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

On the basis of the theory of surface forces and disjoining pressure isothe rms, 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 pres sure in the surrounding media, and the value of the contact angle that char acterizes the state of the hydrophobic part of the substrate. Equilibrium p rofiles of a liquid/gas interface over and near the position of a hydrophob ic spot are calculated taking into account both the distribution of local v alues of disjoining pressure and the local curvature of the interface that determine local capillary pressure. The solutions are obtained using simpli fied forms of isotherms of disjoining pressure both for hydrophilic and hyd rophobic parts of the substrate. The critical width of a hydrophobic spot d ecreases by an order of magnitude with an increase of the contact angle fro m 10 to 90 degrees. (C) 1999 Elsevier Science B.V. All rights reserved.