Optimizing Super-Hydrophobic Surfaces: Criteria for Comparison of Surface Topographies

Two useful criteria are employed for comparing the super-hydrophobicity of surface designs from a wetting point of view. The first is the apparent contact angle at the transition point between the homogeneous (Wenzel) wetting regime and the heterogeneous (Cassie-Baxter) wetting regime. The second criterion is the wetted area of the solid surface in the heterogeneous wetting regime. Four different types of model rough surfaces are compared using the above criteria, in order to demonstrate optimization considerations in the design of super-hydrophobic surfaces. These model surfaces consist of protrusions of cylinders, truncated cones, paraboloids, and hemispheres. It appears that protrusions of very small base radii are not desirable, since they must be very high in order to conform to the first criterion. For higher base radii it seems that rounded protrusions, such as the paraboloids, are advantageous. This is an interesting conclusion, since the protrusions of the Lotus leaf are of a similar geometry. Based on the case of hemispherical protrusions it also appears that more than one geometrical degree of freedom is necessary in order to optimize a super-hydrophobic surface. (c) Koninklijke Brill NV, Leiden, 2009