Wetting of antagonist mixtures: the 'leak out' transition

We measure the contact angle theta(phi) of polymer solutions (polymer volum e fraction phi) on a solid surface. The polymer is repelled at both interfa ces. The pure solvent wets the solid, but the polymer does not. Thus the sp reading 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 (t heta = 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 solve nt. At phi(L), we have a 'leak out transition', and the value of phi(L) res ults from a balance between the osmotic pressure of the polymer solution an d the disjoining pressure of the solvent film. To interpret these results, we have constructed the wetting diagram of a polymer solution, allowing a p hase separation between a film of nearly pure solvent and a polymer solutio n. Because the contact angle is finite at all composition, films of solutio n dewett below a critical thickness e(c)(phi). We observe two regimes of de wetting (i) dry dewetting at composition phi > phi(L). The final state afte r 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 solv ent. These ideas can be extended to other liquid mixtures (polyions, colloi dal suspensions, critical binary mixtures), again assuming that the solute does not adsorb at the interfaces and, more precisely, that the thickness o f the depletion layers is larger than the natural thickness of the solvent film. (C) 2000 Elsevier Science Ltd. All rights reserved.