Fabrication of Spatially Periodic Double Roughness Structures by

Jan 4, 2008 - Fingering and Spinodal Dewetting for Water-Repellent Surfaces. Akihiro Kuroda, Tsukasa Ishihara, Hikari Takeshige, and Kouichi Asakura*...
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J. Phys. Chem. B 2008, 112, 1163-1169

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Fabrication of Spatially Periodic Double Roughness Structures by Directional Viscous Fingering and Spinodal Dewetting for Water-Repellent Surfaces Akihiro Kuroda, Tsukasa Ishihara, Hikari Takeshige, and Kouichi Asakura* Department of Applied Chemistry, Faculty of Science and Technology, Keio UniVersity, 3-14-1, Hiyoshi, Kohoku, Yokohama 223-8522, Japan ReceiVed: August 4, 2007; In Final Form: October 16, 2007

Water-repellent and self-cleaning properties of lotus leaves are considered to be due to its double roughness structure, protrusion structure (∼20 µm) and hairy structure (0.2-1.0 µm). In this study, attempts to fabricate a spatially periodic double roughness structure by two far-from-equilibrium self-organization phenomena, a directional viscous fingering and a spinodal dewetting, were made. A mixture of an octylsilyl titanium dioxide particle having an average diameter of 35 nm suspended in volatile silicone, decamethyl cyclopentasiloxane, and octyl p-methoxycinnamate was spread on a glass plate by dragging an applicator across the top. Formation of a stripe pattern parallel to the direction of dragging, called directional viscous fingering, was sometimes observed. Influences of spreading conditions on the pattern formation were analyzed. In addition, attempts were made to apply the stripe pattern formation to the preparation of a water repellent surface. We have succeeded in preparing a highly water-repellent surface by immersing a glass plate, on which a spatially periodic stripe pattern having a characteristic wavelength of 200-700 µm was formed, in water, after the completion of evaporation of decamethyl cyclopentasiloxane. In this case, dewetting patterns having a characteristic wavelength at around 5 µm were formed at the bottom part of the stripe patterns. Neither the surface on which only the mesoscopic spatially periodic stripe pattern was formed nor the one on which only the microscopic dewetting pattern was formed showed high water-repellent properties, indicating that the coexistence of the two different scales of patterns increased the water-repellent properties of the hydrophobic surface.

1. Introduction Temporal and spatial patterns are ubiquitous in nature. Biological oscillations such as cardiac pulsation and circadian rhythm in the cell cycle are typical examples of the temporal patterns. Also, spatially periodic concentration patterns of pigments can be extensively observed in the body surface of animals. During the last half of 20th century, the mechanism of temporal and spatial pattern formation in far-from-equilibrium systems has been extensively recognized. The concept called dissipative structure proposed by Ilya Prigogine, who was awarded the 1977 Nobel Prize in Chemistry, explained that the pattern formations are from self-organization in far-fromequilibrium systems due to the growth of fluctuations.1-3 Many theoretical and experimental works in this field have been developed to show how these patterns are formed in far-fromequilibrium systems.4 Although, new technology based on the concept of dissipative structure in the field of material science has scarcely been developed, it might lead to a true revolution in this field.5 For the spatial concentration pattern formation in a chemical reaction-diffusion system, Alan Turing proposed the first nonlinear kinetic model in 1952.6 Up until 1990, the Turing pattern has not been experimentally realized.7 Even now, not many material scientists are interested in the Turing pattern formation. Contrary to the chemical reaction-diffusion system, it has been widely recognized that many kinds of patterns are generated when the moving interface becomes unstable to the * Corresponding author. Phone: +81-45-566-1553. Fax: +81-45-5661560. E-mail: [email protected].

