Not spreading in reverse: the dewetting of a liquid film into a single droplet

Wetting and dewetting are both fundamental modes of motion of liquids on solid surfaces. They are critically important for processes in biology, chemistry and engineering, such as drying, coating and lubrication. However, recent progress in wetting, which has led to new fields such as superhydrophobicity and liquid marbles, has not been matched by dewetting. A significant problem has been the inability to study the model system of a uniform film dewetting from a non-wetting surface to a single macroscopic droplet – a barrier which does not exist for the reverse wetting process of a droplet spreading into a film. Here, we report the dewetting of a dielectrophoresis-induced film into a single equilibrium droplet. The emergent picture of the full dewetting dynamics is of an initial regime, where a liquid rim recedes at constant speed and constant dynamic contact angle, followed by a relatively short exponential relaxation of a spherical-cap shape. This sharply contrasts with the reverse wetting process, where a spreading droplet follows a smooth sequence of spherical-cap shapes. Complementary numerical simulations and a hydrodynamic model reveal a local dewetting mechanism driven by the equilibrium contact angle, where contact-line slip dominates the dewetting dynamics. Our conclusions can be used to understand a wide variety of processes involving liquid dewetting, such as drop rebound, condensation and evaporation. In overcoming the barrier to studying single film-to-droplet dewetting our results provide new ways of fluid manipulation and use of dewetting, such as inducing films of prescribed initial shapes and slip-controlled liquid retraction.