Superhydrophobic Surface With Shape Memory Micro/Nanostructure and Its Application in Rewritable Chip for Droplet Storage Tong Lv,†,§ Zhongjun Cheng,*,‡ Dongjie Zhang,†,§ Enshuang Zhang,† Qianlong Zhao,† Yuyan Liu,*,† and Lei Jiang⊥ †
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering and ‡Natural Science Research Center, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, PR China ⊥ Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China S Supporting Information *
ABSTRACT: Recently, superhydrophobic surfaces with tunable wettability have aroused much attention. Noticeably, almost all present smart performances rely on the variation of surface chemistry on static micro/nanostructure, to obtain a surface with dynamically tunable micro/nanostructure, especially that can memorize and keep different micro/nanostructures and related wettabilities, is still a challenge. Herein, by creating micro/nanostructured arrays on shape memory polymer, a superhydrophobic surface that has shape memory ability in changing and recovering its hierarchical structures and related wettabilities was reported. Meanwhile, the surface was successfully used in the rewritable functional chip for droplet storage by designing microstructure-dependent patterns, which breaks through current research that structure patterns cannot be reprogrammed. This article advances a superhydrophobic surface with shape memory hierarchical structure and the application in rewritable functional chip, which could start some fresh ideas for the development of smart superhydrophobic surface. KEYWORDS: superhydrophobic, micro/nanostructure, shape memory polymer, micropattern, rewritable
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Recently, by constructing microscale pillar arrays on the elastic polydimethylsiloxane (PDMS) substrates,20−22 superhydrophobic surfaces with dynamically tunable microstructures have been successfully prepared, these surfaces have exhibited great potential applications in in situ droplet transportation21 and unidirectional water spreading.22 Nevertheless, these research can only control the microstructure at single microscale, which is not the most perfect design since both the natural selection (e.g., lotus leaves,23,24 water stride legs,25 and butterfly’s wings15) and experimental research have proved that for superhydrophobic surfaces, the hierarchical micro/nanostructure is the optimal design.26−30 Meanwhile, such elastic surfaces cannot keep the deformed microstructures and related wetting
n the past decades, smart superhydrophobic surfaces have become a research hotspot due to their wide applications including self-cleaning,1−3 controllable oil/water separation,4,5 cell capture/release,6 and antibioadhesion.7 Generally, such surfaces can be prepared by modifying responsive molecules on substrates with hierarchical micro/nanostructures, which can display various smart wetting performances such as reversible transition between the superhydrophobicity/superhydrophilicity and high/low adhesions under external stimulus including temperature, light, pH, and so on.8−13 Nevertheless, all these smart performances rely on the variation of surface chemistry, and the surface microstructures are static, which lacks sufficient intelligence and therefore can only offer passive functions on the surfaces. In fact, surface microstructures are also important because they can directly endow the surfaces with many particular properties that cannot be obtained by changing surface chemistry, such as directional adhesion,14,15 water collection,16,17 and water directional spreading.18,19 © 2016 American Chemical Society
Received: June 28, 2016 Accepted: September 19, 2016 Published: September 21, 2016 9379
DOI: 10.1021/acsnano.6b04257 ACS Nano 2016, 10, 9379−9386
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ACS Nano performances without the external actions (for instance, mechanical strain and magnetic field), thus limiting their applications. A superhydrophobic surface that can overcome these imperfections would be better to meet the requirement for complex practical applications: for example, creating rewritable structure-dependent micropatterned chips by combining different microstructure shapes on the identical surface, but until now, it is still a challenge. Herein, a superhydrophobic surface made from the epoxybased shape memory polymer (SMP) with particular shape memory hierarchical micro/nanostructure and the application in rewritable droplet-based functional chip is reported. The surface was prepared by replicating hierarchical structured template with SMP, on which the micro/nanostructure can be controlled reversibly between the permanent and temporary shapes. Accordingly, different wetting performances can be obtained on the surface. Furthermore, based on the special memory ability of micro/nanostructure, we showed that the surface can be used as the rewritable platform for droplet storage chip by designing microstructure-depended patterns, which can overcome the disadvantages in present reports that structural patterns are impossible for reprogramming.31,32Although a large number of works have paid attention to SMP materials for their fantastic shape memory performance and related valuable applications, such as biomedical chips,33−36 smart adhesives materials,37−41 and optical devices,42−46 research on the SMP wetting performances is still scarce and commonly on those pillars with single micro- or nanoscale and general hydrophobicity.47,48 Superhydrophobic SMP is extremely rare49,50 and the first to be reported on high-aspectratio micropillars,49 which is hard to be extended because such high-aspect-ratio pillars are typically not mechanical stable and unfavorable for dynamic control.39 As an alternative strategy, herein, we advance a method by creating hierarchical micro/ nanostructure on the short pillars, which can not only realize the low adhesive superhydrophobicity but also be convenient for the dynamic control of the surface microstructure. This work reports a superhydrophobic surface with shape memory hierarchical micro/nanostructure and the application in the rewritable droplet-based chip, which can not only overcome the problem presented in dynamic control of the superhydrophobic surface microstructure but also put forward a concept in the field of application of intelligent superhydrophobic surfaces.
