Langmuir 2008, 24, 11895-11901
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Wetting on Regularly Structured Surfaces from “Core-Shell” Particles: Theoretical Predictions and Experimental Findings Alla Synytska,*,† Leonid Ionov,‡ Victoria Dutschk,† Manfred Stamm,† and Karina Grundke† Leibniz Institute of Polymer Research Dresden e.V., Hohe Strasse 6, D-01069 Dresden, Germany, and Max-Planck-Institute of Molecular Cell Biology and Genetics, Pfotenhauer Strasse 108, 01307 Dresden, Germany ReceiVed April 7, 2008. ReVised Manuscript ReceiVed July 25, 2008 In this paper, we report on a systematic and thorough study of wetting phenomenon on regularly patterned surfaces fabricated from inorganic-organic hybrid “core-shell” particles of different radii (100 nm to 10 µm). Inorganic silica particles were modified through chemical anchoring of polymers and silanes with different hydrophobicities. Modified “core-shell” particles were assembled into regular hexagonally packed structures. The use of regular structured surfaces with specifically designed surface roughness allowed mathematic prediction of the wetting behavior according to existing models and its comparison with experimental observations. It was shown that the character of the wetting behavior varies with the particles size and the chemical nature of the surface immobilized substance. For the regular particle assemblies, an increase in the vertical roughness was achieved with increasing particle radius, but without changing the Wenzel roughness factor.
Introduction Surface wettability plays an important role in nature and numerous industrial applications such as coatings, paintings, adhesives, microfluidic technology, microelectronics, etc.1-7 One of the factors influencing the wettability of solid surfaces is the surface roughness. In many cases, rough surfaces possess completely different properties than flat ones made of the same material. The most brilliant natural example of the roughnessmodulated wetting is the self-cleaning leaves of several plants.8,9 Despite significant progress achieved in the last few decades in engineering of rough self-cleaning surfaces and their characterization,4,8,10-22 the phenomenon of wetting on rough surfaces still remains vague. * To whom correspondence should be addressed. Telephone: +49 (0351) 4658 327. Fax: +49 (0351) 4658 474. E-mail:
[email protected] † Leibniz Institute of Polymer Research Dresden e.V. ‡ Max-Planck-Institute of Molecular Cell Biology and Genetics.
(1) Blossey, R. Nat. Mater. 2003, 2, 301–306. (2) Bico, J.; Quere, D. J. Fluid Mech. 2002, 467, 101–127. (3) Zhang, G.; Wang, D.; Gu, Z.-Z.; Mohwald, H. Langmuir 2005, 21(20), 9143–9148. (4) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5(10), 2097–2103. (5) Kwok, D. Y.; Neumann, A. W. AdV. Colloid Interface Sci. 1999, 81, 167– 249. (6) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2(7), 457–460. (7) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288–289. (8) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (9) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21(3), 956–961. (10) Fu¨rstner, R.; Barthlott, W. Langmuir 2005, 21(3), 956–961. (11) Ming, W.; Wu, D.; vanBenthem, R.; deWith, G. Nano Lett. 2005, 5(11), 2298–2301. (12) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21(8), 3235–3237. (13) Marmur, A. Langmuir 2006, 22(4), 1400–1402. (14) Quere, D.; Lafuma, A.; Bico, J. Nanotechnology 2003, 14(10), 1109– 1112. (15) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512–19517. (16) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777–7782. (17) Takeshita, N.; Paradis, L. A.; Oner, D.; McCarthy, T. J.; Chen, W. Langmuir 2004, 20(19), 8131–8136. (18) Marmur, A. Langmuir 2003, 19(20), 8343–8348. (19) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19, 4999–5003. (20) Patankar, N. A. Langmuir 2003, 19, 1249–1253. (21) Jopp, J.; Gru, H.; Yerushalmi-Rozen, R. Langmuir 2004, 20(23), 10015– 10019.
