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Wetting on Fractal Superhydrophobic Surfaces from “Core-Shell” Particles: A Comparison of Theory and Experiment Alla Synytska,*,† Leonid Ionov,‡ Karina Grundke,† and Manfred Stamm† Leibniz Institute of Polymer Research Dresden e.V., Hohe Strasse 6, 01069 Dresden, and Max-Planck-Institute of Molecular Cell Biology and Genetics, Pfotenhauer Strasse 108, 01307 Dresden, Germany ReceiVed September 23, 2008. ReVised Manuscript ReceiVed December 30, 2008 We report an experimental and theoretical investigation of the wetting behavior of different model polar and nonpolar liquids and their mixtures on superhydrophobic fractal surfaces made of polymer- or silane-coated “core-shell” particles. We compared the experimental results with the theoretical predictions made according to the theories of Onda-Shibuichi (describes wetting on fractal surfaces) and Cassie-Baxter (describes wetting on generic rough composite surfaces). We found that the experimental findings deviate from the behavior predicted by the Onda-Shibuichi model. On the other hand, the wetting properties were found to be close to the predictions made by the Cassie-Baxter model in the hydrophobic region (the intrinsic contact angle on the flat surface is larger than 90°). However, the wetting behavior in the hydrophilic region (the intrinsic contact angle is less than 90°) could not be described by the Onda-Shibuichi or Cassie-Baxter model. The observed inconsistency between the experimental results and theoretical predictions was explained by the formation of metastable states of a liquid droplet on a fabricated fractal surface according to the theory developed by Johnson and Dettre for generic rough surfaces. The entrapments of the liquid droplets in metastable states resulted in superhydrophobic behavior on fractal surfaces as well, made from nonfluorinated material such as polystyrene with a surface free energy of about 30 mJ/m2. This finding is very promising for real industrial applications where fluorinated compounds are willing to be reduced. It can be concluded that employing a texture with fractal geometry is necessary for the design of superhydrophobic coatings. Thereby, extremely lowering the surface free energy of materials by fluorination is not an obligatory factor for the generation of liquid-repellent superhydrophobic materials. We believe that the results we presented in the paper give new insight into the understanding of wetting not only on general superhydrophobic rough surfaces but also on fractal surfaces.
Introduction The lotus leaf, on which a water droplet apparently forms a sphere, unstably sitting on the top, and dirt is easily removed with a rain shower, is an admirable example from nature.1,2 The behavior of liquid droplets on the lotus leaf is known as the “lotus” or self-cleaning effect and is intrinsic to the surfaces which possess very high advancing and receding contact angles (both are more than 150°) and very low sliding angles (lower than 10°). Such surfaces are often called superhydrophobic surfaces and attract much attention due to their high potential implementation as self-cleaning materials, nonsoiling clothing,3 novel waterproof coatings to microfluidic devices,4-10 etc. * To whom correspondence should be addressed. E-mail: synytska@ ipfdd.de. Phone: +49 (0351) 4658 327. Fax: +49 (0351) 4658 474. † Leibniz Institute of Polymer Research Dresden e.V. ‡ Max-Planck-Institute of Molecular Cell Biology and Genetics.
(1) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (2) Koch, K.; Bhushan, B.; Barthlott, W. Soft Matter 2008, 4, 1943–1963. (3) Ramaratnam, K.; Tsyalkovsky, V.; Klep, V.; Luzinov, I. Chem. Commun. 2007, (43), 4510–4512. (4) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2(7), 457–460. (5) Que´re´, D. Nat. Mater. 2002, 1, 14–15. (6) Gao, L.; McCarthy, T. J. Langmuir 2006, 22(7), 2966–2967. (7) Wier, K. A.; McCarthy, T. J. Langmuir 2006, 22(6), 2433–2436. (8) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38(8), 644– 652. (9) Callies, M.; Que´re´, D Soft Matter 2005, 1, 55–61. (10) Blossey, R. Nat. Mater. 2003, 2, 301–306. (11) Gao, L.; McCarthy, T. J. Langmuir 2006, 22(14), 6234–6237. (12) Han, W.; Wu, D.; Ming, W.; Niemantsverdriet, J. W.; Thune, P. C. Langmuir 2006, 22(19), 7956–7959. (13) Feng, B. L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14(24), 1857–1860. (14) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19(14), 5626–5631.
