PHASE-TRANSITIONS OF A CONFINED COMPLEX FLUID

The effect. of confinement on a microemulsion, a disordered, fluid phase with two characteristic length scales, xi the correlation length, and d the wavelength of oil and water density variations, is studied within a simple Ginzburg-Landau theory. We find a qualitative difference between those systems which contain a strong amphiphile as opposed to a weak one. As the distance between walls is increased in the former, a series of first-order transitions occurs which, in principle, can continue without limit, while in the latter, only a small number of such transitions are expected. Such transitions will be manifest in discontinuities in the forces between the walls. In our model calculation, the boundary between strong and weak amphiphiles is 2pixi/d = square-root 3, a value easily accessible experimentally.