The first publications describing the wetting behavior of liquid drops on rough surfaces appeared more than a half-century ago. Wenzel,23 Cassie and Baxter24 were the first who described wetting on rough substrates. The Wenzel model describes a regime when a liquid completely penetrates into the roughness groovessa “homogeneous wetting regime”.18,23,25 The apparent contact angle on a rough surface in the homogeneous regime θW, is given by the Wenzel equation (eq 1)
cos θW ) rs cos θY
(1)
where θW and θY are the apparent contact angle on the rough surface and the Young contact angle on the flat one, respectively. rs is the roughness factor, which was defined as the ratio of the actual area of the solid surface to the geometric/projected one.23 It was later shown by Johnson and Dettre that such surfaces often have high contact angle hysteresis.26,27 The Cassie-Baxter model describes the wetting regime when air is trapped in the microstructures of the surface and liquid sits on top of asperitiessthe “heterogeneous wetting regime”.18,24 The apparent contact angle in the heterogeneous wetting regime, θCB, is given by the Cassie-Baxter equation (eq 2)
cos θCB ) rf fcos θY + f - 1
(2)
where θCB and θY are the contact angle on rough surface according to Cassie and Baxter and the Young contact angle, respectively; f is the fraction of the projected area of the solid surface that is wetted by the liquid; and rf is the roughness ratio of the wetted area. When f ) 1, and rf ) r, the Cassie–Baxter equation turns into the Wenzel equation. It should be noted that the wetting (22) Han, W.; Wu, D.; Ming, W.; Niemantsverdriet, J. W.; Thune, P. C. Langmuir 2006, 22(19), 7956–7959. (23) Wenzel, R. N. Ind. Eng. Chem. Res. 1936, 28, 988. (24) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (25) Marmur, A. Langmuir 2004, 20(9), 3517–3519. (26) Johnson, R. E.; Dettre, R. H. Contact Angle Hysteresis. I. Study of an Idealized Rough Surfaces. In AdVances in Chemistry Series; Gould, R. F., Ed.; American Chemical Society: Washington, DC, 1964; Vol. 43, pp 112-135. (27) Dettre, R. H.; Johnson, R. E. Contact Angle Hysteresis. II. Contact Angle Measurements on Rough Surfaces. In AdVances in Chemistry Series; Gould, R. F., Ed.; American Chemical Society: Washington, DC, 1964; Vol. 43, pp 136144.
10.1021/la8010585 CCC: $40.75 2008 American Chemical Society Published on Web 09/18/2008
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Table 1. Surface Tensions of Used Liquids (T ) 23 °C)
individual liquids
surface tension, γLV (mN/m)
n-hexane n-heptane n-octane methanol n-decane n-dodecane n-hexadecane benzyl alcohol methylene iodide formamide water 3 M CaCl2 in water
19.0 19.7 22.1 22.3 23.4 25.1 27.6 40.0 50.8 58.2 71.8 78.7
mixtures methanol water/methanol water/methanol water/methanol water/methanol water/methanol water/methanol water/methanol water/methanol water/methanol water/methanol water/methanol water/methanol water/methanol water/methanol water
surface tension, γLV mass % methanol (mN/m) 100 77.0 70.4 60.2 50.2 40.0 30.6 25.0 20.0 14.8 10.4 6.0 5.4 3.8 1.8 0
Scheme 1. Scheme of “Grafting to” Approach of Carboxy-Terminated Polymer onto an Anchored Layer of 3-Glycidoxypropyltrimethoxysilane
22.3 25.2 28.9 31.8 34.5 37.8 41.4 44.1 47.7 51.3 55.0 59.7 62.5 62.9 67.8 71.8
regime that yields the lowest contact angle is the more stable one from a thermodynamic point of view, since the Gibbs energy turns out to be a monotonically increasing function of the contact angle.18 Later on, Johnson and Dettre26,27 extended the Wenzel and Cassie-Baxter models, arguing about a large number of metastable configurations of the liquid contact angle on rough surfaces. 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. Moreover, they predicted and demonstrated experimentally a transition from homogeneous wetting regime to heterogeneous one with increase of surface roughness. Interest in wetting on rough surfaces was renewed again in the last several years.18-20,25,28 Utilizing fundamental thermodynamic principles and applying methods for the minimization of the free energy under the relevant constraints, the effects of surface topography on the wetting behavior of droplets were addressed. Patankar20 showed, on the basis of experimental evidence, that there can be two contact angles (Cassie and Wenzel contact angles) on the same rough surface, depending on how a drop is formed. A transition can occur between different states by an external disturbance. Marmur has also discussed theoretically equilibrium wetting on rough surfaces in terms of the “competition” between complete liquid penetration into the roughness grooves and entrapment of air bubbles inside the grooves underneath the liquid.18 He placed the Wenzel and Cassie-Baxter equations into proper mathematical-thermodynamic perspective and defined the conditions for determining the transition between the “homogeneous” and “heterogeneous” wetting regimes. ¨ ner and Extrand presented an alternative approach for O consideration of wetting on rough surfaces using a contact line approach.16,17,29,30 They argued that surfaces with completely different topography can have the same values of fraction of liquid area in contact with the material but completely different contact line structures. Although there are plenty of publications reporting the influence of surface topography on the contact angle, there are still many antagonisms in the interpretation of wettability on rough surfaces: often the experimental observation cannot be explained by only one of these theories. We aim to compare the predictions based (28) Jopp, J.; Grull, H.; Yerushalmi-Rozen, R. Langmuir 2004, 20(23), 10015– 10019. (29) Extrandt, C. W. Langmuir 2002, 18, 7991–7999. (30) Extrand, C. W. Langmui 2006, 22, 1711–1714.