fluctuation in its shape. This phenomenon is called fingering instability, since the growth of fluctuation results in fingering patterns. Formation of snow crystals,8 dendrite growth of bacterial colonies,9 and viscous fingering10-13 are examples of spatial pattern formations lead by the growth of fluctuation. Viscous fingering is a fingering pattern formation through a morphological instability generated at the mobile interface of two immiscible fluids. When a low viscous fluid displaces a high viscous fluid, fluctuation in morphology in the interface grows to lead to fingering instability. There is a critical wavelength in fluctuation above which the fluctuation grows. Since the growth rate of fluctuation is a function of its wavelength, viscous fingering shows characteristic spatial periodicity corresponding to the fastest growing finger. It has been well-known among engineers that spatially periodic stripe patterns form when a thin film of viscous fluid is produced by passing it through a small gap.14-18 The same kind of spatially periodic stripe pattern formations are also observed by peeling two pieces of adhesive tape that are stuck to each another.19-21 In all of these cases, stripe patterns are a trace of viscous fingering called directional viscous fingering16,18 which is generated when air displaces a viscous fluid or adhesive. Here, we have focused on the directional viscous fingering as the procedure to fabricate spatially periodic surface roughness. Surface roughness of hydrophobic material tends to enhance its water-repellent properties.22-27 The earliest theoretical works described two possibilities for the mechanism of the enhancement of water-repellent properties by the surface roughness, that

10.1021/jp076254p CCC: $40.75 © 2008 American Chemical Society Published on Web 01/04/2008

1164 J. Phys. Chem. B, Vol. 112, No. 4, 2008 is, the Wenzel theory28,29 and the Cassie theory.30,31 In the Wenzel theory, water droplets are assumed to fill up the grooves on the surface, while composite contact of the water droplets to the rough surface is assumed in the Cassie theory. Contact angles of water droplets on the same rough surface predicted by the two theories are thus different, indicating that the two distinct contact angles on the same condition are theoretically possible.32,33 The two theories along with energy analysis predicted that double or multiple roughness structures similar to the surface of lotus leaves are appropriate for highly waterrepellent surfaces.34 Mesoscopic protrusion structures (∼20 µm) and microscopic hairy structures (0.2-1.0 µm) coexist on lotus leaves. Many attempts were made to fabricate double or multiple roughness structures in order to prepare super-water-repellent surfaces.35-41 These methods included making replicas of natural plant leaves by molding,35 vapor-induced phase separation,36 electrochemical polymerization,37 combination of photolithography and alignment of carbon nanotube followed by deposition of fluorocarbon film,38 laser-etching,39 preparation of a raspberrylike silica particle followed by its chemical deposition to epoxy film,40 and mineralization from aqueous solution of zinc salt directed by amino acids and peptides.41 All of these methods, however, are unlikely to be available for industrial production processes of water-repellent surfaces because of their high cost of processing and the difficulty in magnifying the scale of production. In this study, not only directional viscous fingering but also spinodal dewetting42-45 was applied to the fabrication of the surface roughness. Dewetting is a rupture of thin liquid film on a solid substrate and occurs when the spreading coefficient changes its sign from positive to negative. The first stage of dewetting is spinodal dewetting, that is, barrierless spontaneous instability for formation of holes by amplification of surface fluctuations. The spinodal dewetting is thus categorized in the concept of dissipative structure. There is also a critical wavelength of fluctuation that grows in spinodal dewetting, and the hole formed is spatially periodic corresponding to the fastest growing fluctuation. The spinodal dewetting is followed by the growth and coalescence of holes to form spatially periodic long cylindrical ridged or droplet patterns. We recently analyzed the dewetting pattern formation of liquid films containing octyl p-methoxycinnamate (OPMC) and an octylsilyl titanium dioxide particle having an average diameter of 35 nm (OSI-TIO2-35) on a glass plate.46 In this study, a suspension of OSI-TIO2-35 in volatile silicone, decamethyl cyclopentasiloxane (DMCPSI), was mixed with OPMC, and the mixture was spread on a glass plate by dragging an applicator using a linear motor coater. Spatially periodic stripe patterns were formed spontaneously in some cases by the directional viscous fingering. We have succeeded in fabricating the spatially periodic double surface roughness by the combination of directional viscous fingering and spinodal dewetting. The resulting surface had a double roughness structure that exhibited highly water-repellent properties. 2. Experimental Section 2.1. Materials. A volatile silicone, DMCPSI, was purchased from Shin-Etsu Chemical Company, Ltd. The oil, OPMC, was purchased from Hoffman-La Roche Ltd. A hydrophobic particle having an average particle size of 35 nm and specific surface area of 40 m2‚g-1, OSI-TIO2-35, was supplied from TAYCA Corporation. The suspension of OSI-TIO2-35 in DMCPSI was mixed with OPMC, and the mixture was diluted with DMCPSI. Viscosity of the suspension was measured using a rotational

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Figure 1. Linear motor coat assembly for performing a dragging coat at constatnt velocity. (a) Linear motor actuator, (b) stage, (c) stopper gate, (d) glass plate, (e) applicator, (f) sample liquid.

