Existence of receding and advancing contact lines

We study a solid plate plunging into or being withdrawn from a liquid bath to highlight the fundamental difference between the local behavior of an advancing or a receding contact line, respectively. It is assumed that the liquid partially wets the solid, making a finite contact angle in equilibrium. In our hydrodynamic description, which neglects the presence of the outer gas atmosphere, an advancing dynamic wetting line persists to arbitrarily high speeds. The receding wetting line, on the other hand, vanishes at a critical speed set by the competition between viscous and surface tension forces. In the advancing case, we apply existing matching techniques to the plunging plate geometry to significantly improve on existing theories. For the receding contact line, we demonstrate for the first time how the local contact line solution can be matched to the far-field meniscus. In doing so, we confirm our very recent criterion for the vanishing of the receding contact line, leading to the formation of a film covering the solid. The results of both the advancing and the receding cases are tested against simulations of the full model equations. (c) 2005 American Institute of Physics.