on all theories for wetting on rough surfaces with the experimental observations. Recently, it was demonstrated that layers made of regularly packed spherical particles may provide a useful model for studying the influence of the surface geometry on the wettability.31-33 The peculiarity of particle layers is the independency of the roughness factor, which is a keynote roughness parameter discussed in the theories of Wenzel and Cassie, of the particle size.31,33 Thereby, this study aims to elucidate the influence of vertical surface roughness (particle size) on liquid contact angle on rough surfaces, keeping the roughness factor constant. In particular, we will systematically investigate the wetting behavior of different model polar and nonpolar liquids and their mixtures on regular arrays from inorganic-organic hybrid “core-shell” particles and compare results with mathematical calculations.
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 the 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 ) 45900 g/mol, PDI ) 1.05) synthesized by anionic polymerization was purchased from Polymer Source (Germany). Random carboxy-terminated copolymer of polystyrene (Aldrich) and 2,3,4,5,6-pentafluoropolystyrene (Aldrich), FPS-COOH (Mn ) 40000 g/mol, PDI ) 2.4), was prepared by free radical polymerization in toluene with the ratio 80:20. Toluene (Aldrich) was distilled after drying over sodium. (Tridecafluoro-1,1,2,2-tetrahydrooctyl) dimethylchlorosilane (FSI) from Gelest (Germany) and 3-glycidoxypropyl trimethoxysilane (GPS) from ABCR (Germany) were used as received. Silica particles of different radius varying from 0.1 to 10 µm were purchased in dry state from Geltech (Germany), Microparticles (Germany), and Duke Scientific Corporation (USA) (Table 1). Millipore grade water, methanol, n-alkanes CnH2n+2, with n ) 6, 7, 8, and 10-13, formamide, methlylene iodide, and benzyl (31) Nakae, H.; Inui, R.; Hirata, Y.; Saito, H. Acta Mater. 1998, 46(7), 2313– 2318. (32) Shiu, J. Y.; Kuo, C. W.; Chen, P. L.; Mou, C. Y. Chem. Mater. 2004, 16(4), 561–564. (33) Synytska, A.; Ionov, L.; Dutschk, V.; Minko, S.; Eichhorn, K.-J.; Stamm, M.; Grundke, K. Prog. Colloid Polym. Sci. 2006, 132, 72–81.
Wetting of Core-Shell Particle Surfaces
Figure 1. Representative topography images of the particle arrays: (a) SEM image for 0.1 µm; (b) SFM image for 0.5 µm; (c) MicroGlider image for 1.2 µm s; (d) MicroGlider image for a 2.5 µm particle layer; (e) MicroGlider image for 2.5 µm large particles in radius (larger scale). Reprinted from ref 33 with permission from Springer.
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. Particle Modification and Surface Characterization. Details of the particle modification, deposition, and characterization by FTIRATR/diffuse reflection IR spectroscopy, capillary penetration experiments, surface topography by scanning electron microscopy (SFM), and optical sensor (MicroGlider) are given elsewhere.33,34 Briefly, the “grafting to” approach was used to anchor polymer chains (carboxy-terminated polystyrene and random carboxyterminated copolymer of polystyrene and 2,3,4,5,6-pentafluoropolystyrene) onto the surface of silica particles (Scheme 1). The synthetic procedure starts with covalent grafting of 3-glycidoxypropyl trimethoxysilane (GPS) onto the surface. The next step of the synthetic procedure consists in grafting of the carboxy-terminated polymers (PS-COOH or FPS-COOH) onto the surface of the GPS-modified particles. The modified silica particles were deposited onto supported silica wafers using a vertical deposition technique.33-35 The quality of the prepared layers was controlled by scanning force (SFM) scanning electron (SEM) microscopies and an optical imaging instrument (MicroGlider).33 Scanning force microscopy (SFM) studies were performed on a Dimension IV (Digital Instruments, USA) microscope. The tapping mode was used to map the surface morphology at ambient conditions. Standard silicon tips with radii of about 10-30 nm, apex angle 65°, and frequency of about 300 kHz as well as ultrasharp ones with tip radius
