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Enhancement of dynamic wetting properties by direct fabrication on robust micro–micro hierarchical polymer surfaces
Applied Surface Science 300 (2014) 117–123
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Enhancement of dynamic wetting properties by direct fabrication on robust micro–micro hierarchical polymer surfaces Donghui Chu a,b,∗ , Akihiko Nemoto a , Hiroshi Ito a a b
Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Yamagata, Japan Chemical Division, Samsung Cheil Industries Inc., 332-2, Gocheon-Dong, Uiwang-Si 437-711, Gyeonggi-Do, South Korea
a r t i c l e
i n f o
Article history: Received 27 November 2013 Received in revised form 4 February 2014 Accepted 6 February 2014 Available online 15 February 2014 Keywords: Polymethylmethacrylate (PMMA) Superhydrophobic Evaporation Geometric parameter
a b s t r a c t Understanding evaporation phenomena on hierarchical surfaces is of crucial importance for the design of robust superhydrophobic polymer structures for various applications. This fabrication method enables precise control of the dimensions to elucidate the dynamic wetting behavior affected by geometric parameters. That behavior exhibits three distinct evaporation modes: a constant contact line (CCL), a constant contact angle (CCA), and mixed mode during the droplet evaporation. The droplet evaporation results show that the sticky CCL mode and the Cassie–Wenzel transition can be prevented by engineering hierarchy integration. Moreover, the CCL–CCA transition point time scale exhibits remarkable dependence on surface dimensions such as the area fraction and solid–liquid contact line. Finally, the fabricated hierarchical structures indicate remarkable superhydrophobic properties, static contact angle above 160◦ and low sliding angle under 10◦ , with good durability in terms of aging effect and mechanical robustness for 2 months. © 2014 Elsevier B.V. All rights reserved.
Introduction Superhydrophobic surfaces, which exhibit high water droplet contact angles greater than 150◦ , have attracted considerable interest in the last decade because of their excellent water-repellent surface properties [1–5]. Such surfaces show great potential for various applications related to self-cleaning, anti-icing repellence, biotechnology, microdevices and nanodevices, and thermal systems [6–16]. An extremely effective method to achieve superior superhydrophobic properties is the development of a hierarchical surface: a nanostructure within a microstructure. Fundamentally, these hierarchies were inspired by a natural structure: a lotus leaf [17,18]. The lotus leaf superhydrophobic behavior is attributed to surface roughness created by randomly dispersed micrometersized surfaces covered with nanometer-sized fibers. Water droplets on such a surface rest on the peak of the surface features and cannot penetrate into the bottom of the microstructure or nanostructure because of the low surface tension. The contact angle of a water droplet is believed to be dependent on the ratio of the volume of entrapped air to the volume of the surface structure. Because the decreased area fraction of the two-level structure is much smaller
∗ Corresponding author at: Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata, Japan. Tel.: +81 238 26 3081; fax: +81 238 26 3081. E-mail address: [email protected] (D. Chu). http://dx.doi.org/10.1016/j.apsusc.2014.02.017 0169-4332/© 2014 Elsevier B.V. All rights reserved.
than that of the single-level structure, the superhydrophobic state is best achieved through the adoption of hierarchical structures. However, the study of the static hydrophobicity alone is insufficient to ascertain the wetting property of a surface. Although the apparent contact angles on the certain superhydrophobic structure are similar, they can show different dynamic phenomena, being either sticky or slippery. They are differentiated by the contact angle hysteresis of the moving water droplet on the surface. On a slippery surface, a water droplet rolls off easily with little contact angle hysteresis, which is known as the “lotus effect”. However, on superhydrophobic sticky surfaces, a water droplet does not roll off, even when the surface is turned upside down, because the water droplet is strongly pinned on the solid surface. Such surfaces are also apparent in nature, designated as a “petal effect” [19]. Although the contact angle hysteresis on a superhydrophobic surface has been studied actively in equilibrium conditions [20–22], the contact angle hysteresis during an in-phase transition process such as evaporation has remained virtually unexplored. Most studies of droplet evaporation specifically assess phase transition patterns on the pinning–depinning transition: from a constant contact line (CCL) mode to a constant contact angle (CCA) mode [23–26] or on the Cassie–Wenzel transition occurring at the late stage of evaporation involved with the CCA mode to mixed mode [27,28]. Despite extensive progress, fundamental understanding of how the geometric parameters affect the wetting transition is lacking.
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A myriad of methods has been developed to fabricate micro–nano hierarchical structures on polymer surfaces [29–40]. Although numerous fabrication methods can generate superhydrophobic surfaces, it is particularly difficult to control the nanostructure part dimensions precisely within the hierarchical structures. Therefore, fabrication methods must be developed to control the dimensions precisely and thereby elucidate the dynamic wetting behavior affected by geometric parameters. That knowledge will be invaluable to develop optimized polymer structures. A hierarchy can also be achieved on another scale by combining micro–micro hierarchical structures that can easily control the dimensions and increase the mechanical robustness of structures [41,42]. It is assumed that micro–micro hierarchical structures can also exhibit highly hydrophobic properties because of the hierarchy effect. Furthermore, the mechanical robustness of micro–micro hierarchical structures is expected to be better than that of the micro–nano hierarchical structures [38,43]. Our research group has developed a simple and highly accurate fabrication method for well-ordered microstructures and micro–micro hierarchical structures using direct fabrication with a precision tooling machine. During the fabrication process, these structures can be controlled precisely in all single-level and hierarchy processing. For this study, four micropillar and hierarchical structures were prepared to assess the enhancement of superhydrophobic properties related to hierarchical integration and the effect of geometric parameters. We demonstrate dynamic wetting properties on the polymer surface during the in-phase transition process governed by wetting hysteresis. To ascertain how the energy transition point is involved in pinning–depinning and how the Cassie–Wenzel transition is affected by the surface morphology, experimentally obtained data related to water droplet evaporation on PMMA surfaces are presented and analyzed.