Scheme 1. Chemical Structures of the SMP Prepolymer Ingredients
of such nanostructures and the roughness of the top surface of the pillar is about 23.5 nm (Figure 1d). After heating and pressing by an external force, the pillars would collapse (Figure 1e), and the average value of the pillar height decreased to approximately 3.6 μm (Figure 1f); the following cooling process can help the surface remain such collapsed structure. Further heating the surface at 120 °C for about 45 s, both the surface morphology (Figure 1g) and pillar size (Figure 1h) recovered to the original states. Furthermore, because of the presence of three-dimensional cross-linking networks, the obtained SMP surface possesses wonderful fatigue durability (Figure S4). The hierarchical structure can be tuned reversibly for several cycles without any deterioration. As displayed in Figure 1i, the average value of pillar height can be varied repeatedly for at least 50 times without any decrease, indicating that the surface has remarkable memory ability in changing and restoring of its hierarchical micro/nanostructure. Moreover, even after one month storage, the responsivity of surface microstructure can still be observed, meaning that the surface possesses an excellent stability. In addition to the confocal microscope, we further examined the surface morphology at different states with the scanning electron microscopy (SEM). Figure 2a clearly displays the original arrays of pillars. The sizes of the pillars are in agreement with the results derived from the confocal microscope. From the magnified image of one pillar (Figure 2d), one can find that the surface of the pillar is not smooth, and nanopits with average width of about 520 nm can be observed. On such a hierarchical micro/nanostructured surface, a high water contact angle (CA) of about 151° can be observed, indicating that the surface has the superhydrophobicity (inset in Figure 2d). After heating and pressing under a certain external pressure, the pillar would collapse, and all the pillars show uniform deformation and orientation after cooling to room temperature (Figure 2b). Noticeably, the nanostructures become unclear because a flat glass substrate was used to press the surface (Figures 2e and S5). Meanwhile, the surface wettability is changed, and the CA reduces to approximately 110° (inset in Figure 2e). After further heating at 120 °C for about 45 s, both the surface micro/nanostructure (Figure 2c,f) and the surface superhydrophobicity (inset in Figure 2f) would be recovered. Furthermore, it is found that the recovery velocity is depended on the heating temperature, and by controlling the heating time at relatively low temperature, various microstructures and wetting performances can be observed on the surface (Figures S6 and S7). It needs to be emphasized that no apparent variation of the surface chemical composition can be
RESULTS AND DISCUSSION Diglycidyl ether of bisphenol A type epoxy resin (DGEBA), mxylylenediamine (MXDA), and n-octylamine (OA) were mixed51,52and cured on a micro/nanostructured substrate to prepare the surface (Scheme 1 and Figures S1 and S2). To demonstrate the shape memory ability of the hierarchical micro/nanostructure, the surface was first subjected to an external pressure with a glass slide at 120 °C (higher than the glass transition temperature (Tg) 77.8 °C for the used SMP, see Figure S3) and then further heated to examine the recovery property (Figure 1a). The confocal microscope was used to investigate the surface micro/nanostructure in situ at different states. From Figure 1b, it can be seen that the as-prepared surface is covered by arrays of pillars. The diameter, height, and the distance between pillars are 10 μm, 10 μm, and 10 μm, respectively. The unsmooth curve corresponding to top part of the pillar in the profile picture indicates that there are nanostructures on the pillars (Figure 1c). Atomic force microscope (AFM) results can further confirm the presence 9380
DOI: 10.1021/acsnano.6b04257 ACS Nano 2016, 10, 9379−9386
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ACS Nano
Figure 1. (a) Schematic illustration of surface hierarchical micro/nanostructure deformation and recovery process. (b and c) 3D confocal microscopy image and related profile picture of the as-prepared SMP surface, respectively. (d) AFM image of the top part of one pillar, indicating the presence of nanostructure. (e and f) 3D confocal microscopy image and corresponding profile picture of the collapsed surface, respectively. (g and h) 3D confocal microscopy image and corresponding profile picture of the recovered surface, respectively. (i) The average pillar height on the surface after several consecutive pressing/recovering processes, indicating that the height of pillar can be reversibly changed without any decrease.