It is already known and widely reported by many researchers that attaining ultrahydrophobic/superhydrophobic artificial surfaces requires combination of both the surface chemical composition (low surface free energy hydrophobic materials) and surface texture (surface roughness, geometry).2,4-35 The (15) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4(7), 1349–1353. (16) Han, J. T.; Xu, X. R.; Cho, K. Langmuir 2005, 21(15), 6662–6665. (17) Ming, W.; Wu, D.; vanBenthem, R.; deWith, G. Nano Lett. 2005, 5(11), 2298–2301. (18) Wu, X.; Shi, G. Nanotechnology 2005, 16(10), 2056–2060. (19) Zhang, X.-T.; Sato, O.; Fujishima, A. Langmuir 2004, 20(14), 6065– 6067. (20) Xie, Q.; Fan, G.; Zhao, N.; Guo, X.; Xu, J.; Dong, J.; Zhang, L.; Zhang, Y.; Han, C. C. AdV. Mater. 2004, 16, 1830–1833. (21) Shi, F.; Chen, X.; Wang, L.; Niu, J.; Yu, J.; Wang, Z.; Zhang, X. Chem. Mater. 2005, 17(24), 6177–6180. (22) Li, Y.; Cai, W.; Cao, B.; Duan, G.; Sun, F.; Li, C.; Jia, L. Nanotechnology 2006, 17, 238–243. (23) J. Ji, J. F.; Shen, J. AdV. Mater. 2006, 18(11), 1441–1444. (24) Vogelaar, L.; Lammertink, R. G. H.; Wessling, M. Langmuir 2006, 22(7), 3125–3130. (25) Zhang, L.; Zhou, Z.; Cheng, B.; DeSimone, J. M.; Samulski, E. T. Langmuir 2006, 22(20), 8576–8580. (26) Ma, M.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193–202. (27) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3(12), 1701–1705. (28) Minko, S.; Muller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125(13), 3896–3900. (29) Cheng, Y. T.; Rodak, D. E.; Wong, C. A.; Hayden, C. A. Nanotechnology 2006, 17(5), 1359–1362. (30) Zhao, N.; Xu, J.; Xie, Q.; Weng, L.; Guo, X.; Zhang, X.; Shi, L. Macromol. Rapid Commun. 2005, 26, 1075–1080. (31) Taurino, R.; Fabbri, E.; Messori, M.; Pilati, F.; Pospiech, D.; Synytska, A. J. Colloid Interface Sci. 2008, 325(1), 149. (32) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53(9), 1466–1467. (33) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777–7782. (34) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15(10), 3395–3399.