Experiments Micropillar and hierarchical structures were fabricated directly on flat polymer surfaces using a precision tooling machine (Robonano ␣-0iB; Fanuc Ltd.). The flat samples used for direct fabrication were prepared using a micro-injection molding machine (AU3E; Nissei Co. Ltd.) with a commercial transparent polymer, polymethylmethacrylate (PMMA, MFR = 3 at 230 ◦ C/3.8 kg, Tg = 115 ◦ C, PM-7200; Cheil Industries Inc.). The single-level micropillar structures were fabricated by placing a needle-type tool in the tooling machine, by setting the machine program parameters, and by then machining micropitted structures. Each linear axis of the pillar structure is inscribed with a linear motion. The cutting tool can fabricate microscopic grooves on the polymer surface at the rate of five grooves per second. The hierarchical structures are prepared using two-step fabrication: a large-scale micropillar structure is formed, followed by formation of a small-scale micropyramidal structure with a smaller cutting tool. The structured area was 1 cm2 . Next, fabricated structures were coated with a—CF2 -based amorphous fluoropolymer using dip-coating (dip-coater, M115S; Asumi Giken Ltd.). A primer solution was prepared for pretreatment to enhance the coating properties. A solution of the primer (CT-P10; Asahi Glass Co. Ltd.) was mixed with a solution (isopropyl alcohol and isobutyl acetate in a ratio of 9:5) diluted to 5% concentration. The structured plate was first coated with a pre-treatment solution at room temperature for 5 min. Then it was dried at room temperature for 30 min. The pre-treated plate was then coated with—CF2 -based amorphous fluoropolymer (CTX109AE, (C6 F10 O)n ; Asahi Glass Co. Ltd.). The schematic structure of this polymer is presented in Fig. 1. The coated plate was dried at room temperature for 1 h. It was then heated at 80 ◦ C for 1 h.
Fig. 1. Schematic structure of the—CF2 -based amorphous fluoropolymer.
Contact angles were measured using the sessile drop method with a contact angle meter (DM 500; Kyowa Interface Science Co. Ltd., Saitama, Japan). Pure DI water (drop volume of about 4 m) was used for the contact angle and for evaporation measurements. All measurements were performed under atmospheric conditions: 23 ± 1 ◦ C, 40 ± 2% humidity, and atmosphere pressure. The values reported herein are averages of at least five measurements of all samples. During evaporation, the evolution of the contact angle and contact radius was measured from the images observed every 30 s for data analysis. Images of patterned surfaces were observed using a scanning electron microscope (FE-SEM, SU-8000; Hitachi High-Technologies Corp.). The structure dimensions were evaluated using a laser spectroscope (LEXT OLS 4000; Olympus Corp., Tokyo, Japan).
Results and discussion Dimensions of fabricated microstructures and hierarchical structures The dimensions and area fractions of fabricated micropillars and hierarchical structures are shown in Table 1. Herein, Mx and my respectively denote the numbers of micropillar structures and micropyramid structures. Hierarchical structures were prepared with four samples, mM1 , mM2 , mM3 , and mM4 , combined individual micropillar structures with fixed micropyramid structures (2.0 m distance, 1.0 m height). The micropillar structures were produced to have equal height (40 m) but with varying pillar width and pillar-to-pillar spacing appearing in a square array for various dimensions in a dewetting Cassie–Baxter state. The dimensions were prepared to have area fractions of the solid–liquid interface: 0.074 in M1 and M2 and 0.184 in M3 and M4 . The width and pillar-to-pillar spacing were then changed in the same area fraction to ascertain the effects of the solid–liquid contact area on hydrophobic properties. The submicro-scale micropyramid structures were applied on the solid–liquid interface parts of micropillars to increase the volume of the three-phase contact line, which can enhance the dewetting properties [1,44]. The micropillar and hierarchical structure dimensions were estimated using laser spectroscopy. The area fraction of the micropillar surface in contact with liquid was calculated according to Eq. (1) below, where X denotes the micropillar width, and D represents the distance between micropillars. area fraction of pillars =
X2 (X + D)2
(1)
Fig. 2 presents SEM images of micropillar and hierarchical structures on a PMMA surface. Well-ordered arrays were observed. Almost all sizes of fabricated structures measured using laser spectroscopy were controlled accurately to less than 10% error.
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Table 1 Dimensionsa and contact angle results of patterned PMMA surfaces. Dimensions (m)
Contact angles
Lower microstructure
Flat M1 M2 M3 M4 mM1 mM2 mM3 mM4
Upper microstructure
˚s
X
D
H
d
z
Theoretical
Static
Tilting
– 0.074 0.074 0.184 0.184 – – – –
– 15 30 15 30 15 30 15 30
– 40 80 20 40 40 80 20 40
– 40 40 40 40 40 40 40 40
– – – – –
– – – – –
2.0
1.0
– 162.5 162.5 152.3 152.3 – – – –
112.2 157.1 155.1 150.4 147.2 161.2 158.3 154.4 151.4
Stuck b 14.0 24.8 35.0 40.4 9.9 16.8 24.1 32.3
a ˚s is the area fraction of the solid surface; X, D, and H respectively stand for the width, spacing, and height of the micro-pillars; d and z respectively represent the spacing and height of the micropyramid. b Droplet does not slip even at an angle greater than 90◦ .
Contact angle results of fabricated structures Contact angles were measured for samples of various dimensions. Table 1 shows the measured static contact angle of the micropillar structures depending on the area fraction fixed with pillar height. It also presents a comparison between single-level micropillar structures and micro–micro hierarchical structures. First, comparison within the micropillar structures reveals the dewetting Cassie formula. Lower area-fraction samples (˚s = 0.074) show higher contact angles (M1 157.1◦ and M2 155.1◦ ) than the higher area-fraction samples do (˚s = 0.184) with M3 150.4◦ and
M4 147.2◦ . Results show fair agreement between the experiment and calculation following the Cassie–Baxter equation [45] based on the equilibrium energy state given by Eq. (2), as cos CB = ˚s cosi − (1 − ˚s ) cos CB
(2)
where is the apparent contact angle for a droplet on a surface, i is the initial contact angle on a smooth surface, and ˚s is the fraction of the solid–liquid interface. Furthermore, the contact angle varies when the pillar width decreases 30 m to 15 m, even at the same area fraction, as shown in Fig. 3(a). Smaller widths M1 and M3 respectively show higher contact angles than those of
Fig. 2. SEM images of coated micro and hierarchical surfaces: (a) M1 , (b) M4 , (c) m1 M1 , (d) m1 M4 , and (e) micropyramid part of m1 M4 .
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Fig. 3. Measurement of static contact angle (a) and tilting angle (b) dependent on dimensions.