Figure 2. Two ways of applying a dragging coat on a glass plate. (a) Setting the gap of the applicator at 0.5 minch ()12.7 µm). (b) Putting the central part of the applicator onto glass plate.

double coaxial cylinder viscometer (MR-3 Soliquidmeter, Rheology, Company, Ltd.). 2.2. Dragging Coat Using Linear Motor Coater. The linear motor coater was assembled from a linear motor actuator (LMS30-C2-L555-5.0, PBA Systems. Pte Ltd.). As shown in Figure 1, a stage was installed on the top of the linear motor actuator so that the sample glass plate placed on the stage moves at constant velocity. The applicator (SA-201, Tester Sangyo Company, Ltd.) was put on the plate attached to the stopper gate bridged over the actuator. The suspension of OSI-TIO235 and OPMC in DMCPSI was dropped beside the applicator on the plate and spread by moving the stage at constant velocity. The dragging velocity was varied from 1.0 × 10-2 m‚s-1 to 1.0 m‚s-1, and two ways of the dragging coat were performed as illustrated in Figure 2. One is setting the gap of application at 0.5 minch ()12.7 µm) and the suspension was spread on a glass plate having a size of 20 cm × 20 cm. The second is putting the central part of the applicator on a glass plate having a size of 5 cm × 20 cm. Total length and length of the gapped part of the applicator is 11.5 cm and 7.5 cm, respectively. Thus, there is no assigned gap between the applicator and the glass plate in the latter case, and the coating was performed by squashing the suspension under the weight of the applicator. Weight of the applicator was 348.6 g. After the dragging coat, the plate was placed in an oven at 60 °C for 1 h to let the DMCPSI evaporate. In this process, OPMC was scarcely removed. In some cases, the plate was immersed in a 35 °C water shower flowing at 4 L‚min-1 for 1 min to generate dewetting patterns and dried in an oven at 60 °C for 10 min. 2.3. Analyisis of Surface Structure. Patterns formed on the surface were observed by an optical microscope (BX51, Olympus Corporation) and a field emission scanning electron microscope (FE-SEM, SIRION, FEI Company). Surface struc-

Spatially Periodic Double Roughness Structures

Figure 3. Relation between shear rate and shearing stress of the suspension in samples A, B, and C used for the dragging coat.

ture was analyzed by a surface profile measuring system (DEKTAK 3030, Sloan Technology Corporation). 2.4. Characterization of Water-Repellent Properties. Waterrepellent properties of the surface were characterized by measuring the contact angle of a 2.0 µL water droplet on the sample surface by contact angle meter (DropMaster 500, Kyowa Interface Science Company, Ltd.). The contact angle was calculated from the CCD camera image of the water droplet by integrated multifunctional analysis software (FAMAS, Kyowa Interface Science Company, Ltd.). 3. Results and Discussion 3.1. Properties of the Suspension for Coating. Three types of suspensions (samples A, B, and C), mixtures of OSI-TIO235, OPMC, and DMCPSI, were prepared by varying the content of DMCPSI. The weight ratio of OSI-TIO2-35/OPMC/DMCPSI was A 3.2:1.0:4.8, B 3.2:1.0:6.8, and C 3.2:1.0:8.8, respectively, so that the ratio of OSI-TIO2-35 and OPMC was kept constant. All of the samples are not Newtonian fluids, since no linear relation between shearing stress and share rate was observed and the viscosity decreased when increasing the share rate as shown in Figure 3. It was, however, clear that the viscosity decreased when increasing the content of DMCPSI,

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Figure 5. Surface structure analysis by surface profile measuring system DEKTAK 3030. Sample C was coated by setting the applicator gap at 12.7 µm. The dragging velocity was 7.0 × 10-1 m‚s-1. The dragging coat conditions were the same as the one for Figure 4a.

