DYNAMICS OF COLLOIDAL HARD-SPHERES IN THIN AQUEOUS SUSPENSION LAYERS - PARTICLE TRACKING BY DIGITAL IMAGE-PROCESSING AND BROWNIAN DYNAMICS COMPUTER-SIMULATIONS

A new experimentally simple technique is introduced for studying dynamical properties of hard sphere colloids in thin aqueous suspension layers by light-microscopy observation supported by computer-aided digital image processing. The thickness of the layers of the colloidal samples confined between two smooth glass plates is accurately adjusted by monodisperse "spacer" spheres which are larger than the diffusing spheres. Tracking of single particles in concentrated phases is accomplished using fluorescence light microscopy where a few dyed particles are mixed with the undyed colloidal spheres of the same size. First results are presented for the self-diffusion coefficient-(i) in very dilute suspensions as a function of the layer thickness to particle size ratio, and (ii) as a function of the sphere volume fraction. For a layer thickness more than twice the particle size our results agree well with experimental and theoretical 3-d results given in the literature. They also agree with Brownian dynamics simulations performed in two dimensions. Our 2-d simulations done on a PC are in good agreement with the corresponding 3-d simulations of Cichocki and Hinsen (Physica A166, 473 (1990)) where a large Cray computer was necessary. © 1993 Academic Press, Inc.