Brownian motion of colloidal particles in a model porous medium

The motion of interacting colloidal particles diffusing in a model porous medium is studied by means of computer simulations using a Brownian dynamics algorithm. The particles are assumed to interact through a repulsive Yukawa pair potential. The porous medium is formed by a fraction of the colloidal particles whose positions are frozen in a given spatial configuration. Varying the molar fraction of the mobile particles, keeping constant the total particle concentration, and/or varying the parameters of the pair potential, we vary the effective porous medium in which the mobile particles diffuse. Here we report results for the mean squared displacement of the mobile particles for different values of their molar fraction and of the Yukawa amplitude. We compare our results with theoretical calculations from two independent approaches to the description of tracer diffusion in colloidal mixtures, by considering the case, in both theories, in which a fraction of the particles has vanishing free-diffusion coefficient. We find that both theories predict the same basic qualitative trends for the mean squared displacement obtained from the Brownian dynamics experiments. For completeness, we also present the static structure of the colloidal suspension permeating our model porous medium.