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 roughness- modulated 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 charac- terization, 4,8,10-22 the phenomenon of wetting on rough surfaces still remains vague. The first publications describing the wetting behavior of liquid drops on rough surfaces appeared more than a half-century ago. Wenzel, 23 Cassie and Baxter 24 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 θ W , is given by the Wenzel equation (eq 1) cos θ W ) r s cos θ Y (1) where θ W and θ Y are the apparent contact angle on the rough surface and the Young contact angle on the flat one, respectively. r s 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, θ CB , is given by the Cassie-Baxter equation (eq 2) cos θ CB ) r f fcos θ Y + f - 1 (2) where θ CB and θ Y 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 r f is the roughness ratio of the wetted area. 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