Self-Diffusion of Colloidal Particles in Two-Dimensional Porous Media

Abstract

Here we report a study of the short-time motion of colloidal particles in two-dimensional porous media. A single layer of colloidal particles is fixed between two glass plates forming a disordered array of obstacles. The void space is filled with an aqueous suspension of colloidal particles of smaller size. The lateral diffusion of the smaller species is measured in real space by optical microscopy. Quantities describing single particle motion, such as the step distribution functions and the particles mean squared displacement, are measured in the presence and absence of obstacle particles. At very short times, the motion of individual particles is qualitatively insensitive to the presence of the obstacles. However, at intermediate times the motion of individual particles is quite sensitive to the local distribution of obstacles. The measured quantities, when averaged over the trajectories of particles diffusing in different pores, show a behavior corresponding to particles diffusing in a homogeneous medium with a lower effective mobility.