Amphiphilic membranes in thin films of a complex fluid: statics and dynamics in lattice models

Abstract

We investigate the equilibrium structure as well as a specific dynamical aspect of thin films of complex fluids containing amphiphilic membranes. We explore the structures of ternary microemulsions, confined in a pore, resulting from various different types of interactions of the confining wall with the molecules. We compute the concentration profiles of the three components, namely, oil, water and amphiphiles, by carrying out extensive Monte Carlo simulations of the Widom model and its recently proposed generalizations on a simple-cubic lattice of size L_x×L_y×L_z under appropriate boundary conditions. We investigate both the wide-gap geometry (L_x=L_y= L_z»1) and narrow-gap geometry (L_x=L_y»L_z). We observe novel concentration profiles in the narrow-gap geometry when L_z is of the order of a few molecular layers (i.e., in micropores). We also compute the lifetime of thermodynamically unstable bilayers of a specific initial conformation in the Widom model of microemulsions. For identical initial conformations of these model bilayers, we study the dependence of the lifetime on (a) the initial concentration of the amphiphilic molecules, (b) the temperature, (c) the curvature elasticity of the amphiphilic membrane, and (d) the thickness of water layers inside the two constituent monolayers. We also compare the models and results in our computer experiment with those in the laboratory experiment on the rupture of the Newton black films and common black films.