Convection and thermodiffusion of colloidal gold tracers by laser light scattering

In adynamic light scattering experiment, we have investigated the time inte nsity correlation function and the profile of the transmitted laser beam fo r organic dispersions of light absorbing colloidal particles containing tin y gold clusters. The correlation functions have been found to show a superp osition of an exponential decay, corresponding to Brownian motion of the tr acers, and well-defined oscillations. These oscillations are caused by conv ection due to local beating of the sample by the incident laser beam, which has been confirmed independently via measurements of the local temperature within the sample. It will be shown how the particle convection velocity, which is the order of 1 mm/s, can be obtained quantitatively from the oscil lating correlation functions. The profile of the transmitted beam allowed u s to determine the Soret coefficient, which is a measure for the thermal di ffusion of the particles. This article shows how tracer diffusion, convecti on, and thermal diffusion can be determined simultaneously by one single ex periment, laser light scattering of light absorbing colloidal particles in dispersion. [S1063-651X(99)09708-1].