
Materials. Branched poly(ethylenimine) (PEI, Mw 25 000) and poly(acrylic acid) (PAA, Mw 100 000) were purchased from SigmaAidrich. Perfluoropolyether lubricant (Krytox GPL 100) and carboxylic acid terminated perfluoropolyether (Krytox 157 FSL) were obtained from DuPont (America). Methyl perfluorobutyl ether (MPE), ethylene glycol, and octane were purchased from J&K (Shanghai, China). Rhodamine B was obtained from TCI (Japan). Gold nanoparticles (carboxylic acid functionalized, 20 nm diameter, 100 μg/mL gold atoms) and 1H,1H,2H,2H-perfluorodecyltrichlorosilane were purchased from Aladdin (Shanghai, China). Deionized water used in all experiments was filtered through a Milli-Q water purification system (Millipore, Billerica, United States). The pH of PEI and PAA aqueous solutions was adjusted utilizing 1.0 M HCl or 1.0 M NaOH as needed. Creation of Microstructured Surfaces. Glass and silicon substrates were immersed in freshly prepared piranha solution (30% H2O2/98% H2SO4 = 3/7 V/V) for 40 min and then flushed with the deionized water thoroughly. PEM films were fabricated by alternately dipping these precleaned substrates into PEI solution (1 mg mL−1, pH 9.0) for 1 min and PAA solution (3 mg mL−1, pH 2.9) for 1 min. Between each dipping step, the substrates were rinsed with the deionized water and blown dry by nitrogen. This process was repeated B
DOI: 10.1021/acsami.6b14081 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. (A) Measurements of liquid repellency after applying shear conditions to the lubricant-infused homogeneous surfaces. (B) Sliding angles and (C) contact angle hysteresis of 20 μL of water droplets for the lubricant-infused microstructured and flat surfaces after spinning at various rates for 60 s. Top-down images of a water droplet (20 μL, dyed with methylene blue) (D) sliding on a lubricant-infused microstructured surface and (E) pinning on a lubricant-infused flat surface (tilting angle = 13°). Both samples were tested after spinning at 3000 rpm for 60 s. microscope (Zeiss Axiovert 200M, Germany), when using fluorescently labeled lubricant to infuse the PEM films. The mean fluorescence intensities on the lubricant-infused surfaces were calculated from corresponding fluorescent photographs using ImageJ. For each sample (15 mm × 25 mm), the fluorescence measurements were performed respectively at 0, 6, and 12 mm from the center of spinning. Each value reported in this article was an average of five independent measurements. Fluorescently labeled lubricant was fabricated according to a reported procedure.35 Briefly, 200 μL of aqueous solution of Rhodamine B (0.1 mg mL−1) was first mixed with carboxylic acid functionalized gold nanoparticles (20 nm diameter, 100 μg/mL gold atoms), and the mixture was rapidly stirred at room temperature overnight. The fluorescently labeled nanoparticles were then extracted using a fluorinated liquid (100 μL of Krytox 157 FSL diluted in 500 μL of MPE), and soon after a pink color appeared in the oil phase. At last, the oil phase was diluted 1:10 in GPL 100 to make the lubricant fluorescently labeled.