since the inclination of the curve was the lowest for sample C and the highest for sample A at every share rate. 3.2. Stripe Pattern Formation by Dragging Coat. Examples of the optical microscope images of the surface after the dragging coat and evaporation of DMCPSI are shown in Figure 4. The formation of a stripe pattern parallel to the dragging direction was observed in these cases. Characteristics of the stripe patterns could be analyzed by the surface profile measuring system. An example of the measurement is shown in Figure 5. From the chart, the characteristic length of a spatially periodic stripe pattern, λ, was calculated by averaging each distance between adjacent top and bottom peaks. Heights of the top and bottom of the stripe pattern, hT and hB, were calculated by averaging each height of the top and bottom. Stripe pattern formation by the dragging coat method was strongly influenced by dragging velocity, dragging procedure, and viscosity of the suspension. Double plots at the same dragging velocity in Figure 6 indicate the generation of the spatially periodic stripe pattern by directional viscous fingering. A critical point on the dragging velocity for the fingering instability, VC, was clearly observed in every case. In the case of setting the gap of the applicator at 12.7 µm, the directional viscous fingering was observed when the dragging velocity was equal to or more than 1.5 × 10-1 m‚s-1 for sample A and 2.5

Figure 4. Examples of optical microscope images of the surface after the dragging coat and evaporation of DMCPSI. Sample C was coated, and the dragging velocity was 7.0 × 10-1 m‚s-1. Coating was performed (a) by setting the applicator gap at 12.7 µm and (b) by squashing the suspension under the weight of the applicator.

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Figure 6. Relation between dragging velocity and height of the top and bottom of the liquid film, hT and hB. Samples A, B, and C were coated. Two ways of coating: setting the gap between the applicator and the glass plate at 12.7 µm and zero, respectively.

Figure 7. Relation between dragging velocity and characteristic length of spatial periodicity of the stripe pattern for the samples A, B, and C.

× 10-1 m‚s-1 for samples B and C, so that the value of VC increased when decreasing the viscosity of the suspension. When the coating was performed by squashing the suspension under the weight of the applicator, the value of VC also increased as viscosity of the liquid decreased. It was 3.0 × 10-2 m‚s-1, 5.0 × 10-2 m‚s-1, and 1.0 × 10-1 m‚s-1 for samples A, B, and C, respectively. The value of VC by squashing the suspension under the weight of the applicator was thus always smaller than by setting the gap of the applicator at 12.7 µm. Hekim et al. reported that VC was inversely proportional to the viscosity and proportional to the gap between the roller and the plate when the roller was rotated on a glass plate wetted by silicone oil.16 Thus, the same tendency was observed on the spontaneous generation of the directional viscous fingering by a dragging coat in this study. After the bifurcation point, the height of the bottom, hB, gradually decreased and then slightly increased as dragging velocity increased, while the height of the top, hT, suddenly increased, decreased, and then slightly increased. The tendency was the same for both dragging procedures, although the values were extremely different with respect to each other. In the case of the squashing coat, both hB and hT were much smaller than

the ones obtained by setting the gap of the applicator at 12.7 µm. The characteristic length of the stripe pattern, λ, decreased and then saturated when increasing the dragging velocity in both coating procedures as shown in Figure 7. The value of λ by the squashing coat was much smaller than the coating made by setting the gap of the applicator at 12.7 µm. The profile of the positions of back and front meniscus in the case of the rotating roller on the glass plate wetted by silicone oil reported by Hekim et al. is the same.16 This indicates that the mechanism of generation of directional viscous fingering by the dragging coat method in this study and the rotating roller on the glass plate wetted by silicone oil is the same. A bifurcation diagram that was similar to Figure 6 was obtained for the relation between the dragging velocity and the wavenumber of spatial periodicity of the stripe pattern, q ()2π/λ), as shown in Figure 8. Bifurcation point increased when decreasing the viscosity of the samples and increasing the gap of the applicator. 3.3. Dewetting Pattern Formation by Water Processing. Attempts to generate a dewetting pattern on the liquid film prepared by the dragging coat method were performed by treating them with water. In our previous study, the plate on which the liquid film was formed was immersed in stationary

