Brownian motion in quasibidimensional colloidal suspensions

Digital video microscopy is used to study the Brownian motion in quasibidimensional colloidal systems, consisting of spherical polystyrene particles suspended in water and confined between two glass plates. This technique allows the direct measurement of the lateral (two-dimensional) probability distribution function, $P(\Delta \mathbf{r},t),$ of the random variable Δr (the particle displacement) at time $t,$ and the mean squared displacement $W\left(t\right).$ We studied the effect of confinement in highly diluted samples, where $W\left(t\right)\mathrm{}$ 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 approximate hydrodynamic theories. Keeping fixed the separation between the plates, we studied the effect of direct and hydrodynamic interactions between the particles by increasing the particle concentration. In this case, the short time dynamics is characterized by means of a theoretical approach that describes self-diffusion in terms of the static structure of the suspension. In all the samples studied, we found negligible deviations of $P(\Delta \mathbf{r},t)$ from Gaussian behavior.