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Wetting Properties of the Multiscaled Nanostructured Polymer and Metallic Superhydrophobic Surfaces Edward Bormashenko,* Tamir Stein, Gene Whyman, Yelena Bormashenko, and Roman Pogreb The College of Judea and Samaria, the Research Institute, 44837, Ariel, Israel ReceiVed June 6, 2006. In Final Form: September 18, 2006 A superhydrophobic surface is produced from industrial grade polymer materials. The surface comprises partly disordered triple-scaled arrays of polyvinylidene fluoride (PVDF) globules. An inherently superhydrophobic metallic surface is produced with polymer template. The mathematical model based on the Cassie-Baxter hypothesis of air trapping under a water drop is built, which gives the apparent contact angle on the manifold-scaled interface. The presence of several scales itself is not a sufficient condition of hydrophobicity of inherently wettable surfaces. The geometrical features favoring the increase of the vapor-water interface fraction are necessary for this phenomenon.
1. Introduction Wetting of textured surface has been studied intensively in the past decade.1-25 Although the main theoretical approaches to the wetting of highly developed relieves were developed by Cassie and Wenzel 50 years ago, the problem still turns to be attractive to investigators. According to the Cassie model, air can remain trapped below the drop, forming “air pockets”. Thus, hydrophobicity is strengthened because the drop sits partially on the air. On the other hand, according to the Wenzel model, the roughness increases the surface area of the solid, which also geometrically modifies hydrophobicity.1 It is conventional to relate so-called moderate hydrophobicity to the Wenzel regime, * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) de Gennes, P. G.; Brochard-Wyart, F.; Que¨re¨, D. Capillarity and Wetting Phenomena; Springer: Berlin, 2003. (2) Gao, L.; McCarthy, Th. J. Langmuir 2006, 22, 2966-2967. (3) Vogelaar, L.; Lammertink, R. G. H.; Wessling, M. Langmuir 2006, 22, 3125-3130. (4) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457-460. (5) Bico, J.; Thiele, U.; Que´re´, D. Colloids Surf., A 2002, 206, 41-46. (6) Sun, M. H.; Luo, C. X.; Xu, L. P.; Ji, H.; Qi, O. Y.; Yu, D. P.; Chen, Y. Langmuir 2005, 21, 8978-8981. (7) Jopp, J.; Gru¨ll, H.; Yerushalmi-Rozen, R. Langmuir 2004, 20, 1001510019. (8) Thiele, U.; Brusch, L.; Bestehorn, M.; Ba¨r, M. Eur. Phys. J. E 2003, 11, 255-271. (9) Herminghaus, S. Europhys. Lett. 2000, 52, 165-170. (10) Shibuichi, A.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512-19517. (11) He, B.; Lee, J.; Patankar, N. A. Colloids Surf., A 2004, 248, 101-104. (12) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1-8. (13) Patankar, N. A. Langmuir 2004, 20, 7097-7102. (14) Van Krevelen, D. W. Properties of Polymers; Elsevier: Amsterdam, 1997. (15) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782. (16) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818-5822. (17) Abdelsalam, M. E.; Bartlett, Ph. N.; Kelf, T.; Baumberg, J. Langmuir 2005, 21, 1753-1757. (18) Han, J. T.; Jang, J.; Lee, D. Y.; Park, J. H.; Song, S.-H.; Ban, D.-Y.; Cho, K. J. Mater. Chem. 2005, 15, 3089-3092. (19) Zhao, N.; Shi, F.; Wang, Z. Q.; Zhang, X. Langmuir 2005, 21, 47134716. (20) Wang, C. H.; Song, Y. Y.; Zhao, H. W.; Xia, X. H. Surf. Sci. 2006, 600, L38-L42. (21) Cui, G. L.; Xu, W.; Zhou, X. H.; Xiao, X. W.; Jiang, L.; Zhu, D. B. Colloids Surf., A 2006, 272, 63-67. (22) Zhai, Lei.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349-1353. (23) Porcheron, F.; Monson, P. A. Langmuir 2006, 22, 1595-1601. (24) Jeong, H. E.; Lee, S. H.; Kim, J. K.; Suh, K. Y. Langmuir 2006, 22, 1640-1645. (25) Baldacchini, T.; Carey, J. E.; Mazur, E. Langmuir 2006, 22, 4917-4919.
