A Rapid One-Step Fabrication of Patternable Superhydrophobic

Feb 24, 2014 - We present a facile and inexpensive approach without any fluorinated chemistry to create superhydrophobic surface with exceptional liqu...
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A Rapid One-Step Fabrication of Patternable Superhydrophobic Surfaces Driven by Marangoni Instability Sung-Min Kang,† Sora Hwang,† Si-Hyung Jin,† Chang-Hyung Choi,† Jongmin Kim,† Bum Jun Park,‡ Daeyeon Lee,§ and Chang-Soo Lee*,† †

Department of Chemical Engineering, Chungnam National University, Yuseong-gu, Daejeon 305-764, South Korea Department of Chemical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, South Korea § Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ‡

S Supporting Information *

ABSTRACT: We present a facile and inexpensive approach without any fluorinated chemistry to create superhydrophobic surface with exceptional liquid repellency, transportation of oil, selective capture of oil, optical bar code, and self-cleaning. Here we show experimentally that the control of evaporation is important and can be used to form superhydrophobic surface driven by Marangoni instability: the method involves in-situ photopolymerization in the presence of a volatile solvent and porous PDMS cover to afford superhydrophobic surfaces with the desired combination of micro- and nanoscale roughness. The porous PDMS cover significantly affects Marangoni convection of coating fluid, inducing composition gradients at the same time. In addition, the change of concentration of ethanol is able to produce versatile surfaces from hydrophilic to superhydrophobic and as a consequence to determine contact angles as well as roughness factors. In conclusion, the control of evaporation under the polymerization provides a convenient parameter to fabricate the superhydrophobic surface, without application of fluorinated chemistry and the elegant nanofabrication technique.

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INTRODUCTION

The main idea of the conventional methods involves the roughening of the surface of hydrophobic materials, the chemical modification of the surface with fluorinated chemicals of low surface energy (bottom-up approach),4,13,14 and physical formation of micro/nanostructure through silicon fabrication techniques including etching, deposition, or photolithography (top-down approach).15−18 However, these approaches typically involve multistep, time-consuming procedures, harsh preparation conditions, limited substrates, specialized chemical modifiers or reagents, and equipment. In addition, they are expensive and only applicable to small area or specific materials. As a result, practical applications of such functional materials have not been fully realized, and there is a clear need for an inexpensive and broadly applicable approach toward superhydrophobic coatings. Recently, alternative approaches have been developed to enable the straightforward fabrication of superhydrophobic surfaces using the phase separation of polymers.19−21 The major route of the phase separation method is coating of a polymer and then treating the coating with a nonsolvent for the polymer at a specific temperature and under certain humidity conditions. Thus, the relevant selection of solvents and the precise control of temperature or relative humidity may impose limitation to scale-up for creating large-area uniform coatings or

The fabrication of superhydrophobic surface is a fast growing area in both scientific and technological fields because of their unique water-repellent and self-cleaning properties and their potential for practical applications ranging from biotechnology to self-cleaning commodity materials such as rain- or snowproof glass, stain-resistant textiles, self-cleaning traffic signs, microfluidic devices, and functional separation equipment.1−12 Superhydrophobicity is explained by the Cassie−Baxter model according to which air is trapped in the microgrooves of the rough surface and water droplets rest on the microstructured surface.2,4,5 Nature utilizes the extreme water-repellent properties of superhydrophobic surfaces in many plants and animals.1−3 Well-known examples include lotus leaves and water striders that are able to walk on the surface of water. Based on the understanding of the relationship between surface energy and roughness of natural nonwetting surfaces, a number of approaches inspired by nature open routes toward fabrication of artificial superhydrophobic surfaces; the specific micro−nano binary structures dramatically increase the surface roughness and minimize the contact area between the structures and the liquid. For water repellency, especially surface roughness and low surface energy are essential. A water drop gently deposited on the surface shows a contact angle above 150° and rolls off easily, demonstrating the surface’s superhydrophobicity. © 2014 American Chemical Society