M2 and M4 (157.1◦ from 155.1◦ to 150.4◦ from 147.2◦ ). Controlling the pillar width dimension is also more effective to increase the hydrophobic property than pillar-to-pillar space is. For example, only one dimension is controlled in the width or pillar-to-pillar spacing to lower the area fraction, the difference M4 to M1 (9.9 degree, ˚s 0.184 to ˚s 0.074 in same spacing) shows a higher increase than that of M3 to M1 (6.7 degree, ˚s 0.184 to ˚s 0.074 in same pillar width). Presumably, the narrow solid–liquid contact line lowers the surface energy of the solid surface. Therefore, the static contact angle is affected also by the pillar width, and not solely by the area fraction [24,46]. Second, the fabricated hierarchical structures indicate better contact angle results than those of micropillar structures, as depicted in Fig. 3(a). The graph pattern resembles that of the micropillar structures. The comparison reveals the increase of the static contact angle as a result of hierarchical integration. In the Cassie state, the surface contains air traps into which liquid is incapable of penetrating, so that a composite interface is formed between the solid and liquid. Therefore, a structure in the lower area fraction will behave as a stable Cassie surface with higher contact angle and extremely low sliding angle compare with a higher area fraction. Similarly, the presence of a micropyramid structure on the micropillar structure can enhance the stability of dewetting property, which formed a markedly low area fraction and a continuous three-phase contact line. To assess the sliding properties and to find the sliding angles of water droplets on the fabricated surfaces, a sliding test was conducted. The sliding angle is determined at that when the droplet on a tilted surface starts rolling. It can also be expressed in terms of the hysteresis: the difference between the advancing angle and receding angle [47]. Table 1 and Fig. 3(b) present results of the sliding
angle. The patterns in the micropillar and hierarchical structures follow the dewetting Cassie state in direct proportion with the area fraction. Furthermore, hierarchical structures show lower sliding angles than those of micropillar structures, which is ascribed to the presence of a narrow solid–liquid interface on micropyramid parts. On the superhydrophobic surface of ˚s 0.074 with a lower solid fraction and pillar width (15 m), indicating the smallest sliding angle of 9.9◦ . Compared with the static contact angle values, those of sliding angles vary considerably depending on the dimensions. As the area fraction (˚s ) and pillar width (X) decrease from the higher values, dewetting values represented by the sliding angle increased about 69% in micropillars (from M4 to M1 ) and 65% in the hierarchical structures (from mM4 to mM1 ). Durability of fabricated structures We tested the durability of the micropillar and hierarchical superhydrophobic surfaces under ambient conditions. Fig. 4 shows the robustness of all fabricated structures over a period of 2 months: the left-hand and right-hand sides of the y axis respectively show results of the static contact angle and sliding angle, for which no noticeable changes or degradations were observed. In all samples, the contact angles (the static contact angle and sliding angle) varied within 1% of the values over the time period. These results indicate that the superhydrophobic property can be maintained with no aging effect. The fabricated structures maintain well-ordered surface conditions without mechanical damage despite the PMMA brittleness. Moreover, results show that the coated surfaces retain their properties against aging effect in terms of aging time. Contact angles were detected with at least five measurements at different time points.
Fig. 4. Measurements of static contact angle and tilting angle during 2 months:.
(a) micropillar structure and (b) hierarchical structure.
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Fig. 5. Time evolutions of normalized contact radius and contact angle on micropillar structures: (a) M1 , (b) M2 , (c) M3 , and (d) M4 , and hierarchical structures; (e) mM1 , (f) mM2 , (g) mM3 , and (f) mM4 .
Droplet evaporation test This experiment examined dynamic wetting transitions involved in CCL–CCA transition and in the Cassie–Wenzel transition at the late stage of evaporation. Both are remarkably dependent on geometric parameters. Water droplets on all structures exhibited three distinct evaporation modes simultaneously: a CCL mode with a decreasing contact angle in the early stage, a CCA mode with a shrinking contact line, and a mixed mode with diminishing contact line and contact angle (Fig. 5). Although all eight
surfaces show the same pattern with three evaporation stages, results show that the CCL–CCA transition is affected strongly by the geometric parameters and engineering hierarchical structure. The existence of a CCL stage might be explained by consideration of the local forces at the three-phase contact line [23,24]. At the beginning of the evaporation process, the droplet boundary is initially pinned to the substrate with a strong pinning force (FP ). Therefore, the contact radius of the droplet remains constant while the contact angle decreases gradually. The droplet pinning mode persists until it reaches a receding angle ( c ) at the end of
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Fig. 6. Images of the evaporation droplet on the M2 (a) and mM2 (b) surfaces.
the CCL mode. Driven by the energy minimization principle, the contact angle remains a constant value of = c [48]. Consequently, the droplet boundary depins; then the droplet starts to shrink. The depinning force (FD ) per unit length of the apparent droplet boundary can be expressed as shown in Eq. (3). FD = (cos r − cos i )
(3)
Therein, stands for the surface tension of the liquid ( ≈ 72 mN/m for water at 23 ◦ C); i represents the initial contact angle. As presented in Table 2, the depinning force calculated according to Eq. (3) declines dramatically with the decrease of the solid–liquid contact area involved with area fraction and pillar width. The depinning force can be reduced more than two times by control of the dimensions of the micropillar and hierarchical structures. The smallest value of depinning force is indicated in hierarchical mM1 samples with a smaller area fraction and pillar width (˚s 0.074, 15 m pillar width). All four hierarchical structures show a lower value than that of the micropillar structures. Furthermore, the transition point data show that the surface with lower depinning force shows the faster CCL–CCA transition point in the time scale, except for the M2 surface. Therefore, the decrease of the critical depinning force pushed the CCL–CCA transition point to the left-hand side of the time scale. Compared to the corresponding properties of the micropillar structure, hierarchical structures imply a narrow distribution at the transition point. Fig. 5 portrays the representative evaporation graph examples of the fabricated structures with the time periods of 0 days and 2 months. It can be recognized easily that most droplet patterns behave indistinguishably on each surface. Time evolutions of the contact angle and contact radius represent the durability of patterned surfaces in terms of the dynamic wetting property. The faster disappearance of the sticky CCL mode is ascribed to the low depinning force enabled by the reduction of solid–liquid contact area in the micropillar, but also the presence of hierarchical structures. Because of such enhanced wetting
Conclusion
Table 2 CCL–CCA transition point and depinning force. SPL no.
Micropillar structures Transition pointa
1 2 3 4 a b
0.43 0.54 0.50 0.51
± ± ± ±
0.03 0.10 0.02 0.03
Hierarchical structures FD b
Transition point
FD
6.20 8.40 13.25 15.13
0.40 ± 0.02 0.41 ± 0.02 0.41 ± 0.02 0.45 ± 0.01
5.57 6.38 9.36 12.63
Average value of time period in 2 months. Depinning force (mN/m).