10.1021/la803120d CCC: $40.75 2009 American Chemical Society Published on Web 02/10/2009
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surface roughness affects the hydrophobic behavior according to two possible wetting mechanisms: (1) liquid penetrates into the cavities (Wenzel regime, “homogeneous wetting”),32,36 (2) grooves are occupied by trapped air (Cassie-Baxter regime,35 “heterogeneous wetting” 36). The transition between the Wenzel and Cassie-Baxter regimes depends on the intrinsic wettability (intrinsic contact angle on a flat homogeneous surface).36,37 Numerous investigations of the wettability of superhydrophobic surfaces of plants and insects revealed that surfaces with dualsize (micro and nano) hierarchical structures2,6,11-31,34,38 are essential for the design of superhydrophobic materials. However, in most of the reports, the researchers discuss the wetting property of rough surfaces considering general roughness parameters such as the root-mean-square (rms) surface roughness or Wenzel roughness factor (rs). Moreover, superhydrophobic rough surfaces are described very generally. On the other hand, it is well-known that such surfaces can possess regular geometry, random (irregular) or hierarchical rough structures with a certain level of fractality. Unfortunately, there are only a few reports where the effects of fractality on wetting were studied.31,39-41 Shibuichi and Onda39,40 were the first who investigated wetting on fractal wax surfaces both theoretically and experimentally. They developed the model for wetting on fractal surfaces and compared it with the experimental results on waxy soft films. Our goal is to investigate the wetting behavior of different model polar and nonpolar liquids and their mixtures on fractal surfaces made of polymer- or silane-coated “core-shell” colloidal particles, which are relevant for industrial application. In fact, we aim to compare the experimental wetting results with the classical Cassie-Baxter and Wenzel wetting models on generic rough surfaces and the model developed by Onda and Shibuichi for fractal ones.
Experimental Section Materials. Highly polished single-crystal silicon wafers with native silicon oxide layers (Semiconductor Processing, Germany) were used as substrates. They were precleaned by rinsing three times in dichloromethane in an ultrasonic bath for 5 min followed by washing in a mixture of deionized water, ammonia solution (25%), and hydrogen peroxide (30%) in a 1:1:1 volume ratio at 60 °C for 1 h. After washing, the substrates were thoroughly rinsed with deionized reagent grade water and then dried with nitrogen flux. Carboxy-terminated polystyrene (PS-COOH) (Mn ) 45 900 g/mol, PDI ) 1.05) synthesized by anionic polymerization was purchased from Polymer Source (Germany). (Tridecafluoro-1,1,2,2tetrahydrooctyl)dimethylchlorosilane (FSI) from Gelest (Germany) and (3-glycidoxypropyl)trimethoxysilane (GPS) from ABCR (Germany) were used as received. Silica particles (200 nm in diameter) were purchased in the dry state from Kisker (Germany). Millipore grade water, methanol, n-alkanes CnH2n+2, with n ) 6, 7, 8, and 10-13, formamide, methlylene iodide, and benzyl alcohol for contact angle measurements were used as received from Fluka and Aldrich (Germany). We also have prepared a 3 M solution of CaCl2 (Merck) for additional contact angle measurements.42 (35) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (36) Marmur, A. Langmuir 2003, 19(20), 8343–8348. (37) Johnson, R. E.; Dettre, R. H. In Contact Angle Hysteresis. I. Study of Idealized Rough Surfaces; Gould, R. F., Ed.; Advances in Chemistry Series, Vol. 43; American Chemical Society: Washington, DC, 1964; pp 112-135. ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777–7782. (38) O (39) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512–19517. (40) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12(9), 2125–2127. (41) Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. J. Colloid Interface Sci. 1998, 208, 287–294. (42) Synytska, A.; Ionov, L.; Dutschk, V.; Stamm, M.; Grundke, K. Langmuir 2008, 24(20), 11895–11901.
