Switch of lamellae orientation in slits

Effect of hydrophilicity of the confining walls on lamellar phases in oil-water-surfactant mixtures is studied in a slit geometry. In contrast to strongly hydrophilic or hydrophobic walls, which induce parallel orientation of lamellae, the lamellae can be oriented perpendicularly to the neutral walls when the material properties and the termodynamical state of the sample are suitably chosen. When the elastic energy associated with compression or decompression of the lamellae parallel to very weakly hydrophilic walls is sufficiently large, then changes of the film thickness lead to a switch from the parallel to the perpendicular orientation of the lamellae. Our general arguments are confirmed by explicit mean-field calculations in a lattice vector model.