property induced by geometric parameters and engineering hierarchy structures, the sticky CCL mode is quickly diminished. Furthermore, we verified the enhanced evaporation dynamic patterns at CCA-mixed mode transition point ascribed to the hierarchy structures for which the presence of the small microstructure on the large-scale microstructure was able to prevent the Cassie–Wenzel transition. Fig. 6(a) and (b) present four timedependent images, respectively depicting the behavior in the time evolution of the evaporation droplets, M2 surface (Fig. 5(b)), and mM2 surface (Fig. 5(f)). For both cases, the pattern of light that is visible beneath the droplets indicates that the initial droplets (first images, CCL) rest on the structures and form air-pockets in a three-phase contact line, indicating the Cassie–Baxter state [27]. The droplet remains in the composite interface, maintaining the large volume of three-phase contact line, until the late stage of CCA mode. At the CCA-mixed transition point, the pattern of light appears to show a shadow beneath the droplet (third images, CCAmixed). It is assumed that the droplet is no longer sustained upon the structures but that it has penetrated into the bottom of the pattern, indicating the energy transition Cassie–Baxter state to Wenzel state. The evaporation pattern of the M2 surface (Fig. 5(b)) shows an extremely short lifetime in CCA mode, followed by an abrupt increase of the contact radius at the CCA-mixed transition point. The discontinuous increase in the contact radius represents a collapse of the water droplet to the bottom of the pattern. However, for the hierarchical structure (Fig. 5(f)), the presence of the small micropyramid structure on the micropillar structure lowers the surface energy. Therefore, the stable CCA mode is maintained until the late stage of evaporation without a droplet collapse occurring. It is noteworthy that, although we observe a Wenzel state at the late stage of evaporation in the hierarchical mM2 surface, the time scale of the stable Cassie–Baxter state is extended to more than 0.9 normalized evaporation time. The hierarchical structure formation can suppress the Cassie–Baxter transition.
Evaporation of a water droplet on a fabricated superhydrophobic polymer surface (contact angle >150◦ ) produces a different dynamic wetting pattern that is remarkably dependent on geometric parameters. We investigated the full spectrum of contact line dynamics during droplet evaporation. Dynamic dewetting is enhanced by hierarchy integration in comparison with micropillar structures, indicating a faster pinning–depinning transition at the early stage of evaporation and stable CCA mode until the late stage of evaporation with maintenance of the dewetting Cassie
D. Chu et al. / Applied Surface Science 300 (2014) 117–123
state. The obtained experimental data closely approximate those obtained from theoretical analysis in the depinning force. Moreover, the surface with a low contact angle hysteresis is observed to enhance evaporation patterns not only for acceleration of a transition to CCA mode, but also for the formation of a composite droplet without transition to the sticky Wenzel state. Finally, the fabricated micropillar and hierarchical structures show superhydrophobic properties, with a static contact angle above 150◦ and a low sliding angle, in addition to good durability in terms of aging effect and mechanical robustness over a two-month period. Acknowledgment This research was supported financially by Cheil Industries Inc., Korea. References [1] Y. Kwon, N. Patankar, J. Choi, J. Lee, Design of surface hierarchy for extreme hydrophobicity, Langmuir 25 (2009) 6129–6136. [2] J.W. Krumpfer, P. Bian, P. Zheng, L. Gao, T.J. McCarthy, Contact angle hysteresis on superhydrophobic surfaces: an ionic liquid probe fluid offers mechanistic insight, Langmuir 27 (2011) 2166–2169. [3] J.Y. Shiu, P. Chen, Addressable protein patterning via switchable superhydrophobic microarrays, Adv. Funct. Mater. 17 (2007) 2680–2686. [4] N. Yoshida, Y. Abe, H. Shigeta, A. Nakajima, H. Ohsaki, K. Hashimoto, T. Watanabe, Sliding behavior of water droplets on flat polymer surface, J. Am. Chem. Soc. 128 (2006) 743–747. [5] S.H. Kim, S.Y. Lee, S.M. Yang, Janus microspheres for a highly flexible and impregnable water-repelling interface, Angew. Chem. Int. Ed. 49 (2010) 2535–2538. [6] R. Blossey, Self-cleaning surfaces—virtual realities, Nat. Mater. 2 (2003) 301–306. [7] R. Fürstner, W. Barthlott, C. Neinhuis, P. Walzel, Wetting and self-cleaning properties of artificial superhydrophobic surfaces, Langmuir 21 (2005) 956–961. [8] Y. Liu, J.H. Xin, C.H. Choi, Cotton fabrics with single-faced superhydrophobicity, Langmuir 28 (2012) 17426–17434. [9] K.K. Varanasi, D. Tao, J.D. Smith, H. Ming, B. Nitin, Frost formation and ice adhesion on superhydrophobic surfaces, Appl. Phys. Lett. 97 (2010) 234102. [10] L. Mishchenko, B. Hatton, V. Bahadur, J.A. Taylor, T. Krupenkin, J. Aizenberg, Design of ice-free nanostructured surfaces based on repulsion of impacting water droplets, ACS Nano 4 (2010) 7699–7707. [11] M.A. Sarshar, C. Swarctz, S. Hunter, J. Simpson, C.H. Choi, Effects of contact angle hysteresis on ice adhesion and growth on superhydrophobic surfaces under dynamic flow conditions, Colloid Polym. Sci. 291 (2013) 427–435. [12] C.H. Choi, C.J. Kim, Droplet evaporation of pure water and protein solution on nanostructured superhydrophobic surfaces of varying heights, Langmuir 25 (2009) 7561–7567. [13] J.B. Boreyko, C.H. Chen, Self-propelled dropwise condensate on superhydrophobic surfaces, Phys. Rev. Lett. 103 (2009) 184501. [14] S. Lee, W. Kim, K. Yong, Overcoming the water vulnerability of electronic devices: a highly water-resistant ZnO nanodevice with multifunctionality, Adv. Mater. 23 (2011) 4398–4402. [15] X. Chen, J. Wu, R. Ma, M. Hua, N. Koratkar, S. Yao, Z. Wang, Nanograssed micropyramidal architectures for continuous dropwise condensation, Adv. Funct. Mater. 21 (2011) 4617–4623. [16] N. Miljkovic, R. Enright, E.N. Wang, Effect of droplet morphology on growth dynamics and heat transfer during condensation on superhydrophobic nanostructured surfaces, ACS Nano 6 (2012) 1776–1785. [17] M. Sun, C. Luo, L. Xu, H. Ji, Q. Ouyang, D. Yu, Y. Chen, Artificial lotus leaf by nanocasting, Langmuir 21 (2005) 8978–8981. [18] H. Tavana, A. Amirfazli, A.W. Neumann, Fabrication of superhydrophobic surfaces of n-hexatriacontane, Langmuir 22 (2006) 5556–5559. [19] N.A. Malvadkar, M.J. Hancock, K. Sekeroglu, W.J. Dressick, M.C. Demirel, An engineered anisotropic nanofilm with unidirectional wetting properties, Nat. Mater. 9 (2010) 1023–1028. [20] N.A. Patankar, Hysteresis with regard to Cassie and Wenzel states on superhydrophobic surfaces, Langmuir 26 (2010) 7498–7503. [21] T. Koishi, K. Yasuoka, S. Fujikawa, T. Ebisuzaki, X.C. Zeng, Coexistence and transition between Cassie and Wenzel state on pillared hydrophobic surface, PNAS 106 (2009) 8435–8440.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhancement of dynamic wetting properties by direct fabrication on robust micro–micro hierarchical polymer surfaces Donghui Chu a,b,∗ , Akihiko Nemoto a , Hiroshi Ito a a b
Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Yamagata, Japan Chemical Division, Samsung Cheil Industries Inc., 332-2, Gocheon-Dong, Uiwang-Si 437-711, Gyeonggi-Do, South Korea
a r t i c l e
i n f o
Article history: Received 27 November 2013 Received in revised form 4 February 2014 Accepted 6 February 2014 Available online 15 February 2014 Keywords: Polymethylmethacrylate (PMMA) Superhydrophobic Evaporation Geometric parameter
a b s t r a c t Understanding evaporation phenomena on hierarchical surfaces is of crucial importance for the design of robust superhydrophobic polymer structures for various applications. This fabrication method enables precise control of the dimensions to elucidate the dynamic wetting behavior affected by geometric parameters. That behavior exhibits three distinct evaporation modes: a constant contact line (CCL), a constant contact angle (CCA), and mixed mode during the droplet evaporation. The droplet evaporation results show that the sticky CCL mode and the Cassie–Wenzel transition can be prevented by engineering hierarchy integration. Moreover, the CCL–CCA transition point time scale exhibits remarkable dependence on surface dimensions such as the area fraction and solid–liquid contact line. Finally, the fabricated hierarchical structures indicate remarkable superhydrophobic properties, static contact angle above 160◦ and low sliding angle under 10◦ , with good durability in terms of aging effect and mechanical robustness for 2 months. © 2014 Elsevier B.V. All rights reserved.
