BROWNIAN-MOTION IN QUASIBIDIMENSIONAL COLLOIDAL SUSPENSIONS

Digital video microscopy is used to study the Brownian motion in quasi bidimensional colloidal systems, consisting of spherical polystyrene p articles suspended in water and confined between two glass plates. Thi s technique allows the direct measurement of the lateral (two-dimensio nal) probability distribution function, P(Delta r,t), of the random va riable Delta r (the particle displacement) at time t, and the mean squ ared displacement W(t). We studied the effect of confinement in highly diluted samples, where W(t) is found to be a linear function of time. The hydrodynamic interactions between the colloidal particles and the glass walls are found to be more important than predicted by approxim ate hydrodynamic theories. Keeping fixed the separation between the pl ates, we studied the effect of direct and hydrodynamic interactions be tween the particles by increasing the particle concentration. In this case, the short time dynamics is characterized by means of a theoretic al approach that describes self-diffusion in terms of the static struc ture of the suspension. In all the samples studied, we found negligibl e deviations of P(Delta r,t) from Gaussian behavior.