Received: July 13, 2013 Published: February 24, 2014 2828

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Figure 1. Fabrication of superhydrophobic surface. (A) Formation of superhydrophobic surface containing micro- and nanostructure driven by Marangoni flow (case 1) and generation of smooth surface when glass cover is applied (case 2). (B) Static contact angle of water on the fabricated polymeric surface with various cover materials. (C) Schematic diagram of the fabrication process, depicting Marangoni flow induced by heterogeneous evaporation. Evaporation (solid arrows) occurs over the entire interface between coating solution and cover. If the rate of evaporation process is almost identical, the surface would be preserved to its inherent property during evaporation and photopolymerization. However, the rate of evaporation is heterogeneous beneath the PDMS cover, inducing Marangoni flow. Thus, a capillary flow (dotted arrows), from the lower surface tension area to higher interfacial tension area, is induced to replenish coating fluid, which ultimately leads to the formation of binary micro/ nanostructures.

selective patterning of the superhydrophobic surfaces on substrates. Patterning superhydrophobic surface as another technical issue is especially important for numerous applications including water harvesting, bioassay, microfluidics, and fluid transport.16,22,23 However, microscale patterning of superhydrophobic structures typically requires multiple steps and cannot be scaled up easily. Also, most of the previous methods tend to focus on patterning hydrophilic regions on superhydrophobic surfaces, rather than directly patterning superhydrophobic structure on a hydrophilic surface. Directly patterning superhydrophobic features on glass, for example, would allow for subsequent functionalization of glass using the silane chemistry, which would provide versatility in controlling the surface wettability and properties for a variety of applications. Thus, the development of methods to enable

straightforward patterning of superhydrophobic features is highly desirable. Here, we present an inexpensive and broadly applicable method that facilitates the preparation of patternable superhydrophobic surfaces with exceptional water repellency using Marangoni instability and photopolymerization. We demonstrate one-step formation of a superhydrophobic surface using a trimethylpropane triacrylate (TMPTA) monomer solution in ethanol as a coating solution on the substrate, following a “spread, cover, and photopolymerization (SCP)” route. The coating solution containing photoinitiator is directly spread onto a clean slide glass, followed by the placement of a PDMS cover on top of the precursor solution and subsequently photopolymerization triggered by UV irradiation. A superhydrophobic film is immediately obtained without any additional treatment, where uneven evaporation and cluster formation of the polymer take place to afford superhydrophobic 2829

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Figure 2. Effect of ethanol concentration on the fabrication of superhydrophobic surface. SEM images at (A) 10%, (B) 20%, (C) 30%, (D) 40%, (E) 50%, (F) 60%, (G) 70%, (H) 80%, and (I) 90% ethanol. The precursor solution comprises TMPTA and ethanol. The inset image show a static water droplet on each surface. (J) Static water contact angle on each film. (K) Analysis of roughness at each film. Scale bars are 300 nm.

formation of porous polymer structures that have micro- and nanoscale hierarchical structures. To better understand the formation mechanisms of the superhydrophobic morphology from the precursor solution, we examine the influence of solvent evaporation on the fabrication of superhydrophobic surfaces using different types of covers. The concentration of ethanol is kept constant at 50%. The effect of different covers on creating superhydrophobic surfaces is summarized in Figure 1B and Figure S1. We find that the water contact angle increases when a PDMS or a PDMS/glass cover is used, which demonstrates the use of the PDMS cover has a significant influence on the formation of superhydrophobic surfaces. There is little difference in water contact angles when the PDMS and PDMS/glass covers are used. However, in case of maximized retardation of solvent evaporation with a glass cover and free evaporation without the use of any covers, we cannot obtain a superhydrophobic surface. In the case of the PDMS or PDMS/glass cover, the result of high contact angle value (above 160°) implies that large volumes of air is trapped under a water droplet in the surfaces, resulting from high surface roughness. In other words, increasing the surface roughness of the film plays a major role in rendering the surface superhydrophobic. The photo-

surfaces with the desired combination of micro- and nanoscale roughness. Consequently, the morphology of the binary architecture in the superhydrophobic surface can be simply and efficiently controlled by changing the concentration of ethanol and the use of PDMS cover. We also demonstrate that these superhydrophobic surfaces can be readily patterned on glass using a simple molding technique. We envision that these surfaces will be useful in fluid handling and transportation, optical sensing, medicine, and as self-cleaning and antifouling materials operating in extreme.

