Fall and rise of small droplets on rough hydrophobic substrates

¹  Interdisciplinary Centre for Advanced Materials Simulation (ICAMS), Ruhr-Universität Bochum Stiepeler Strasse 129, 44801 Bochum, Germany, EU
²  Max-Planck Institut für Eisenforschung - Max-Planck Str. 1, 40237 Düsseldorf, Germany, EU

Received: 26 August 2009
Accepted: 1 October 2009

Abstract

Liquid droplets on patterned hydrophobic substrates are typically observed either in the Wenzel or the Cassie state. Here we show that for droplets of comparable size to the roughness scale an additional local equilibrium state exists, where the droplet is immersed in the texture, but not yet contacts the bottom grooves. Upon evaporation, a droplet in this state enters the Cassie state, opening the possibility of a qualitatively new self-cleaning mechanism. The effect is of generic character and is expected to occur in any hydrophobic capillary wetting situation where a spherical liquid reservoir is involved.