Contact angle hysteresis on nano-structured surfaces

We present results from an experimental study on the phenomenon of contact angle hysteresis on solid surfaces decorated by a random array of nanometric hollows. For weak values of the areal density of defects φ_d, the hysteresis H increases linearly with φ_d. This evolution is described by a pinning–depinning process of the contact line by individual defects. At higher values of φ_d, a collective pinning effect appears and H decreases with increasing φ_d. In the linear regime, our experimental results are compared to theoretical predictions for contact angle hysteresis induced by a single isolated defect on the solid surface. We suggest that the crossover from the individual to the collective pinning effects could be interpreted in terms of an overlapping of wetting cross sections. Finally, we analyse the influence of both the size and the morphology (hollows/hillocks) of defects on the anchorage of the contact line.