Introduction Superhydrophobic surfaces, which exhibit high water droplet contact angles greater than 150◦ , have attracted considerable interest in the last decade because of their excellent water-repellent surface properties [1–5]. Such surfaces show great potential for various applications related to self-cleaning, anti-icing repellence, biotechnology, microdevices and nanodevices, and thermal systems [6–16]. An extremely effective method to achieve superior superhydrophobic properties is the development of a hierarchical surface: a nanostructure within a microstructure. Fundamentally, these hierarchies were inspired by a natural structure: a lotus leaf [17,18]. The lotus leaf superhydrophobic behavior is attributed to surface roughness created by randomly dispersed micrometersized surfaces covered with nanometer-sized fibers. Water droplets on such a surface rest on the peak of the surface features and cannot penetrate into the bottom of the microstructure or nanostructure because of the low surface tension. The contact angle of a water droplet is believed to be dependent on the ratio of the volume of entrapped air to the volume of the surface structure. Because the decreased area fraction of the two-level structure is much smaller
∗ Corresponding author at: Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata, Japan. Tel.: +81 238 26 3081; fax: +81 238 26 3081. E-mail address: [email protected] (D. Chu). http://dx.doi.org/10.1016/j.apsusc.2014.02.017 0169-4332/© 2014 Elsevier B.V. All rights reserved.
than that of the single-level structure, the superhydrophobic state is best achieved through the adoption of hierarchical structures. However, the study of the static hydrophobicity alone is insufficient to ascertain the wetting property of a surface. Although the apparent contact angles on the certain superhydrophobic structure are similar, they can show different dynamic phenomena, being either sticky or slippery. They are differentiated by the contact angle hysteresis of the moving water droplet on the surface. On a slippery surface, a water droplet rolls off easily with little contact angle hysteresis, which is known as the “lotus effect”. However, on superhydrophobic sticky surfaces, a water droplet does not roll off, even when the surface is turned upside down, because the water droplet is strongly pinned on the solid surface. Such surfaces are also apparent in nature, designated as a “petal effect” [19]. Although the contact angle hysteresis on a superhydrophobic surface has been studied actively in equilibrium conditions [20–22], the contact angle hysteresis during an in-phase transition process such as evaporation has remained virtually unexplored. Most studies of droplet evaporation specifically assess phase transition patterns on the pinning–depinning transition: from a constant contact line (CCL) mode to a constant contact angle (CCA) mode [23–26] or on the Cassie–Wenzel transition occurring at the late stage of evaporation involved with the CCA mode to mixed mode [27,28]. Despite extensive progress, fundamental understanding of how the geometric parameters affect the wetting transition is lacking.
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A myriad of methods has been developed to fabricate micro–nano hierarchical structures on polymer surfaces [29–40]. Although numerous fabrication methods can generate superhydrophobic surfaces, it is particularly difficult to control the nanostructure part dimensions precisely within the hierarchical structures. Therefore, fabrication methods must be developed to control the dimensions precisely and thereby elucidate the dynamic wetting behavior affected by geometric parameters. That knowledge will be invaluable to develop optimized polymer structures. A hierarchy can also be achieved on another scale by combining micro–micro hierarchical structures that can easily control the dimensions and increase the mechanical robustness of structures [41,42]. It is assumed that micro–micro hierarchical structures can also exhibit highly hydrophobic properties because of the hierarchy effect. Furthermore, the mechanical robustness of micro–micro hierarchical structures is expected to be better than that of the micro–nano hierarchical structures [38,43]. Our research group has developed a simple and highly accurate fabrication method for well-ordered microstructures and micro–micro hierarchical structures using direct fabrication with a precision tooling machine. During the fabrication process, these structures can be controlled precisely in all single-level and hierarchy processing. For this study, four micropillar and hierarchical structures were prepared to assess the enhancement of superhydrophobic properties related to hierarchical integration and the effect of geometric parameters. We demonstrate dynamic wetting properties on the polymer surface during the in-phase transition process governed by wetting hysteresis. To ascertain how the energy transition point is involved in pinning–depinning and how the Cassie–Wenzel transition is affected by the surface morphology, experimentally obtained data related to water droplet evaporation on PMMA surfaces are presented and analyzed.
