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-sc attering medium is presented. We report dynamic multiple-light-scatter ing experiments and a theoretical model, based on diffusing photon-den sity 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 pa rticle sizes in Brownian motion. These model objects, with purely dyna mic but no static scattering contrast, can be visualized by space-reso lved measurements of the time autocorrelation function g(2)(tau) of th e scattered light intensity at the sample surface. Maximum contrast oc curs at a parameter-dependent finite correlation time tau. The physica l origin of this effect is-outlined Our data are in excellent quantita tive agreement with the model, with no adjustable parameter. (C) 1997 Optical Society of America.