Confined fluid between two walls: The case of micelles inside a soap film

We study by light scattering methods the behavior of soap films drawn from solutions of different surfactant concentration. For concentrations just above the critical micelle concentration, micelles induce a depletion attraction and thus a reduction in equilibrium film thickness. For larger surfactant concentrations we observe stepwise thinning, due to the organization of micelles in layers parallel to the film plane. Our results show that the behavior of the micelles in the confining geometry of the film is analogous to that of ordinary fluids confined between walls. The ability to go from a gaslike to a liquidlike configuration of micelles by changing surfactant concentration allows us to test model predictions of density oscillations in confined liquids.