Experiments Micropillar and hierarchical structures were fabricated directly on flat polymer surfaces using a precision tooling machine (Robonano ␣-0iB; Fanuc Ltd.). The flat samples used for direct fabrication were prepared using a micro-injection molding machine (AU3E; Nissei Co. Ltd.) with a commercial transparent polymer, polymethylmethacrylate (PMMA, MFR = 3 at 230 ◦ C/3.8 kg, Tg = 115 ◦ C, PM-7200; Cheil Industries Inc.). The single-level micropillar structures were fabricated by placing a needle-type tool in the tooling machine, by setting the machine program parameters, and by then machining micropitted structures. Each linear axis of the pillar structure is inscribed with a linear motion. The cutting tool can fabricate microscopic grooves on the polymer surface at the rate of five grooves per second. The hierarchical structures are prepared using two-step fabrication: a large-scale micropillar structure is formed, followed by formation of a small-scale micropyramidal structure with a smaller cutting tool. The structured area was 1 cm2 . Next, fabricated structures were coated with a—CF2 -based amorphous fluoropolymer using dip-coating (dip-coater, M115S; Asumi Giken Ltd.). A primer solution was prepared for pretreatment to enhance the coating properties. A solution of the primer (CT-P10; Asahi Glass Co. Ltd.) was mixed with a solution (isopropyl alcohol and isobutyl acetate in a ratio of 9:5) diluted to 5% concentration. The structured plate was first coated with a pre-treatment solution at room temperature for 5 min. Then it was dried at room temperature for 30 min. The pre-treated plate was then coated with—CF2 -based amorphous fluoropolymer (CTX109AE, (C6 F10 O)n ; Asahi Glass Co. Ltd.). The schematic structure of this polymer is presented in Fig. 1. The coated plate was dried at room temperature for 1 h. It was then heated at 80 ◦ C for 1 h.
Fig. 1. Schematic structure of the—CF2 -based amorphous fluoropolymer.
Contact angles were measured using the sessile drop method with a contact angle meter (DM 500; Kyowa Interface Science Co. Ltd., Saitama, Japan). Pure DI water (drop volume of about 4 m) was used for the contact angle and for evaporation measurements. All measurements were performed under atmospheric conditions: 23 ± 1 ◦ C, 40 ± 2% humidity, and atmosphere pressure. The values reported herein are averages of at least five measurements of all samples. During evaporation, the evolution of the contact angle and contact radius was measured from the images observed every 30 s for data analysis. Images of patterned surfaces were observed using a scanning electron microscope (FE-SEM, SU-8000; Hitachi High-Technologies Corp.). The structure dimensions were evaluated using a laser spectroscope (LEXT OLS 4000; Olympus Corp., Tokyo, Japan).
Results and discussion Dimensions of fabricated microstructures and hierarchical structures The dimensions and area fractions of fabricated micropillars and hierarchical structures are shown in Table 1. Herein, Mx and my respectively denote the numbers of micropillar structures and micropyramid structures. Hierarchical structures were prepared with four samples, mM1 , mM2 , mM3 , and mM4 , combined individual micropillar structures with fixed micropyramid structures (2.0 m distance, 1.0 m height). The micropillar structures were produced to have equal height (40 m) but with varying pillar width and pillar-to-pillar spacing appearing in a square array for various dimensions in a dewetting Cassie–Baxter state. The dimensions were prepared to have area fractions of the solid–liquid interface: 0.074 in M1 and M2 and 0.184 in M3 and M4 . The width and pillar-to-pillar spacing were then changed in the same area fraction to ascertain the effects of the solid–liquid contact area on hydrophobic properties. The submicro-scale micropyramid structures were applied on the solid–liquid interface parts of micropillars to increase the volume of the three-phase contact line, which can enhance the dewetting properties [1,44]. The micropillar and hierarchical structure dimensions were estimated using laser spectroscopy. The area fraction of the micropillar surface in contact with liquid was calculated according to Eq. (1) below, where X denotes the micropillar width, and D represents the distance between micropillars. area fraction of pillars =
X2 (X + D)2
(1)
Fig. 2 presents SEM images of micropillar and hierarchical structures on a PMMA surface. Well-ordered arrays were observed. Almost all sizes of fabricated structures measured using laser spectroscopy were controlled accurately to less than 10% error.
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Table 1 Dimensionsa and contact angle results of patterned PMMA surfaces. Dimensions (m)
Contact angles
Lower microstructure
Flat M1 M2 M3 M4 mM1 mM2 mM3 mM4
Upper microstructure
˚s
X
D
H
d
z
Theoretical
Static
Tilting
– 0.074 0.074 0.184 0.184 – – – –
– 15 30 15 30 15 30 15 30
– 40 80 20 40 40 80 20 40
– 40 40 40 40 40 40 40 40
– – – – –
– – – – –
2.0
1.0
– 162.5 162.5 152.3 152.3 – – – –
112.2 157.1 155.1 150.4 147.2 161.2 158.3 154.4 151.4
Stuck b 14.0 24.8 35.0 40.4 9.9 16.8 24.1 32.3
a ˚s is the area fraction of the solid surface; X, D, and H respectively stand for the width, spacing, and height of the micro-pillars; d and z respectively represent the spacing and height of the micropyramid. b Droplet does not slip even at an angle greater than 90◦ .
Contact angle results of fabricated structures Contact angles were measured for samples of various dimensions. Table 1 shows the measured static contact angle of the micropillar structures depending on the area fraction fixed with pillar height. It also presents a comparison between single-level micropillar structures and micro–micro hierarchical structures. First, comparison within the micropillar structures reveals the dewetting Cassie formula. Lower area-fraction samples (˚s = 0.074) show higher contact angles (M1 157.1◦ and M2 155.1◦ ) than the higher area-fraction samples do (˚s = 0.184) with M3 150.4◦ and
M4 147.2◦ . Results show fair agreement between the experiment and calculation following the Cassie–Baxter equation [45] based on the equilibrium energy state given by Eq. (2), as cos CB = ˚s cosi − (1 − ˚s ) cos CB
(2)
where is the apparent contact angle for a droplet on a surface, i is the initial contact angle on a smooth surface, and ˚s is the fraction of the solid–liquid interface. Furthermore, the contact angle varies when the pillar width decreases 30 m to 15 m, even at the same area fraction, as shown in Fig. 3(a). Smaller widths M1 and M3 respectively show higher contact angles than those of
Fig. 2. SEM images of coated micro and hierarchical surfaces: (a) M1 , (b) M4 , (c) m1 M1 , (d) m1 M4 , and (e) micropyramid part of m1 M4 .
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Fig. 3. Measurement of static contact angle (a) and tilting angle (b) dependent on dimensions.
