Self-diffusion in dilute quasi-two-dimensional hard sphere suspensions: Evanescent wave light scattering and video microscopy studies

Abstract

We report measurements of the diffusion coefficient of a dilute quasi-two-dimensional hard sphere colloidal suspension using two independent experimental techniques: evanescent wave dynamic light scattering (EWDLS) and digital video microscopy (DVM). The system studied consists of 1 μm sterically stabilized, uncharged, poly(methylmethacrylate) spheres in a very thin cell (∼3 μm). In principle, EWDSL and DVM yield identical information about the system, albeit by different analyses. When both methods are used to study the same system it is possible to directly compare measurements of preaveraged statistical dynamical quantities with their microscopic counterparts. Our EWDLS measurements yield the effective diffusion coefficient as a function of wave number and the mean square particle displacement, for fixed wave number, as a function of time. The DVM measurements generate particle trajectories; Fourier decomposition of the trajectories yields the dynamic scattering function, which is found to be in quantitative agreement with the same function measured by EWDLS. Analysis of the observed intermediate scattering function indicates that, as predicted by Cichocki and Felderhof [J. Phys. Condens. Matter 6, 7287 (1994)], in this quasi-two-dimensional system the time dependence of the evolution of the effective diffusion coefficient from its short time value to its long time value has the form (lnt)/t. To our knowledge, these results are the first experimental verification of the predicted temporal evolution of the diffusion coefficient for Brownian motion in a quasi-two-dimensional liquid. © 1996 The American Physical Society.