For this study, we used (PEI/PAA)8 films to modify the surfaces of substrates in the rest of this work. To further manipulate these microstructures, the sensitivity of the PEI/PAA films was then investigated. We recently demonstrated that humidity (gaseous water) can serve as an mild and effective trigger for activating the transition of microporous PEM films.34,43 Here, a remarkable change happened to the transparency of a (PEI/PAA)8 film, after it was immersed into a bath of deionized water for 10 min (Figure 1C). The enhancement in its optical performance could be related to the evolution of the surface microstructures and subsequent reduction of light scattering. To verify this, AFM examination was performed to the (PEI/PAA)8 films that were immersed in water for various times. Figure 1D shows that the film surfaces clearly and rapidly become smoother on annealing in water. The root mean square roughness of the (PEI/PAA)8 films without annealing is 1547 nm, while the corresponding values are 519 and 126 nm after water annealing for 3 and 10 min, respectively. The thickness measurements of PEI/PAA films in drying condition were also performed before and after annealing in water. It was found that the film thickness was lowered a small fraction (∼5%) within 10 min of annealing (Figure S1, Supporting Information). For the change of the film thickness, one possibility is that a portion of polyelectrolytes is able to be released into water, leading to the thinning of the (PEI/PAA)8 films. However, their densifying in water can be also possible. For the PEI/PAA multilayer films, both PEI and PAA are weak polyelectrolytes with low charge density, and their multilayer assembly is a network that is not tightly crosslinked.34,44 Such a loose network could be easily disturbed under certain condition, and a subsequent reconstruction might lead to its internal densifying. In this study, the evolution of these wormlike microstructures could mainly stem from the plasticizing effect of water.45−47 As a solvent with high dielectric
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RESULTS AND DISCUSSION Construction and Smoothing of Microstructured Surfaces. As the basis of our research, PEI/PAA PEM films were fabricated using the LbL assembly. Figure 1A shows the evolution of film thickness with respect to the number of bilayers. The PEI/PAA films exhibit an exponential growth mode in the initial four bilayers and thereafter a rapid linear growth. At the same time, many wormlike microstructures with uniform sizes were created spontaneously. The size of these microstructures increases when more bilayers were deposited onto the substrates (Figure 1B). According to previous studies,36−38 the rapid buildup of the PEI/PAA films and the formation of these microstructured surfaces can both be ascribed to “in” and “out” diffusion of polyelectrolytes in the process of the film construction. Compared to other existing technologies, LbL deposition can readily yield microstructured films onto objects with arbitrary material, shape, and size.39−42 C
DOI: 10.1021/acsami.6b14081 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. Mean fluorescence intensity on the lubricant-infused (A) microstructured and (B) flat surfaces after spinning at various rates for 60 s. To reveal the distribution uniformity of lubricant, the fluorescence measurements were performed respectively at 0, 6, and 12 mm from the center of spinning. In both diagrams, a.u. is actually short for arbitrary unit. Schematic illustrations of the evolution of (C) a lubricated microstructured surface and (D) a lubricated flat surface under shear conditions.
constant (78.36 F/m, 25 °C), water tends to increase the screening of charges along the polyelectrolyte chains and raise the possibility of splitting electrostatic cross-linking.48 What’s more, water incorporation can also bring additional free volume to the movement of polymer segments.45,49 As a result, the mobility of the polyelectrolyte chains is highly enhanced, and the interdiffusion of polyelectrolytes enables the substance transfer from “hills” into “valleys”, thus smoothing the microstructures present on the film surfaces. Liquid Repellency of Lubricant-Infused Homogeneous Surfaces. Both the microstructured films (i.e., (PEI/ PAA)8 without water immersion) and the flat films (i.e., (PEI/ PAA)8 immersed in water for 10 min) were thermally crosslinked, fluorosilanized, and finally infused with a fluorinated liquid (Krytox GPL 100), resulting in two types of lubricantinfused surfaces. Note that the sensitivity of the PEI/PAA films to water was eliminated after thermal cross-linking and that their surface morphologies can be preserved throughout subsequent processing and measurements. To investigate the stability of the lubricant-infused surfaces, various spinning rates (from 500 to 8000 rpm, 60 s) were then applied to the samples through a spin coater. This was then followed by measuring the sliding angles and contact angle hysteresis of water (surface tension = 72.4 mN/m at 298.15 K) to assess their liquid repellency properties (illustrated in Figure 2A). To unify the experimental conditions, the properties of the lubricant-infused surfaces were tested at 10 mm from the center of spinning. As shown in Figure 2B,C, both types of the lubricant-infused surfaces showed similar water repellency (contact angle hysteresis