M2 and M4 (157.1◦ from 155.1◦ to 150.4◦ from 147.2◦ ). Controlling the pillar width dimension is also more effective to increase the hydrophobic property than pillar-to-pillar space is. For example, only one dimension is controlled in the width or pillar-to-pillar spacing to lower the area fraction, the difference M4 to M1 (9.9 degree, ˚s 0.184 to ˚s 0.074 in same spacing) shows a higher increase than that of M3 to M1 (6.7 degree, ˚s 0.184 to ˚s 0.074 in same pillar width). Presumably, the narrow solid–liquid contact line lowers the surface energy of the solid surface. Therefore, the static contact angle is affected also by the pillar width, and not solely by the area fraction [24,46]. Second, the fabricated hierarchical structures indicate better contact angle results than those of micropillar structures, as depicted in Fig. 3(a). The graph pattern resembles that of the micropillar structures. The comparison reveals the increase of the static contact angle as a result of hierarchical integration. In the Cassie state, the surface contains air traps into which liquid is incapable of penetrating, so that a composite interface is formed between the solid and liquid. Therefore, a structure in the lower area fraction will behave as a stable Cassie surface with higher contact angle and extremely low sliding angle compare with a higher area fraction. Similarly, the presence of a micropyramid structure on the micropillar structure can enhance the stability of dewetting property, which formed a markedly low area fraction and a continuous three-phase contact line. To assess the sliding properties and to find the sliding angles of water droplets on the fabricated surfaces, a sliding test was conducted. The sliding angle is determined at that when the droplet on a tilted surface starts rolling. It can also be expressed in terms of the hysteresis: the difference between the advancing angle and receding angle [47]. Table 1 and Fig. 3(b) present results of the sliding
angle. The patterns in the micropillar and hierarchical structures follow the dewetting Cassie state in direct proportion with the area fraction. Furthermore, hierarchical structures show lower sliding angles than those of micropillar structures, which is ascribed to the presence of a narrow solid–liquid interface on micropyramid parts. On the superhydrophobic surface of ˚s 0.074 with a lower solid fraction and pillar width (15 m), indicating the smallest sliding angle of 9.9◦ . Compared with the static contact angle values, those of sliding angles vary considerably depending on the dimensions. As the area fraction (˚s ) and pillar width (X) decrease from the higher values, dewetting values represented by the sliding angle increased about 69% in micropillars (from M4 to M1 ) and 65% in the hierarchical structures (from mM4 to mM1 ). Durability of fabricated structures We tested the durability of the micropillar and hierarchical superhydrophobic surfaces under ambient conditions. Fig. 4 shows the robustness of all fabricated structures over a period of 2 months: the left-hand and right-hand sides of the y axis respectively show results of the static contact angle and sliding angle, for which no noticeable changes or degradations were observed. In all samples, the contact angles (the static contact angle and sliding angle) varied within 1% of the values over the time period. These results indicate that the superhydrophobic property can be maintained with no aging effect. The fabricated structures maintain well-ordered surface conditions without mechanical damage despite the PMMA brittleness. Moreover, results show that the coated surfaces retain their properties against aging effect in terms of aging time. Contact angles were detected with at least five measurements at different time points.
Fig. 4. Measurements of static contact angle and tilting angle during 2 months:.
(a) micropillar structure and (b) hierarchical structure.
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Fig. 5. Time evolutions of normalized contact radius and contact angle on micropillar structures: (a) M1 , (b) M2 , (c) M3 , and (d) M4 , and hierarchical structures; (e) mM1 , (f) mM2 , (g) mM3 , and (f) mM4 .
Droplet evaporation test This experiment examined dynamic wetting transitions involved in CCL–CCA transition and in the Cassie–Wenzel transition at the late stage of evaporation. Both are remarkably dependent on geometric parameters. Water droplets on all structures exhibited three distinct evaporation modes simultaneously: a CCL mode with a decreasing contact angle in the early stage, a CCA mode with a shrinking contact line, and a mixed mode with diminishing contact line and contact angle (Fig. 5). Although all eight
surfaces show the same pattern with three evaporation stages, results show that the CCL–CCA transition is affected strongly by the geometric parameters and engineering hierarchical structure. The existence of a CCL stage might be explained by consideration of the local forces at the three-phase contact line [23,24]. At the beginning of the evaporation process, the droplet boundary is initially pinned to the substrate with a strong pinning force (FP ). Therefore, the contact radius of the droplet remains constant while the contact angle decreases gradually. The droplet pinning mode persists until it reaches a receding angle ( c ) at the end of
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Fig. 6. Images of the evaporation droplet on the M2 (a) and mM2 (b) surfaces.
the CCL mode. Driven by the energy minimization principle, the contact angle remains a constant value of = c [48]. Consequently, the droplet boundary depins; then the droplet starts to shrink. The depinning force (FD ) per unit length of the apparent droplet boundary can be expressed as shown in Eq. (3). FD = (cos r − cos i )
(3)
Therein, stands for the surface tension of the liquid ( ≈ 72 mN/m for water at 23 ◦ C); i represents the initial contact angle. As presented in Table 2, the depinning force calculated according to Eq. (3) declines dramatically with the decrease of the solid–liquid contact area involved with area fraction and pillar width. The depinning force can be reduced more than two times by control of the dimensions of the micropillar and hierarchical structures. The smallest value of depinning force is indicated in hierarchical mM1 samples with a smaller area fraction and pillar width (˚s 0.074, 15 m pillar width). All four hierarchical structures show a lower value than that of the micropillar structures. Furthermore, the transition point data show that the surface with lower depinning force shows the faster CCL–CCA transition point in the time scale, except for the M2 surface. Therefore, the decrease of the critical depinning force pushed the CCL–CCA transition point to the left-hand side of the time scale. Compared to the corresponding properties of the micropillar structure, hierarchical structures imply a narrow distribution at the transition point. Fig. 5 portrays the representative evaporation graph examples of the fabricated structures with the time periods of 0 days and 2 months. It can be recognized easily that most droplet patterns behave indistinguishably on each surface. Time evolutions of the contact angle and contact radius represent the durability of patterned surfaces in terms of the dynamic wetting property. The faster disappearance of the sticky CCL mode is ascribed to the low depinning force enabled by the reduction of solid–liquid contact area in the micropillar, but also the presence of hierarchical structures. Because of such enhanced wetting
Conclusion
Table 2 CCL–CCA transition point and depinning force. SPL no.
Micropillar structures Transition pointa
1 2 3 4 a b
0.43 0.54 0.50 0.51
± ± ± ±
0.03 0.10 0.02 0.03
Hierarchical structures FD b
Transition point
FD
6.20 8.40 13.25 15.13
0.40 ± 0.02 0.41 ± 0.02 0.41 ± 0.02 0.45 ± 0.01
5.57 6.38 9.36 12.63
Average value of time period in 2 months. Depinning force (mN/m).
