Wetting of antagonist mixtures: the 'leak out' transition

We measure the contact angle theta(phi) of polymer solutions (polymer volume fraction phi) on a solid surface. The polymer is repelled at both interfaces. The pure solvent wets the solid, but the polymer does not. Thus the spreading coefficient S(phi) is positive for phi = 0 and negative for phi = 1. Naively we could expect a wetting transition at the concentration phi(w) such that S(phi(w)) vanishes. We show here that theta(phi) has a plateau (theta = theta(L)) for phi below a critical value phi(L) larger than phi(w). For phi > phi(L), theta(phi) increases monotonously. In the plateau regime, the solution droplet is in equilibrium with a precursor film of pure solvent. At phi(L), we have a 'leak out transition', and the value of phi(L) results from a balance between the osmotic pressure of the polymer solution and the disjoining pressure of the solvent film. To interpret these results, we have constructed the wetting diagram of a polymer solution, allowing a phase separation between a film of nearly pure solvent and a polymer solution. Because the contact angle is finite at all composition, films of solution dewett below a critical thickness e(c)(phi). We observe two regimes of dewetting (i) dry dewetting at composition phi > phi(L). The final state after dewetting are multitude of droplets on a dry solid, (ii) 'wet' dewetting for phi < phi(L), where the final droplets coexist with a film of pure solvent. These ideas can be extended to other liquid mixtures (polyions, colloidal suspensions, critical binary mixtures), again assuming that the solute does not adsorb at the interfaces and, more precisely, that the thickness of the depletion layers is larger than the natural thickness of the solvent film. (C) 2000 Elsevier Science Ltd. All rights reserved.