whereas the Cassie scenario results in strong water-repellent surface properties. However, the Cassie regime has been reported recently for slightly hydrophobic interfaces; moreover, coexistence of Cassie and Wenzel regimes at the same surfaces has been reported.4,24 Lack of the reproducible experimental data in the field has to be emphasized. The phenomenon of superhydrophobicity when apparent contact angle becomes close to 180° was reported recently by different groups.2,3,8 Various sophisticated techniques (UV, soft lithography,11 and temperature-directed capillary molding24) and materials (perfluoroacrylates4 and alkylketene dimers10) were applied for manufacturing super-water-repellent surfaces. Superhydrophobic surfaces are also found in nature.9,12 The biological expedience of the phenomenon, called the lotus effect, consists of the possibility of self-cleaning of plant leaves because of the rolling of drops without water spreading on the leaf surface.9,12 The underlying physical problem was how hydrophobicity can develop on materials which are partially wettable.9 This phenomenon has been explained by forming the large waterair interface under a water droplet in consequence of the air trapping in pockets of a highly textured substrate. Since the surface energy of the water-air interface is large, the droplet tends to decrease the underlying area increasing the contact angle. Despite significant experimental and theoretical efforts, reproducible inexpensive manufacturing of superhydrophobic surfaces remains problematic. We report triple-scaled superhydrophobic surfaces manufactured with industrial grade polyvinylidene fluoride (PVDF) particles, dispersed at the polyethylene substrate. PVDF is inherently a hydrophilic material;14 its calculated contact angle equals 80-86°, and a measured value on our nanometrically flat samples was 75°. Surfaces, prepared according to our process and comprising nanometrically scaled PVDF beads, demonstrated apparent contact angle as high as 160° (see Figure 1a). Among the recent achievements, metallic superhydrophobic surfaces attract significant attention. Metallic surfaces are wellknown as “high-energy interfaces”, for which the chemical binding is about 1 eV, and on which nearly every liquid spreads.1 Superhydrophobic metallic surfaces are usually obtained with a monolayer of n-dodecanthiol assembled on textured metallic surfaces.15-17 We will demonstrate in our paper that it is possible to form inherently hydrophobic metallic interface, when the later is micrometrically textured, with a contact angle as high as 150°, Figure 1b.
10.1021/la061622m CCC: $33.50 © 2006 American Chemical Society Published on Web 10/24/2006
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Figure 2. Schemes of the manufacturing of highly developed surfaces. Figure 1. (a) Droplets deposited on the polymer triple-scaled relief; (b) droplets on the gold-coated relief.
2. Experimental Section We obtained highly developed polymer surfaces by two different techniques (Figure 2a and b). 1. Tilted substrates were coated with solutions comprising a mixture of PVDF beads with a solution of polycarbonate in chlorinated solvent. Polycarbonate is soluble in the chlorinated solvents, whereas PVDF is not; thus, colloidal suspension was formed, was dripped on the quartz glass substrates as depicted in Figure 2a, and was dried with a room-temperature air current. The structure of the dry film of thickness ∼30 µm was studied by means of scanning electron microscopy and showed interconnected colloidal arrays (ICA), such as presented in Figure 3a and b. These arrays are microscopically scaled aggregates comprised of PVDF particles embedded in PC that filled the porosity between PVDF beads. The average dimension of these aggregates was determined as 3-10 µm. The aggregates incorporate 104-106 PVDF nanoparticles. The relief (coated with gold for the purposes of SEM inspection) displayed in Figure 3 is mentioned further as relief A. Polyvinylidene fluoride nanobeads were supplied by Aldrich, molecular weight Mw ) 534 000, Tg ) 38.0 °C, density F ) 1.74 g/cm3. The average diameter of particles was established as 130 nm. Polycarbonate (PC) Lexan 141 was supplied by GE Plastics. Solvent chloroform CHCl3 (pure for analysis) was supplied by Karlo Erba Reagenti. At the first stage, solutions containing 2-5 wt % of PC dissolved in 98-95 wt % of chloroform were prepared. Then, particles of PVDF (2 wt %) were added under stirring (PVDF is insoluble in chlorinated solvents). Two types of substrates (quartz glass and polypropylene (PP)) were coated, such as depicted in Figure 2a. The slope of the substrate was R ) 19-22°. 2. Under the second technique, a powder comprising PVDF beads has been spread uniformly on the low-density polyethylene film of thickness 100 µm and has been covered with a crimpled surface (see
Figure 2b). This sandwich has been exposed to hot pressing; the final thickness of the PVDF layer was established as 70 µm. The polyethylene substrate had been softened under the pressing and trapped single PVDF particles (which were still solid under the pressing temperature) and globular aggregates comprised of PVDF beads. PVDF aggregates composed of nanometrically scaled beads, frozen in the polyethylene matrix, form a highly developed interface depicted in Figure 4a and b; this random triple-scaled interface is mentioned below as relief B. Pressed stamp indentations form “channels” important for hydrophobicity strengthening, as it will be explained below. Hot pressing has been carried out under t ) 85 °C. The characteristic distance between indentations of the riffled stamp was 100 µm, and the depth of the indentations was 20 µm. At the next stage, reliefs A and B were coated with 360 Å gold films by a sputtering procedure in argon atmosphere. An SPI sputter coater was used. A thickness of coating was determined by time of sputtering. Then, bidistilled water droplets were dripped carefully on the coated templates. The volume of the droplets was 2-5 µL.