Langmuir, Vol. 25, No. 5, 2009 3133 Particle Modification and Surface Characterization. Details of the particle modification and characterization by FTIR-ATR/ diffuse reflection IR spectroscopy and capillary penetration experiments and surface topography by scanning electron microscopy (SEM) and an optical sensor (MicroGlider) are given elsewhere.42-44 Deposition of Particles. Rough fractal surfaces were obtained by evaporation of concentrated particle suspensions in ethanol (5 wt %) or by strong suppression of a core-shell particle layer using the Langmuir-Blodgett layer (LBL) technique. Surface Topography. The surface topography was examined by using the optical imaging device MicroGlider (Fries Research & Technology GmbH, Bergisch Gladbach, Germany) and environmental scanning electron microscopy (ESEM) performed on a DSM 982 Gemini (Zeiss, Germany). The roughness characteristics were obtained from 250 × 250 µm2 (MicroGlider) scale images. The resolution of each MicroGlider image taken was 500 × 500 lines. The root mean square roughness (Rq), mean peak to valley height roughness (Rz), and waviness (Wz) were calculated from MicroGlider 250 µm2 large images with a help of the Mark III software. Rq is the standard deviation of the feature height (z) values within a given area. Rz is the sum from above of the highest profile point and the depth of the lowest profile valley of the roughness profile within a single measuring distance. Additionally, the roughness factor (rs), which is the ratio between the real (true, actual) surface area and the geometric surface area, and the fractal dimension as well were also calculated from MicroGlider data. The definition of a fractal is any point set whose fractal dimension is strictly greater than its topological dimension. A fractal surface is a surface for which the lateral and vertical scaling behaviors are not identical but are determined by a scaling law. We applied the box counting method (MicroGlider software FRT Mark III) to the cross section of the investigated films to find the fractal dimension of the prepared coatings. Surface Tension Measurements. The equilibrium surface tension of the liquids and solutions used was evaluated by the pendant drop method using a cell integrated into a FibroDAT 1100 device (Fibro Systems, Sweden). The experimentally determined values of the surface tension liquids used are given elsewhere.42 Contact Angle Measurements. Advancing and receding contact angles were measured by the sessile drop method using a conventional drop shape analysis technique (Kru¨ss DSA 10, Hamburg, Germany). For high measuring accuracy, the optical device was calibrated with a certified calibration kit obtained from Fibro System (Sweden). This tool has a steel ball with well-defined shape parameters. The tolerance of ( 5 µm will give the following maximum error margins: height, (0.01 mm; base, (0.01 mm; volume, (0.01 mL; contact angle, (0.3°. High-purity deionized reagent grade water, methanol, watermethanol mixtures, n-alkanes CnH2n+2, with n ) 6, 7, 8, and 10-13, formamide, methlylene iodide, and benzyl alcohol were chosen as model liquids to investigate the wettability of structured rough layers prepared from polymer-modified particles as was previously noted elsewhere.42 The advancing contact angle (θA) was measured by supplying the liquid into the drop at constant velocity. When the pump was reversed, the drop volume started to decrease linearly and the receding contact angle (θR) was measured. Liquid droplets (10 µL) were dropped carefully onto the sample surface, and the average value of 10 measurements, made at different positions of the same sample, was adopted as the average value of the contact angles of the substrates. The error of the mean contact angle values, calculated as the standard deviation, did not exceed 2° and 3° for advancing and receding contact angles, respectively. All contact angle measurements were carried out at 24 ( 0.5 °C and a relative humidity of 40 ( 3%, which were kept constant. It should be noted that additional contact angle measurements on several samples were performed in a closed chamber where the relative humidity was about 80%. No noticeable effect (43) Synytska, A.; Ionov, L.; Dutschk, V.; Minko, S.; Eichhorn, K.-J.; Stamm, M.; Grundke, K. Prog. Colloid Polym. Sci. 2006, 132, 72–81. (44) Synytska, A. Ph.D. Thesis, Technische Universita¨t Dresden, 2005.
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Figure 1. Representative ESEM micrographs of the generated fractal surface obtained by drying a particle suspension. Fractal arrays are prepared from 200 nm diameter silica particles decorated by an FSI shell.
Figure 2. (a) Topographical MicroGlider image with (b) the corresponding cross-section and (c) optical image of the water droplet on the prepared film from FSI-decorated 200 nm large particles.
between wetting behaviors on designed structured surfaces obtained under relative humidities of 40% and 80% was detected.
