DIFFUSIVE WAVE ILLUMINATION - LIGHT-SCATTERING STUDY OF COLLOIDAL DYNAMICS IN OPAQUE MEDIA

Microscopic movements of latex particles in the water-filled voids of a glass bead packing are investigated with a variant of dynamic light- scattering, diffusive wave illumination. We are able to measure simult aneously diffusive and convective motion of the scatterers within the pore spaces of the completely opaque medium. The influence of increase d seepage velocity on the diffusion coefficient and on the speed distr ibution of the particles is studied. We find an exponential distributi on of the convective speed. From the average speed we determine the to rtuosity of the packing. Using this investigation system as an illustr ation, we address the methodological aspects of diffusive wave illumin ation and discuss its possibilities and limitations: (1) the possibili ty of tuning the amount of local oscillator mixed to the dynamic signa l, (2) the inclusion of residual dynamics of the porous medium into th e fitting model, and (3) a study of the influence of increased particl e concentration on the signal correlation function. (C) 1998 Optical S ociety of America.