Spatially Periodic Double Roughness Structures

Figure 8. Bifurcation diagram as a relation between dragging velocity and wavenumber of spatial periodicity of the stripe pattern for the samples A, B, and C.

water at 35 °C for 10 min.46 In order to speed up the formation of the dewetting patterns, the water processing was carried out by immersing the plate into a 35 °C water shower flowing at 4 L‚min-1 for 1 min. Examples of FE-SEM images of the surface before and after the water processing are shown in Figure 9. In these cases, sample C was spread at 7.0 × 10-1 m‚s-1, so that the sample surfaces before the water processing for these FESEM images are the same as the ones for the optical microscope images shown in Figure 4. Formation of the dewetting pattern was observed only when the liquid film coated by squashing the suspension under the weight of the applicator was treated with water. In addition, the dewetting pattern was generated only at the bottom part of the stripe pattern. In all other experiments, by varying the dragging velocity and changing the suspension samples, the dewetting pattern was generated only at the bottom part of the stripe pattern formed by the squashing coat. In every case, the values of hB and hT were scarcely changed by the water processing. In a previous report, we have shown that there is a critical thickness of the liquid film consisting of OSI-TIO2-35 and OPMC below which the dewetting occurs. The liquid film having a weight ratio of OSI-TIO2-35/OPMC ) 7:3 underwent dewetting when the film weight was 0.100 mg‚cm-1 but not when it was 0.200 mg‚cm-1.46 Since the density of the

J. Phys. Chem. B, Vol. 112, No. 4, 2008 1167 suspension is 1.67 g‚cm-3, the film thickness corresponded to 0.60 and 1.20 µm, respectively. In the case of the liquid film prepared by setting the gap of the applicator at 12.7 µm, the minimum liquid film thickness, 1.50 µm, was realized at the bottom part of the stripe pattern as shown in Figure 6. It was generated by spreading the sample C at 8.0 × 10-1 m‚s-1, and even this thinnest part was thicker than the critical film thickness for dewetting, so that all liquid film prepared by setting the gap of the applicator at 12.7 µm did not undergo dewetting. On the other hand, the thickness of the bottom part of the stripe pattern formed by the squashing coat method was always less than 0.60 µm. The top part of the stripe pattern, however, was thicker than the critical thickness. The dewetting structure was thus formed only at the bottom part of the stripe pattern. The simple method to fabricate double roughness structures was thus established by the combination of coating the hydrophobic viscous liquid on a glass plate to generate the directional viscous fingering and the water processing to generate a dewetting pattern. The double roughness structure is a coexistence of the mesoscopic spatially periodic stripe pattern having characteristic wavelength of 200-700 µm and the microscopic spatially periodic dewetting pattern having a characteristic wavelength at around 5 µm. Although both scales are about 10 times larger than the scales of two types of surface roughness on the lotus leaf, the water-repellent properties were evaluated. 3.4. Water Repellent Property. The contact angle of water on the flat surface of the liquid film having a weight ratio of OSI-TIO2-35/OPMC ) 3.2:1.0 was determined to be 95 ( 2° and independent of the film thickness. All of the samples before the water processing possessed almost the same water-repellent properties as the flat surface shown in Figure 10, indicating that the mesoscopic stripe pattern alone cannot enhance the water-repellent properties. In addition, every sample on which only a microscopic dewetting pattern was formed as prepared in our previous report46 did not show any strong water-repellent properties, indicating that the microscopic dewetting pattern alone cannot enhance the water-repellent properties. The water processing did not influence the water-repellent properties of the samples coated by setting the gap of the applicator at 12.7 µm, since no surface structural change was generated by the water processing. On the other hand, the water-repellent properties of the surfaces prepared by squashing the suspension under the weight of the applicator was enhanced by the water processing. In addition, it was strongly influenced by the velocity

Figure 9. Examples of FE-SEM images of the surface before and after the water processing. Sample C was spread at 7.0 × 10-1 m‚s-1.

