Capillary movement of nearly axisymmetric sessile drops

Abstract

A sessile drop placed on a weekly heterogeneous solid surface adopts a configuration which is nearly that of a spherical cap (in the absence of marked gravitational distortion). However, the fine structure modifying the meniscus and triple line and caused by surface free energy perturbations may leave capillary forces unbalanced at the drop periphery. Conditions for overall stability of the drop are considered. In the absence of stability, the dynamics of drop movement and triple-line modification leading to the engulfment of heterogeneities is examined. Overall behaviour is of an exponentially decaying nature approaching thermodynamic equilibrium. This is relatively slow compared with the initial phenomenon of drop spreading. Drop motion, or 'crawling', may be compared with the phagocytosis of bacteria by macrophages. Whether this similarity is only apparent or whether the surface energetic aspects of the two phenomena are directly comparable remains an open question.