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RESULTS AND DISCUSSION

Our strategy based on a simple “spread, cover, and photopolymerization (SCP)” route to generate a superhydrophobic surface is schematically shown in Figure 1A. Interestingly, we have found that a superhydrophobic surface showing rough topology and exhibiting a water contact angle above 160° can be fabricated by placing a PDMS cover above the precursor solution during photopolymerization (case 1). If a nonporous glass cover is used during photopolymeziation, we are able to obtain a hydrophilic surface rather than a superhydrophobic or hydrophobic one (case 2). The topology of the hydrophilic surface is smooth and flat. These results indicate that the key step in the generation of superhydrophobic surface is the 2830

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surfaces are fabricated with high water contact angles (>160°) (Figure 2D−I). Owing to the extremely low adhesion of water to the coating, it is difficult to deposit water drops on the superhydrophobic surfaces; that is, water droplets immediately roll off upon their placement on these superhydrophobic surfaces, which indicates that our fabrication method induces superhydrophobic properties in a simple and rapid manner. Phase-shifting interferometry images of surface morphologies of the surface and their corresponding roughness also highlight the influence of the concentration of ethanol on both the surface hydrophobicity and the surface morphology (Figure S2 and Figure 2J). As we expected, roughness analysis shows that the surface roughness increases from low to high concentrations of ethanol. While the surfaces obtained at low concentration of ethanol (below 30%) are not structured, high concentration of ethanol leads to the formation of microand/or nanostructure. When the concentration of ethanol is above 40%, coral-like surface morphologies are generated with large numbers of micrometer-scale spherulites decorated with nanometer-sized fine structures, achieving high roughnesses (above 1.4 μm). In other words, an increase in the concentration of ethanol results in an increase in the formation of binary structure with micro- and nanosized spherical aggregates. It is interesting to see that the polymerized films without significant surface roughnesses are hydrophilic. We believe the polymerized film under limited evaporation provides re-entrant microstructures that renders these surfaces superhydrophobic.27 In addition, we have investigated the bouncing of a droplet as dynamic effect of superhydrophobic surface because the ability of water to bounce on a surface provides an indication of the surface properties (Figure S3). The result also confirms the hydrophobicity of a surface, with a relationship established between water contact angle and number of bounces, which is dependent on the surface microstructure.28,29 In short, the increase in the concentration of ethanol under the photopolymerization of the monomer has two major consequences: (1) inhomogeneous or limited evaporation of the volatile solvent, ethanol, through a porous PDMS cover provides the Maragoni instability in the precursor solution; (2) the evaporation results in the localization of monomers, which form nuclei for the formation of micro/nanoclusters and, as a consequence, binary structures are formed. The resulting surface morphology and roughness have a significant effect on the water-repellent properties. Our method provides a straightforward and simple method to generate superhydrophobic surface. The superhydrophobic surfaces prepared using our SCP method can be used for diverse applications owing to its excellent water repellency. We present three examples of applications using our superhydrophobic surface in selfcleaning, selective oil removal, and one-step patterning. Lotus leafs, natural superhydrophobic surfaces, are well-known examples of self-cleaning surfaces.1,3,6 When a water droplet is placed on a particle-contaminated superhydrophobic surface, the droplet captures the particles on the surface while it is moving around on the surface. The adsorption of the particles is due to the strong attachment of particles to the droplet surface.30 We confirm the effectiveness of the superhydrophobic surface in self-cleaning toward particle contamination. When the surfaces contaminated with coffee, cream, and sugar powders are rinsed with water, the liquid forms into droplets

