Colloids and Surfaces A: Physicochemical and Engineering Aspects
Wetting of textured surfaces
Section snippets
The Kao experiment
Shibuichi et al. from the Kao Corporation recently made a remarkable series of experiments where they could show how texturing a surface modifies the contact angle, as a function of the chemical wettability of the solid [1]. Their results are shown in Fig. 1, where the value of the measured apparent contact angle θ* on a rough surface is plotted versus θ, the Young contact angle determined on a flat surface of the same chemical composition. Both angles are expressed by their respective cosine.
Super-hydrophobic solids
For hydrophobic solids (θ>π/2), the dry solid has a surface energy γSV lower than the wet one γSL (as derived from the Young equation γcos θ=γSV−γSL, denoting the liquid/vapour surface tension by γ). Thus, the surface energy can be lowered if air is trapped below the drop. For roughnesses of the order of 30 μm (a typical scale for some plant leaves [4]), these air pockets can be directly observed through the drop, which acts as a magnifying glass. A drop eventually sits on a patchwork of solid
Super-hydrophilic solids
For hydrophilic solids, the situation is quite different because the solid/liquid contact is favoured (γSL<γSV). Thus, the solid/liquid interface is likely to follow the roughness of the solid, which leads to a Wenzel contact angle Eq. (4). Since we have r>1 and θ<π/2, Eq. (4) implies θ*<θ: the solid roughness makes the solid more wettable. The linear relation found in Eq. (4) is in good agreement with the Kao data Fig. 1, in the first part of the hydrophilic side. We can even deduce from the
Applications to porous media
Very often, the properties of a porous medium are (over)simplified by considering that such a medium can be considered as an array of capillary tubes. For example, both the capillary rise in a tube and in a porous material are often found to follow the so-called Washburn law (the height increases as the square root of time, for small heights [10]), from which an equivalent tube radius can be deduced for characterizing the porous structure. This approach is sometimes useful, but cannot explain
Conclusions
We have shown that the simplest possible description of the wetting (or dewetting) of a textured surface implies two dimensionless parameters, namely the surface roughness and a surface fraction characterizing the ratio between the two levels of such a surface. The apparent contact angle could be calculated on such a surface, as a function of these parameters and of the Young contact angle, fixed by the chemical nature of the solid and the liquid.
In the hydrophobic case, it was found that if
Acknowledgements
It is a pleasure to thank S. Herminghaus, C. Marzolin and C. Tordeux for highly valuable discussions and collaborations.
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