The spreading of macroscopic droplets

Some experimental results on the macroscopic spreading of hanging and sessile drops on smooth surfaces are presented. The results for sessile drops nicely corroborate the main aspects of the spreading theory of de Gennes and Joanny. However, it is shown that one assumption of the theory, namely the retainment of a self-similar shape during spreading, which is approximately true for sessile drops, cannot be used for hanging drops, for which no theory is available. We propose a numerical resolution of the hydrodynamic equations which relaxes the necessity of self-similarity. The calculation involves the assumption that the shape of a (sessile or hanging) drop at any given time is in quasi-equilibrium with itself and can therefore be calculated through the Laplace equation. The calculation is indeed capable of describing the spreading of both sessile and hanging drops in detail. Spreading of sessile drops on rough surfaces may also be interpreted in the spirit of the theory of de Gennes and Joanny. Evidence is presented that the kinetics of the macroscopic foot which develops at the edge of a drop spreading on a rough surface is related to the heterogeneous distribution of the macroscopic contact angle and obeys simple equations.