polymerization under the limited evaporation condition (via the use of a PDMS or a PDMS/glass cover) results in different surface wettabilities because the porous structure of PDMS likely plays a decisive role in fabricating superhydrophobic coatings with high contact angles. For instance, only PDMS and PDMS/glass covers provide limited evaporation environment during the photopolymerization. Thus, this phenomenon indicates that the limited evaporation of solvent in photopolymerization can play a vital role in the generation of superhydrophobic surfaces (>160°) with different surface morphology. We believe that the results can be explained by Marangoni instability induced by the limited evaporation of the solvent during photopolymerization. Figure 1C outlines the proposed mechanism for the formation of binary micro/nanostructures via the Marangoni flow, driven by the combined solvent evaporation and photopolymerization. First, a film of the coating solution is spread on the substrate and covered with a PDMS slab. Subsequently, the precursor solution under PDMS is exposed to UV irradiation triggering photopolymerization. As volatile ethanol unevenly evaporates through the highly permeable PDMS cover during photopolymerization, the evaporation of the solvents induces cooling near the surface and leads to the formation of temperature gradient between the top layer (T1) and bottom surface (T2). Finally, the gradients in composition and temperature are simultaneously induced in the thin precursor solution, as shown in Figure 1C because interfacial tension is a function of solution temperature and concentration. The gradients change the interfacial tension, inducing flow near the surface of the precursor film. The high interfacial tension area pulls the solution from the low interfacial tension area; a surface tension gradient (dotted arrow in Figure 1C scheme) and hence a Marangoni flow develop and drive the aggregation of polymers toward the high interfacial tension area to form micro/nanostructures.24−26 Therefore, in this case, the Marangoni flow is mainly attributed to the difference in the interfacial tension, which is responsible for the formation of binary micro/nanostructures. This hypothesis is also supported by the use of different covers (Figure 1B). The quantification of the changes in the water contact angle and surface roughness under different conditions provides a first step toward understanding the principle of superhydrophobic surface formation. First, to investigate the role of the ethanol in the formation of superhydrophobic surfaces, we determine water contact angle and roughness as a function of ethanol concentration, averaged over five experimental results (Figure 2). The fabrication of the thin film with different concentration of ethanol is performed, and the surface properties of the prepared films are examined with a scanning electron microscope (SEM), water contact angle measurements, and phase-shift interferometry. The surface morphology and water contact angle of the polymerized surfaces depend significantly on the concentration of ethanol in the precursor solution. When the concentration of ethanol is below 20%, little surface features are observed. The surface formed with low concentration of ethanol solvent (10 and 20% w/v) shows small increase of hydrophobicity with a water contact angle of 74° and 83°, respectively (Figure 2A,B). The polymerized surface at 30% ethanol is structured; however, the surface roughness is not sufficient to induce superhydrophobic properties (Figure 2C). When the concentration of ethanol is raised above 40% ethanol, superhydrophobic 2831

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As a last example, we demonstrate one-step complex pattern formation of superhydrophobic surfaces using micromolding in capillaries (MIMIC) (Figure 4 and Figure S5).34,35 By slightly

and collect dirt from the surface while it freely moves around on the surface (Figure 3A).

Figure 4. Schematic diagram for selective superhydrophobic patterning using micromolding in capillary (MIMIC): (A) PDMS micromold placement, (B, C) filling the micromold with a precursor solution by capillary action, (D) the curing of prepolymer by UV radiation, (E) removing the PDMS micromold. (F) Selective patterning of superhydrophobic surface onto substrate. (G) Demonstration of infosurface using selective patterning of superhydrophobic surface. Scale bar: 300 μm.

Figure 3. Diverse applications of superhydrophobic surface: (A) selfcleaning, oil capture in (B) oil and water, and (C) water and oil twophase system.

modifying the SCP method, we can achieve a single-step patterning of superhydrophobic surfaces directly on glass. First, we prepare a lithography mask that contains information in the form of a barcode or a Quick Response (QR) code, which is then used to fabricate a PDMS micromold. The micromold, which can be used repeatedly, is placed on a glass surface, and the precursor solution is introduced to one side of the micromold by capillary action (Figure 4A−F). Through capillarity, the solution is wicked into the patterned region between the mold and the glass surface. The precursor is polymerized under UV irradiation to form a patterned superhydrophobic surface (Figure 4F). Remarkably and unexpectedly, these superhydrophobic patterns exhibit strong fluorescence although no fluorescent molecules have been added to the structure. The intensity of fluorescence observed over a wide range of wavelength correlates strongly with the roughness of the surface (Figure S5), which can be tuned by the concentration of ethanol in the precursor solution prior to photopolymerization. In short, the rougher the surface is the stronger the fluorescence intensity. Also, the fluorescence from these patterns is very stable and can be observed for 6 months after sample preparation with little protection from ambient light and oxygen. The strong fluorescence may be attributed to autofluorescence of the photopolymerized polymer; however, we do not clearly understand its physical mechanism and origin. The stable and strong fluorescence in this patternable superhydrophobic surface, nevertheless, presents a new opportunity to generate what we call “infosurface” that contains information in the form

