Mixed fluid bilayers: Effects of confinement

A theoretical model for binary mixtures in fluid membranes is developed to study possible modulated phases in confined geometries. It is shown that two-component fluid bilayers can exhibit modulations of both membrane shape and composition when subject to confinement. In particular, lyotropic smectic phases can exhibit a transition to an undulated structure similar to that of the Helfrich-Hurault instability. In the case of binary mixtures, however, the resulting structure is stable. These results are also extended to freely suspended films, for which a transition to a modulated phase is predicted as a function of the film thickness.