Microscopic origin of macroscopic wetting

Two asymptotic angles should be associated with a liquid droplet in equilibrium on a solid surface: the macroscopic wetting angle theta at a scale large compared to the size of a molecule and the microscopic angle theta(0) at the leading edge. When a short-range repulsion is acting near the leading edge, theta(0) > theta, while when a short-range attraction is present, theta(0) < theta. If the values calculated for cos theta and cos theta(0) come out to be larger than unity (an impossibility), no wetting angle can exist and the spreading will occur as single molecules if the area available is large enough. If cos theta > 1 but \cos theta(0)\ < 1, then a planar droplet (pancake) will form. Regarding the dynamics of wetting, it is suggested that the slip velocity near the leading edge of the droplet has its origin in the nonuniform interaction potential to which the molecules of water located on the solid surface are subjected, and in the dependence of the pressure in the liquid at the solid surface on the distance to the leading edge. The gradient of the chemical potential thus generated is responsible for the slip velocity. (C) 2002 Elsevier Science B.V. All rights reserved.