Viscous drops rolling on a tilted non-wettable solid

Laboratoire de Physique de la Matière Condensée, URA 792 du CNRS, Collège de France 75231 Paris Cedex 05, France

Received: 10 May 1999
Accepted: 3 September 1999

Abstract

A viscous liquid drop sliding down an inclined solid that it partially wets runs all the faster since it is large. Here we examine what happens in the limit of very high contact angles, on a so-called super-hydrophobic surface. It is shown that a droplet rolls instead of sliding, which leads to a surprising law for the velocity as a function of the drop radius: the smaller the droplet, the larger the running velocity. A recent model of Mahadevan and Pomeau allows us to propose an explanation for this paradoxical behaviour.