3. Results and Discussion The strongest hydrophobicity has been achieved with a surface topography displayed in Figure 4a and b. Three spatial scales are inherent for this topography: 100 µm, the distance between channels; 10 µm, the characteristic size of the PVDF beads aggregate; and 0.1 µm, the diameter of a single PVDF particle. The apparent contact angles are summarized in Table 1. The theory explaining such transition from hydrophility to hydrophobicity occurring on the fractal-like surfaces was proposed recently by Herminghaus,9 where the expression for the apparent contact angle θ* has been derived starting from the Young formula
cos θE )
γSV - γSL γ
(1)
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Figure 3. SEM picture (gold coating) of the surface relief: (a) on the basis of the aggregates comprised of PDVF nanobeads (relief A) and (b) single aggregate of PDVF nanobeads.
Here, θE is the microscopic contact angle, and γSV, γSL, and γ are the surface tension coefficients on the solid-vapor, solidliquid, and liquid-vapor interfaces, respectively. The expression for cos θ* was obtained9 from eq 1 on substitution of γSL by the properly determined energy per unit area of the textured solidliquid interface. Below, we follow a more traditional way ascending to Bico et al.5 on the basis of thermodynamics. Consider the surface textured on two scales with the air trapped in “large” and “small” pockets. Let w be the fraction of large pocket liquidair interface in the underlying substrate surface, and let V be the fraction of small pocket cross section in the relief surface, and let g(θE) be a geometric factor that is the ratio between the relief surface and its projection onto the substrate. For the relief of Figures 3 and 5, g(θE) is a decreasing function of θE. Further, in spirit of ref 5, when a drop of liquid moves in all directions by the distance dx, the apparent surface under the drop increases by ldx where l is the length of the intersection line of the three interfaces. From these, new liquid-vapor interfaces appear over large pockets, wldx, and over small pockets, gV(1 - w)ldx. At last, g2(1 - V)(1 - w)ldx is a new liquid-solid interface replacing
Figure 4. SEM images (gold coating) of the random triple-scaled interface composed of PVDF: (a) bead aggregates deposited on PE substrates (relief B) and (b) detailed SEM image of the channel surrounded with PVDF beads aggregates.
Figure 5. The model of the double-scaled surface (R′/R ) a′/a).
a solid-vapor one. Therefore, the variation of the energy per unit area is
dE ) (w+ gV(1 - w))γ + g2(1 - V)(1 - w)(γSL - γSV) + γ cos θ* (2) where the last term accounts for an increase of the upper liquidvapor interface of the drop. From the minimum condition dE ) 0, we get on account of eq 1
cos θ*) g(1 - w)(g cos θE(1 - V) - V) - w
(3)
For the convex profile of Figure 3 and an acute contact angle, the equilibrium of a liquid-air interface is possible if the latter
Table 1. Wetting Properties of the Textured Surfaces apparent contact angle
relief A
gold-coated relief A
relief B
gold-coated relief B
measured calculated
95 ( 5° 94°
95 ( 5° 97°
160 ( 5° 172°
150 ( 5° 144°
Wetting Properties of Polymer and Metallic Surface
Figure 6. Scheme presenting evaporation of water droplet deposited at the superhydrophobic surface depicted in Figure 4. (a) Kinetics of droplet evaporation, (b) advancing θ1 and receding θ2 contact angles, and acute contact angle θ3 observed at the final stage of evaporation.