Results and Discussion The idea of this research is a detailed experimental and theoretical investigation of the wettability of fractal superhydrophobic surfaces made from well-defined spherical hybrid core-shell particles. As we showed previously,42 200 nm large silica particles covered by the FSI “shell” are the most promising systems for this purpose. It was demonstrated that the wetting behavior on even regular arrays from these materials approaches the “heterogeneous” Cassie-Baxter wetting regime when air is trapped beneath a droplet of liquid and liquid escapes from cavities and grooves occupied by trapped air.42 Thereby, in the present work, we prepared fractal surfaces by casting an ethanol suspension of 200 nm large silica particles on planar silica substrates. Then the suspension was allowed to dry at ambient conditions, and afterward, FSI was deposited on the particle layer from its vapor at elevated temperature. As expected, irregular arrays from agglomerates of FSI-decorated silica particles on the substrate were obtained (Figure 1). Then we analyzed the surface roughness and fractal parameters of the fabricated particle layers using ESEM and MicroGlider techniques, respectively (Figures 1, 2a, and 4). The root-meansquare roughness (Rq) and mean peak to valley height roughness
(Rz) of the obtained arrays were calculated to be 1.5 and 2.8 µm, respectively. The measured value of the roughness factor (rs),46 which is the ratio between the real (true, actual) surface area and the geometric surface area, was found to be in the range of 1.7.39,40 Using the box counting method in MicroGlider software FRT Mark III to the cross section, we explored the fractal properties of arrays made from core-shell particles and proved the surface fractality. The fractal dimension of the cross section (Dcross) has been calculated to be 1.45. Thus, the fractal dimension of the real surface was evaluated as D = Dcross + 1 ) 2.45. The upper and lower limit scales of the fractal behavior on the surface are the size of asperities and size of the particles, respectively. Thereby, the upper and lower limit scales of the fractal structures are roughly 2-3 µm and 200 nm, respectively. To experimentally investigate the wetting behavior on fractal structures from FSI-modified silica particles in detail, we used water-methanol mixtures with different compositions and several individual liquids with different surface tensions and polarities similarly to our previous work.42 First, the contact angle measurements demonstrated that the generated fractal surface was superhydrophobic. Both water advancing and receding contact angles were in the range of 160° (Figure 2c). The water (45) Spelt, J. K.; Neumann, A. W. Langmuir 1987, 3(4), 588–591. (46) We note that the experimentally measured value of the roughness factor is usually underestimated due to the resolution limits of Micro Glider.
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droplet freely rolled off the surface, which indicated zero sliding angles. A more detailed investigation of the wetting behavior revealed that the prepared fractal surface repels some other liquids as well. Repellent properties were found for the liquids with very low surface tensions, which are expected to wet it (0 < cos θflat < 0.5). In particular, benzyl alcohol (surface tension of 40 mN/ m) and a 30.6 wt % water-methanol mixture (surface tension of 41 mN/m) with an intrinsic contact angle of about 72° were repelled from the surface. We performed a detailed comparison between the experimental findings and theoretical predictions of the wetting behavior on fractal and generic rough composite surfaces. For this, we used well-known wetting models/regimes on generic composite and fractal surfaces. They are described by the Cassie-Baxter equation (1) and an equation developed by Onda and Shibuichi (2), respectively,39,41 where rs and fs are the roughness factor and fraction of the solid surface under the liquid droplet, θrough, θflat, and θfractal are the contact angles on rough, flat, and fractal surfaces, respectively, L and l are the upper and lower limit scales of the fractal behavior on the surface, respectively, and D is the fractal dimension, which was measured to be 2.45.
