Imaging of dynamic heterogeneities in multiple-scattering media

A new method of visualizing objects with distinct internal dynamics of the constituent scattering particles embedded in a liquid multiple-scattering medium is presented. We report dynamic multiple-light-scattering experiments and a theoretical model, based on diffusing photon-density waves for concentrated colloidal suspensions in Brownian motion, as a background medium into which is inserted a capillary containing (i) the same suspension under flow, or (ii) suspensions of different particle sizes in Brownian motion. These model objects, with purely dynamic but no static scattering contrast, can be visualized by space-resolved measurements of the time autocorrelation function g(2)(tau) of the scattered light intensity at the sample surface. Maximum contrast occurs at a parameter-dependent finite correlation time tau. The physical origin of this effect is-outlined Our data are in excellent quantitative agreement with the model, with no adjustable parameter. (C) 1997 Optical Society of America.