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Figure 10. Contact angle of water on the surfaces before and after the water processing. Samples A, B, and C were coated. Two ways of coating: setting the gap between the applicator and the glass plate at 12.7 µm and zero, respectively.

of dragging. The contact angle of the water suddenly increased when the velocity of dragging was above the bifurcation point for the generation of the directional viscous fingering. In these cases, a double roughness structures, that is, coexistence of mesoscopic spatially periodic stripe pattern by the directional viscous fingering and microscopic dewetting pattern by the spinodal dewetting, were formed. It is thus clear that fabricating the double roughness structure is essential to make the surface of the liquid film highly water-repellent. 4. Conclusion Recent theoretical and experimental works showed that double or multiple roughness structures similar to the surface of lotus leaves are appropriate for highly water-repellent surfaces.32-41 Here, we have developed a simple method to fabricate double roughness structures; coexistence of the mesoscopic (200-700 µm) spatially periodic stripe pattern by the directional viscous

fingering and the microscopic (∼5 µm) dewetting pattern by the spinodal dewetting, for preparing highly water-repellent surfaces. Both phenomena are categorized as a dissipative structure, that is, growth of fluctuations in a nonequilibrium system.1-4 Although the concept of dissipative structure was awarded the Nobel Prize in Chemistry in 1977, industrial technology based on the dissipative structure methods have not been developed. The dissipative structure basis for new technology may lead to a true revolution in the field of material science,5 since the concept itself is quite new in this field. The method to prepare a highly water-repellent surface by dragging a coat of viscous hydrophobic liquid followed by the water processing is presented in this article. It is thus very simple, and magnification in order to scale production is easily possible. Acknowledgment. We thank Keio University Center for Research Promotion’s Grant Programs for Researchers and Grant-in-Aid for the 21st Century COE program “Keio Life

Spatially Periodic Double Roughness Structures Conjugate Chemistry” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan for supporting this work. We also thank Dr. Koichi Terasaka for his help on the measurement of the viscosity and thank Dr. Daniel P. Predecki for his useful discussions. References and Notes (1) Nicolis, G.; Prigogine, I. Self-organization in Non-equilibrium Systems; Wiley: New York, 1977. (2) Kondepudi, D. K.; Prigogine, I. Modern Thermodynamics, From Heat Engines to DissipatiVe Structure; John Wiley & Sons: New York, 1998. (3) Epstein, I. R.; Pojman, J. A. An Introduction to Nonlinear Chemical Dynamics, Oscillations, WaVes, Patterns, and Chaos; Oxford University Press: New York, 1998. (4) Epstein, I. R.; Pojman, J. A.; Steinbock, O. Chaos 2006, 16, 037101. (5) Walgraef, D. Spatio-Temporal Pattern Formation; Springer-Verlag: New York, 1997. (6) Turing, A. M. Philos. Trans. R. Soc. London 1952, 237B, 37. (7) Castets, V.; Dulos, E.; Boissonade, J.; De Kepper, P. Phys. ReV. Lett. 1990, 64, 2953. (8) Libbrecht, K. G. Rep. Prog. Phys. 2005, 68, 855. (9) Ben-Jacob, E.; Cohen, I.; Levine, H. AdV. Phys. 2000, 49, 395. (10) Hele-Shaw, H. Nature 1898, 58, 33. (11) Saffman, P.; Taylor, G. Proc. R. Soc. London, Ser. A 1958, 245, 312. (12) Casademunt, J. Chaos 2004, 14, 809. (13) Paterson, L. J. Fluid Mech. 1981, 113, 513. (14) Pearson, J. R. A. J. Fluid Mech. 1960, 7, 481. (15) Taylor, G. I. J. Fluid Mech. 1963, 16, 595. (16) Hakim, V.; Rabaud, M.; Thome, H.; Couder Y. Directional Growth in Viscous Fingering. In New Trends in Nonlinear Dynamics and PatternForming Phenomena; Coullet, P., Huerre, P., Eds.; Plenum Press: New York, 1990; p 327. (17) Pitts, E.; Greiller, J. J. Fluid Mech. 1961, 11, 33. (18) Rabaud, M.; Michalland, S.; Couder, Y. Phys. ReV. Lett. 1990, 64, 184. (19) McEwan, A. D.; Taylor, G. I. J. Fluid Mech. 1966, 26, 1.

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