Bonn, Juli 2006 A New Model for Contact Angle Hysteresis

We present a model which explains several experimental observations relating contact angle hysteresis with surface roughness. The model is based on the balance between released energy and dissipation, and it describes the stick{slip behavior of drops on a rough surface using ideas similar to those employed in dry friction, elasto{ plasticity and fracture mechanics. The main results of our analysis are formulas giving the interval of stable contact angles as a function of the surface roughness. These formulas show that the dierence between advancing and receding angles is much larger for a drop in complete contact with the substrate (Wenzel drop) than for one whose cavities are lled with air (Cassie-Baxter drop). This fact is used as the key tool to interpret the experimental evidence. Keywords. PACS.68.08.Bc- Wetting. PACS.68.03.Cd- Surface tension and related phenomena