descends below the equatorial plane of spheres. It is seen that indentations in the SEM picture are formed by balls of two types with the radii R and R′ (Figure 5) on different size scales, so the Herminghaus mechanism9 can be realized. For numerical estimation, the double-scaled system of spheres shown in Figure 5 was chosen. Put for simplicity w ) V, then from simple trigonometry w )1 - (2πR2 sin2 θE)/((2R + a)2x3), g ) 1/sin2(θE/2), where R is the radius of balls and a is the shortest distance between their surfaces. For θE ) 75° and a ) R, modeling the relief B, we have from eq 3 θ* ) 172°. In the case of a single scale, eq 3 is converted into cos θ* ) (1 - w)cos θE - w from ref 5, that gives the much smaller value θ* ) 86°. To model the experimental situation, we also considered the close-packed spheres case on the large scale, w(a ) 0), and incompact packing on the small scale, V(a ) R), relief A. For gold-coated reliefs, the local contact angle was changed to experimentally established one 60° (see also ref 17). The comparison of the data in Table 1 shows a satisfactory agreement with the experimental data. Close packing of spheres on both scales leads to the complete wetting for the considered contact angles. This is explained by small fraction of a liquid-air interface and large fraction of a liquid-solid interface under the droplet.
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Thus, the manifold-scaling makeup of the surface is not a sufficient condition of hydrophobicity of inherently wettable surfaces. The geometrical features favoring the increase of the vapor-water interface fraction are necessary for this phenomenon. It was instructive to study the kinetics of water droplet evaporation deposited on the superhydrophobic surface depicted in Figure 5. This investigation supplied information concerning advancing θ1 and receding θ2 apparent contact angles (Figure 6). The contact angle hysteresis was estimated as 15-18°. The hysteresis is relatively large for Cassie-Baxter wetting situation. At first glance, our experimental observations contradict recent results presented by Gao and McCarthy.2 They asserted that texturing on two scales, microscopic and nanometric, brings into existence extremely low contact angle hysteresis. However, in the case of PVDF, the local contact angle is much lower, θE ) 75°, than 103° on flat silicon.2 Such a low microscopic angle leads the large fracture 1 - V of the water-PVDF interface on the nanometric scale since more than a half of the nanosphere is in contact with water (see Figure 5). Therefore, the movement of the contact line is hindered and the contact angle hysteresis is substantial. A similar situation has been observed by Baldacchini et al.25 for hexadecane. Contrary to this, in ref 2 the surface of the nanopattern covered with water on tops of posts is very small. For θE )104° and θ* ) 176° from ref 2, we obtain putting, for example, w ) V, g ) 1 in eq 3, the water-air interface fracture V ) 0.95 that corresponds seemingly to the microphotographs.2 Polyethylene and polypropylene possess the close values of a local contact angle (103-107°), so the proposed simple method of the superhydrophobic surface production may give patterns on these materials with a low hysteresis and higher apparent angles. Somewhat surprisingly, the acute apparent angle θ3 has been observed at the final stage of the evaporation, when an initially pinned droplet jumps, as is displayed in Figure 6B. This may be ascribed to the extremely small size of a drop at the final evaporation stage compatible with the distance between channels in Figure 4a. Under these circumstances, the drop can penetrate the channel.
4. Conclusions The simple and inexpensive technique of producing superhydrophobic surfaces is proposed which is based on hot pressing of PVDF beads on polyethylene substrate. The obtained partially ordered triple-scaled surface shows superhydrophobicity with the apparent contact angle as high as 160° although with substantial hysteresis ∼20° between advancing and receding angles. After covering the obtained surface with a gold layer, the measured apparent contact angle was 150°. Thus, both surfaces exhibit the superhydrophobicity on inherently wetting materials. The effect is explained on the basis of the Cassie-Baxter model of air trapping. The corresponding mathematical model takes into account the existence of two scales of the surface and may be easily generalized the arbitrary number of scales. The comparison of the data in Table 1 shows a satisfactory agreement of the model results with the experimental data. The manifold-scaled interface makeup by itself is not a sufficient condition of hydrophobicity of inherently wetted surfaces. The geometrical features favoring the increase of the vapor-water interface fraction are necessary for this phenomenon. LA061622M