observed during the whole process, meaning that the change of wetting property is mainly governed by the change of surface microstructure (Table S1). From these results, it can be concluded that due to the special shape memory ability, not only different micro/nanostructures but also various wetting performances that governed by the microstructure can be memorized and displayed on the surface. To obtain a surface with the optimal shape memory ability and the superhydrophobicity, microflat and hierarchical micro/ nanostructured arrays with fixed pillar width (10 μm) and height (10 μm) and different space between pillars (5 μm, 10 μm, 20 μm, 30 μm) were prepared and investigated. As shown in Figures S8 and S9, all these surfaces have good memory ability in changing/recovering the microstructure. Figure 3a shows the wetting performances of the flat pillars without nanostructure. It can be seen that as the spacing (distance between adjacent pillars) is increased, the CA is increased, and
Figure 2. SEM images viewed at a tilt angle of about 40°: (a−c) The as-prepared surface, collapsed surface, and recovered surface, respectively. (d−f) The magnified images corresponding to (a), (b), and (c), respectively. Insets are the shapes of a water droplet on the surface at related states.
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DOI: 10.1021/acsnano.6b04257 ACS Nano 2016, 10, 9379−9386
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Figure 3. Statistic of CAs and SAs on different surfaces with various spaces between pillars: (a) Microscale flat pillar structured surfaces and (b) hierarchical micro/nanostructured surfaces. From the two figures, it can be found that the introduction of nanostructure onto the pillars can effectively enhance the surface hydrophobicity and decrease the surface adhesion. (c and d) Reversible variation of CAs and SAs on the surface with spacing of 20 μm through repeatedly pressing and recovering process, demonstrating smart controllability in surface wettability. Insets are the shape of a water droplet on the surface in related conditions.
The obtained surface has such an excellent ability in changing and recovering the micro/nanostructure and related wettabilities, which is attributed to the excellent shape memory performance of the material.53−56 To have a good understanding of the shape memory ability of surface micro/ nanostructure and tunable wetting performance, the possible mechanism for the variation of surface microstructure and wettability was analyzed carefully. According to the Professor Xie’s report,57 two structural requirements are necessary for SMP. One is the reversible thermal transition for temporary shape fixing and recovery, which can allow the molecular chain mobility to be suppressed and activated for entropy trapping and releasing respectively; the other is the cross-linking network that sets the permanent shape. In this work, as shown in Figure 4a, after polymerization between DGEBA, OA, and MXDA, cross-linking network including chemical crosslinking (formed by the reaction between epoxy groups and −NH2) and physical cross-linking (formed by the tail-to-tail associations of alkyl chains)58 can be formed. Meanwhile, as displayed in Figure S3, the as-prepared polymer has apparent thermal reversible phase transition ability. Therefore, as shown in Figure S10, the as-prepared material has an excellent shape memory performancem and the shape recovery ratio is as high as 96%. Such an excellent shape memory ability allows the surface to memorize different microstrucutre shapes and display various wetting performances. For the obtained hierarchical structured surface, the arrays of pillar structures are the permanent shape (Figure 4c), and the conformation of molecular chains has the highest entropy (Figure 4a), that is, the molecular chains are in the thermodynamically stable state.59 On such a micro/nanostructured surface, a water droplet is prone to reside in the Cassie state60 (inset in Figure
the sliding angle (SA) is decreased, which can be ascribed to the decrease of solid/liquid contact area (more details see discussion in Supporting Information). It is worth noting that although superhydrophobicity can be observed on surfaces with large spacing (20 and 30 μm), low adhesive property is still unavailable (even on the surface with spacing of 30 μm, the SA is still as high as 21°), indicating that on these surfaces with flat pillars, it is difficult to obtain low adhesive superhydrophobicity. Fortunately, nature selections such as lotus leaves and legs of water striders give us the inspiration that hierarchical micro/ nanostructure can effectively enhance the surface hydrophobicity and weaken the surface adhesion.23−25 Through adding nanostructure onto the pillars, it can be seen that the surface hydrophobicity can be further enhanced, and the adhesion can be decreased apparently. As shown in Figure 3b, all the micro/nanostructured surfaces have the superhydrophobicity with CAs larger than 150° (Figure 3b). Meanwhile, compared with those surfaces with flat pillars, surfaces with hierarchical structured pillars have decreased SAs when the spacing is identical, and extremely low adhesive superhydrophobicity can be obtained when the spacing is ≥20 μm (SAs are