cos θrough ) rs fs cos θflat + fs - 1
(1)
cos θfractal ) (L/l)D-2 cos θflat
(2)
To estimate the wetting behavior on prepared arrays using the Cassie-Baxter equation (equilibrium wetting regime on a heterogeneous composite surface), we made the following assumptions. First, liquids wet only the top layer of the aggregates, leaving entrapped air in the grooves between them at cos θflat < 0. Therefore, the fraction of solid surface under the liquid droplet (fs) can be estimated as 0.03-0.11 by taking into account that the rough surface is composed of aggregates of 200 nm large particles and the average distance between the aggregates is 2-3 µm. The roughness factor of the aggregate apex is roughly equal to that of densely packed particles and is rs cos θflat < 0 ≈ 1.9.47 Second, the wetting behavior at cos θflat > 0 can be estimated considering that liquid fully wets the aggregates and fills the grooves between them. In this case, the roughness factor of the surface was estimated as a product of the roughness factor of densely packed particles and the roughness factor measured by MicroGlider and is rs cos θflat > 0 ≈ 3.3.39,40,48 The predictions made according to both the Cassie-Baxter and Onda-Shibuichi models are summarized in Figure 3. We found that the wetting behavior at cos θflat < 0 correlates very well with the predictions made according to the Cassie-Baxter heterogeneous wetting regime. On the other hand, the wetting behavior at cos θflat > 0 cannot be described by any of the discussed models, neither by the Cassie-Baxter model nor by the Onda-Shibuichi model. Both models predict that the contact angle on the rough surface must be lower than 90° (cos θrough > 0). The experimental investigation revealed indeed that the contact angle on the fractal surface is higher than 90° (cos θrough < 0) at 0.5 > cos θflat > 0. This is a rather unexpected wetting behavior. Furthermore, both models predict that the liquid contact angle on the fractal surface must be lower than that of the flat surface at cos θA,flat > 0. To explain the difference in the wetting property of the fractal surface from the predicted behavior at cos θA,flat > 0, we refer (47) Because the width of the light beam used in Micro Glider is about 1-2 µm, Micro Glider revealed a “smoothed” topography where 200 nm particles are not resolved. (48) Since Micro Glider is unable to resolve nanometer and submicrometer structures and reveals smoothened features, we made a correction to the “real” roughness factor value by multiplying the roughness factor measured by MG (rs ) 1.7) and the theoretical one (rs ) 1.9).
Figure 3. Experimentally measured and theoretically predicted wetting behavior on the fractal surface prepared from FSI-coated 200 nm diameter silica particles. Cosine of the (9) advancing (θA,fractal/flat) and (0) receding (θR,fractal/flat) contact angles of water/methanol mixtures of the fractal surface vs those on the flat surface. The blue solid line indicates the wetting behavior predicted according to the model of Onda-Shibuichi. The predictions made according to the Cassie-Baxter model are described by the red dashed line and pink area. The pink area comprises the wetting area according to the Cassie-Baxter model at fs ) 0.03-0.11.
to the theory developed by Johnson and Dettre37,49 on rough surfaces. They showed that the values of the advancing and receding contact angles experimentally measured on rough substrates could differ from the equilibrium values predicted by the Cassie-Baxter model. Johnson and Dettre37,49 suggested that there are many metastable configurations of the contact angle of a liquid on a rough surface. Each metastable state is separated from an adjacent one by the energetic barrier, and the probability of transition between metastable states is inversely proportional to the height of the asperities. The height of the energy barrier is approximately directly proportional to the height of part of the asperities, which is in contact with the liquid and almost independent of the distance between the asperities. At the same time, the height of the energy barrier is related to the intrinsic hydrophobicity of the grafted substance as well as the particle size. The liquid droplet is unable to “jump” to the next equilibrium state when the vibration energy is less than the height of the energy barrier. We argue that a decrease of the intrinsic contact angle leads to stronger penetration of a liquid into the pores between the particles and, as a result, to an increase of the height of the energetic barriers between the metastable states. Thereby, we suppose that the deviations from the theoretical wetting behavior are caused by the entrapments of the droplets in metastable states. Finally, we show that entrapments of the liquid droplets in metastable states can result in superhydrophobic behavior on fractal surfaces, which intrinsically are moderately or less hydrophobic compared to fluorinated materials. For this, we prepared fractal arrays composed of polystyrene-coated 200 nm large silica particles and obtained by suppression of the layer using the Langmuir-Blodgett technique (Figure 4). The surface possesses a structure hierarchy similar to that of surfaces fabricated from FSI-decorated particles. The fractal dimension of the generated arrays was calculated to be 2.3. Note that the intrinsic water contact angle on the flat polystyrene -modified surface is 89.5°, which corresponds to a surface free energy of about 30 mJ/m2 (Table 1). (49) Dettre, R. H.; Johnson, R. E. In Contact Angle Hysteresis. II. Contact Angle Measurements on Rough Surfaces; Gould, R. F., Ed.; Advances in Chemistry Series, Vol. 43; American Chemical Society: Washington, DC, 1964; pp 136-144.