We examine the capability of the superhydrophobic surface to selectively capture oil and transport the fluid. Such a material is especially important because the number of environmental accidents involving oil spills has been on the rise in the recent years; for example, the Deep Horizon oil spill in the Gulf of Mexico seriously damaged the ocean in 2010. Functional materials that can remedy these situations are in high demand.31−33 We demonstrate the selective oil capture process in two-phase systems. For example, we have made a two-phase solution composed of olive oil (dyed with a red dye) and water (dyed with a blue dye). Because the density of olive oil is lower than that of water, olive oil forms the superphase, whereas water forms the subphase. Figure 3B illustrates selective oil absorption by simply immersing the superhydrophobic surface into the water phase through the oil phase. Interestingly, the color of the polymer surface turns red, indicating that the polymer surface selectively absorbs the oil. Conversely, we have made a different two-phase system composed of transparent FC-40 as the oil phase and water containing a red food dye. In this case, the oil phase is the subphase because its density is higher than that of water. Although the superhydrophobic surface comes in contact with water first during its immersion, the surface does not show any red color and only absorb transparent FC-40 oil while retaining its original opaque appearance (Figure 3C). In addition, we have performed a removal test of n-decane using this surface as a sponge. When a droplet of decane is placed on the surface, the decane drop is rapidly absorbed into the surface (Figure S4A) and transports across the surface (Figure S4B). 2832

