Polymers as dewetting agents

We study the stability of a film of poly(dimethylsiloxane) with an oligomer as the solvent. On a silanated Si-wafer, the solvent wets, but the polymer does not. We find that at all polymer volume fraction phi, films below a critical thickness e(c)(phi) are unstable. We observe two regimes of dewetting vs phi: (i) classical "dry" dewetting for phi > phi(L), where the film ruptures in a multitude of droplets on a dry solid; (ii) unusual "wet" dewetting for phi < phi(L), where the final droplets coexist with a film of pure solvent. In this new regime, e(c)(phi) is found to scale like phi(0.80+/-0.01). We interpret these results via an equilibrium between droplets of solution and a solid covered by a film of pure solvent. This equilibrium occurs if phi < phi(L), the critical concentration. At phi = phi(L) We have a "leak out" transition of the solvent. All these results can be extended to other complex systems, where wetting processes and phase separation are coupled.