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Figure 4. Representative ESEM images of fractal layers prepared from 200 nm large silica particles decorated with a polystyrene shell. Table 1. Advancing (θA) and Receding (θR) Water Contact Angles and Surface Free Energy Values for Grafted PS and FSI on Planar Silicon Wafer Substrates polymer
θA, deg
θR, deg
∆θ ) θA θR, deg
PS FSI
89.5 ( 0.2 113.0 ( 0.2
82.0 ( 1.0 107.0 ( 1.2
7.5 6.0
γsva, mJ/m2 28.9 18.3
a Values of the surface solid free energy obtained from the advancing contact angles using EQS45 considering a measured value of the water surface tension of 71.8 mN/m.
Water contact angle measurements revealed that the generated fractal surface made from PS-decorated silica particles approaches superhydrophobicity. Both water advancing and receding contact angles were measured to be in the range of 150°. The sliding angle was found to be 3°, which led to unstable water droplets. The wettability of fractal arrays made from PS core-shell particles was investigated with different liquids and water-methanol mixtures as well. Notably, similarly to FSI-coated particles, freely rolling drops of benzyl alcohol (surface tension of 40 mN/m) were observed on polystyrene-modified surfaces.
Conclusions In summary, we have fabricated fractal superhydrophobic surfaces using 200 nm large silane- and polystyrene-coated silica particles and investigated their wetting behavior in detail. Relying on all existing theories of wetting on rough fractal and generic structured surfaces, we undertook efforts for the mathematic prediction of the wetting properties of fabricated fractal arrays and its comparison with experimentally observed findings. We found that the experimental results deviate from the theoretical model of Onda and Shibuichi in the whole hydrophilic and hydrophobic wetting regions. On the other hand, the wetting
property of the obtained fractal arrays in the hydrophobic region (the intrinsic contact angle on the flat surface is larger than 90°) is found to be close to that of the Cassie-Baxter model on rough composite surfaces. However, the wetting behavior in the hydrophilic or moderately hydrophobic regions (the intrinsic contact angle is less than 90°) could not be described by any of these models. The observed inconsistency between the experimental results and theoretical predictions was explained by the formation of metastable states of a liquid droplet on a fractal surface according to the theory developed by Johnson and Dettre for generic rough surfaces. Interestingly, entrapments of the liquid droplets in metastable states can result in superhydrophobic behavior on fractal surfaces as well, made from nonfluorinated material such as polystyrene with a surface free energy of about 30 mJ/m2. This finding is very promising for real industrial applications where fluorinated compounds are willing to be reduced. It can be concluded that employing a texture with a fractal geometry is necessary for the design of superhydrophobic coatings. Thereby, extremely lowering the surface free energy of materials by fluorination is not an obligatory factor for the generation of liquid-repellent superhydrophobic materials. We believe that the results we presented in this paper give new insight into the understanding of wetting not only on general superhydrophobic rough surfaces but also fractal ones. We hope that the obtained knowledge is interesting for both academic research and technological applications. Acknowledgment. We gratefully acknowledge the German Ministry of Education, Research (BMBF) (Project No. 01 RC 0040), and IPF for financial support. LA803120D