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(6) Liu, K. S.; Yao, X.; Jiang, L. Recent developments in bio-inspired special wettability. Chem. Soc. Rev. 2010, 39, 3240−3255. (7) Verho, T.; Bower, C.; Andrew, P.; Franssila, S.; Ikkala, O.; Ras, R. H. A. Mechanically durable superhydrophobic surfaces. Adv. Mater. 2011, 23, 673−678. (8) Anastasiadis, S. H. Development of functional polymer surfaces with controlled wettability. Langmuir 2013, 29, 9277−9290. (9) Celia, E.; Darmanin, T.; de Givenchy, E. T.; Amigoni, S.; Guittard, F. Recent advances in designing superhydrophobic surfaces. J. Colloid Interface Sci. 2013, 402, 1−18. (10) Dash, S.; Garimella, S. V. Droplet evaporation dynamics on a superhydrophobic surface with negligible hysteresis. Langmuir 2013, 29, 10785−10795. (11) Lai, Y. K.; Pan, F.; Xu, C.; Fuchs, H.; Chi, L. F. In situ surfacemodification-induced superhydrophobic patterns with reversible wettability and adhesion. Adv. Mater. 2013, 25, 1682−1686. (12) Tuberquia, J. C.; Jennings, G. K. Surface initiation from adsorbed polymer clusters: A rapid route to superhydrophobic coatings. ACS Appl. Mater. Interfaces 2013, 5, 2593−2598. (13) Wu, W. L.; Zhu, Q. Z.; Qing, F. L.; Han, C. C. Water repellency on a fluorine-containing polyurethane surface: Toward understanding the surface self-cleaning effect. Langmuir 2009, 25, 17−20. (14) Wolfs, M.; Darmanin, T.; Guittard, F. Versatile superhydrophobic surfaces from a bioinspired approach. Macromolecules 2011, 44, 9286−9294. (15) Zhao, H.; Law, K. Y.; Sambhy, V. Fabrication, surface properties, and origin of superoleophobicity for a model textured surface. Langmuir 2011, 27, 5927−5935. (16) Zhai, L.; Berg, M. C.; Cebeci, F. C.; Kim, Y.; Milwid, J. M.; Rubner, M. F.; Cohen, R. E. Patterned superhydrophobic surfaces: Toward a synthetic mimic of the Namib Desert beetle. Nano Lett. 2006, 6, 1213−1217. (17) Kietzig, A. M.; Hatzikiriakos, S. G.; Englezos, P. Patterned superhydrophobic metallic surfaces. Langmuir 2009, 25, 4821−4827. (18) Deng, X.; Mammen, L.; Butt, H. J.; Vollmer, D. Candle soot as a template for a transparent robust superamphiphobic coating. Science 2012, 335, 67−70. (19) Zhang, Y.; Wang, H.; Yan, B.; Zhang, Y. W.; Yin, P.; Shen, G. L.; Yu, R. Q. A rapid and efficient strategy for creating super-hydrophobic coatings on various material substrates. J. Mater. Chem. 2008, 18, 4442−4449. (20) Zhao, N.; Xie, Q. D.; Weng, L. H.; Wang, S. Q.; Zhang, X. Y.; Xu, J. Superhydrophobic surface from vapor-induced phase separation of copolymer micellar solution. Macromolecules 2005, 38, 8996−8999. (21) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Transformation of a simple plastic into a superhydrophobic surface. Science 2003, 299, 1377−1380. (22) Han, S.; Bae, H. J.; Kim, J.; Shin, S.; Choi, S. E.; Lee, S. H.; Kwon, S.; Park, W. Lithographically encoded polymer microtaggant using high-capacity and error-correctable QR code for anti-counterfeiting of drugs. Adv. Mater. 2012, 24, 5924−5929. (23) Pretsch, T.; Ecker, M.; Schildhauer, M.; Maskos, M. Switchable information carriers based on shape memory polymer. J. Mater. Chem. 2012, 22, 7757−7766. (24) Erbil, H. Y. Evaporation of pure liquid sessile and spherical suspended drops: A review. Adv. Colloid Interface Sci. 2012, 170, 67− 86. (25) Cai, Y. J.; Newby, B. M. Z. Porous polymer films templated by Marangoni flow-induced water droplet arrays. Langmuir 2009, 25, 7638−7645. (26) Lu, S. Y.; Chen, H. L.; Wu, K. H.; Chen, Y. Y. Formation of nanowire striations driven by marangoni instability in spin-cast polymer thin films. Langmuir 2007, 23, 10069−10073. (27) Tuteja, A.; Choi, W.; Ma, M. L.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing superoleophobic surfaces. Science 2007, 318, 1618−1622. (28) Richard, D.; Quere, D. Bouncing water drops. Europhys. Lett. 2000, 50, 769−775.

of superhydrophobic patterns that can be readily detected using fluorescence microscopy. These structures can be especially useful in applications that require protection against moisture.

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CONCLUSIONS To enable commercialization of superhydrophobic coatings, a simple and efficient fabrication process using accessible and cost-effective materials and simple preparation conditions are highly desirable. This study demonstrates a facile, simple, and fast fabrication route to obtain readily patternable superhydrophobicity by using Marangoni instability. The superhydrophobic surfaces could be created through a simple “spread, cover, and photopolymerize (SCP)” procedure, without need for the use of any fluorinated materials and sophisticated instruments. The technique described here can be employed to mimic the special micro−nano binary structure of the lotus leaf under ambient conditions, especially suitable for creating surfaces with complex shapes and patterns, such as QR and barcodes. We have also demonstrated the successful application of the superhydrophobic surfaces in selective oil capture and transportation. Our approach opens a truly simple, rapid, and efficient route to the generation of superhydrophobic surfaces, which may be extended to a wide variety of polymers. Also, it contributes to the realization of superhydrophobic surfaces with an eco-friendly approach and opens new perspectives on the application of Marangoni instability.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental procedure and additional experimental results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (C.-S.L.). Author Contributions

S.-M.K. and S.H. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MEST) (No. 2011-0017322). D.L. acknowledges the support from the NSF (CAREER DMR-1055594).

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REFERENCES

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