property induced by geometric parameters and engineering hierarchy structures, the sticky CCL mode is quickly diminished. Furthermore, we verified the enhanced evaporation dynamic patterns at CCA-mixed mode transition point ascribed to the hierarchy structures for which the presence of the small microstructure on the large-scale microstructure was able to prevent the Cassie–Wenzel transition. Fig. 6(a) and (b) present four timedependent images, respectively depicting the behavior in the time evolution of the evaporation droplets, M2 surface (Fig. 5(b)), and mM2 surface (Fig. 5(f)). For both cases, the pattern of light that is visible beneath the droplets indicates that the initial droplets (first images, CCL) rest on the structures and form air-pockets in a three-phase contact line, indicating the Cassie–Baxter state [27]. The droplet remains in the composite interface, maintaining the large volume of three-phase contact line, until the late stage of CCA mode. At the CCA-mixed transition point, the pattern of light appears to show a shadow beneath the droplet (third images, CCAmixed). It is assumed that the droplet is no longer sustained upon the structures but that it has penetrated into the bottom of the pattern, indicating the energy transition Cassie–Baxter state to Wenzel state. The evaporation pattern of the M2 surface (Fig. 5(b)) shows an extremely short lifetime in CCA mode, followed by an abrupt increase of the contact radius at the CCA-mixed transition point. The discontinuous increase in the contact radius represents a collapse of the water droplet to the bottom of the pattern. However, for the hierarchical structure (Fig. 5(f)), the presence of the small micropyramid structure on the micropillar structure lowers the surface energy. Therefore, the stable CCA mode is maintained until the late stage of evaporation without a droplet collapse occurring. It is noteworthy that, although we observe a Wenzel state at the late stage of evaporation in the hierarchical mM2 surface, the time scale of the stable Cassie–Baxter state is extended to more than 0.9 normalized evaporation time. The hierarchical structure formation can suppress the Cassie–Baxter transition.
Evaporation of a water droplet on a fabricated superhydrophobic polymer surface (contact angle >150◦ ) produces a different dynamic wetting pattern that is remarkably dependent on geometric parameters. We investigated the full spectrum of contact line dynamics during droplet evaporation. Dynamic dewetting is enhanced by hierarchy integration in comparison with micropillar structures, indicating a faster pinning–depinning transition at the early stage of evaporation and stable CCA mode until the late stage of evaporation with maintenance of the dewetting Cassie
D. Chu et al. / Applied Surface Science 300 (2014) 117–123
state. The obtained experimental data closely approximate those obtained from theoretical analysis in the depinning force. Moreover, the surface with a low contact angle hysteresis is observed to enhance evaporation patterns not only for acceleration of a transition to CCA mode, but also for the formation of a composite droplet without transition to the sticky Wenzel state. Finally, the fabricated micropillar and hierarchical structures show superhydrophobic properties, with a static contact angle above 150◦ and a low sliding angle, in addition to good durability in terms of aging effect and mechanical robustness over a two-month period. Acknowledgment This research was supported financially by Cheil Industries Inc., Korea. References [1] Y. Kwon, N. Patankar, J. Choi, J. Lee, Design of surface hierarchy for extreme hydrophobicity, Langmuir 25 (2009) 6129–6136. [2] J.W. Krumpfer, P. Bian, P. Zheng, L. Gao, T.J. McCarthy, Contact angle hysteresis on superhydrophobic surfaces: an ionic liquid probe fluid offers mechanistic insight, Langmuir 27 (2011) 2166–2169. [3] J.Y. Shiu, P. Chen, Addressable protein patterning via switchable superhydrophobic microarrays, Adv. Funct. Mater. 17 (2007) 2680–2686. [4] N. Yoshida, Y. Abe, H. Shigeta, A. Nakajima, H. Ohsaki, K. Hashimoto, T. Watanabe, Sliding behavior of water droplets on flat polymer surface, J. Am. Chem. Soc. 128 (2006) 743–747. [5] S.H. Kim, S.Y. Lee, S.M. Yang, Janus microspheres for a highly flexible and impregnable water-repelling interface, Angew. Chem. Int. Ed. 49 (2010) 2535–2538. [6] R. Blossey, Self-cleaning surfaces—virtual realities, Nat. Mater. 2 (2003) 301–306. [7] R. Fürstner, W. Barthlott, C. Neinhuis, P. Walzel, Wetting and self-cleaning properties of artificial superhydrophobic surfaces, Langmuir 21 (2005) 956–961. [8] Y. Liu, J.H. Xin, C.H. Choi, Cotton fabrics with single-faced superhydrophobicity, Langmuir 28 (2012) 17426–17434. [9] K.K. Varanasi, D. Tao, J.D. Smith, H. Ming, B. Nitin, Frost formation and ice adhesion on superhydrophobic surfaces, Appl. Phys. Lett. 97 (2010) 234102. [10] L. Mishchenko, B. Hatton, V. Bahadur, J.A. Taylor, T. Krupenkin, J. Aizenberg, Design of ice-free nanostructured surfaces based on repulsion of impacting water droplets, ACS Nano 4 (2010) 7699–7707. [11] M.A. Sarshar, C. Swarctz, S. Hunter, J. Simpson, C.H. Choi, Effects of contact angle hysteresis on ice adhesion and growth on superhydrophobic surfaces under dynamic flow conditions, Colloid Polym. Sci. 291 (2013) 427–435. [12] C.H. Choi, C.J. Kim, Droplet evaporation of pure water and protein solution on nanostructured superhydrophobic surfaces of varying heights, Langmuir 25 (2009) 7561–7567. [13] J.B. Boreyko, C.H. Chen, Self-propelled dropwise condensate on superhydrophobic surfaces, Phys. Rev. Lett. 103 (2009) 184501. [14] S. Lee, W. Kim, K. Yong, Overcoming the water vulnerability of electronic devices: a highly water-resistant ZnO nanodevice with multifunctionality, Adv. Mater. 23 (2011) 4398–4402. [15] X. Chen, J. Wu, R. Ma, M. Hua, N. Koratkar, S. Yao, Z. Wang, Nanograssed micropyramidal architectures for continuous dropwise condensation, Adv. Funct. Mater. 21 (2011) 4617–4623. [16] N. Miljkovic, R. Enright, E.N. Wang, Effect of droplet morphology on growth dynamics and heat transfer during condensation on superhydrophobic nanostructured surfaces, ACS Nano 6 (2012) 1776–1785. [17] M. Sun, C. Luo, L. Xu, H. Ji, Q. Ouyang, D. Yu, Y. Chen, Artificial lotus leaf by nanocasting, Langmuir 21 (2005) 8978–8981. [18] H. Tavana, A. Amirfazli, A.W. Neumann, Fabrication of superhydrophobic surfaces of n-hexatriacontane, Langmuir 22 (2006) 5556–5559. [19] N.A. Malvadkar, M.J. Hancock, K. Sekeroglu, W.J. Dressick, M.C. Demirel, An engineered anisotropic nanofilm with unidirectional wetting properties, Nat. Mater. 9 (2010) 1023–1028. [20] N.A. Patankar, Hysteresis with regard to Cassie and Wenzel states on superhydrophobic surfaces, Langmuir 26 (2010) 7498–7503. [21] T. Koishi, K. Yasuoka, S. Fujikawa, T. Ebisuzaki, X.C. Zeng, Coexistence and transition between Cassie and Wenzel state on pillared hydrophobic surface, PNAS 106 (2009) 8435–8440.
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