Finite size effects on textured surfaces: recovering contact angles from vagarious drop edges

A clue to understand wetting hysteresis on superhydrophobic surfaces is the relation between receding contact angle and surface textures. When the surface textures are large, there is a significant distribution of local contact angles around the drop. As seen from the cross section, the apparent contact angle oscillates as the triple line recedes. Our experiments demonstrate that the origin of these oscillations is a finite size effect. Combining side and bottom views of the drop, we take into account the 3D conformation of the surface near the edge to evaluate an intrinsic contact angle from the oscillations of the apparent contact angle. We find that for drops receding on axisymmetric textures the intrinsic receding contact angle is the minimum value of the oscillation while for a square lattice it is the maximum.