Wetting films on locally heterogeneous surfaces: hydrophilic surface with hydrophobic spots

On the basis of the theory of surface forces and disjoining pressure isotherms, a critical width has been calculated for a hydrophobic spot on a flat hydrophilic substrate at which wetting film ruptures. The critical size of a spot depends on the parameters of the isotherms used, relative vapor pressure in the surrounding media, and the value of the contact angle that characterizes the state of the hydrophobic part of the substrate. Equilibrium profiles of a liquid/gas interface over and near the position of a hydrophobic spot are calculated taking into account both the distribution of local values of disjoining pressure and the local curvature of the interface that determine local capillary pressure. The solutions are obtained using simplified forms of isotherms of disjoining pressure both for hydrophilic and hydrophobic parts of the substrate. The critical width of a hydrophobic spot decreases by an order of magnitude with an increase of the contact angle from 10 to 90 degrees. (C) 1999 Elsevier Science B.V. All rights reserved.