DIFFUSING-WAVE SPECTROSCOPY - THE TECHNIQUE AND SOME APPLICATIONS

We discuss the entension of dynamic light scattering to very strongly scattering media, where the propagation of light is described by the d iffusion approximation, allowing the distribution of the light paths t o be determined. The temporal evolution of the length of each of these paths, due to the dynamics of the scattering medium, is calculated, a nd an expression for the temporal autocorrelation function of the inte nsity fluctuations of the scattered light is obtained. This relates th e measured decay of the autocorrelation function to the dynamics of th e medium. This technique is called diffusing wave spectroscopy (DWS). To extend its utility, we consider the consequences of interactions be tween the scattering particles on the light scattering. To illustrate its applications, we consider several examples of new physics that can be investigated using DWS. We study the transient nature of hydrodyna mic interactions between a particle and the surrounding fluid. We are able to probe the decay of the velocity correlation function of the pa rticles, and we demonstrate its algebraic decay, with a t(-3/2) rime d ependence. We also show that the time-dependent self diffusion coeffic ient exhibits an unexpected scaling behavior, whereby all the data, fr om samples of different volume fractions, can be scaled onto a single curve. Finally, we discuss the applications of DWS to the study of the dynamics of foams, and show how it can be used to probe the rearrange ment of the bubbles within the foam as they coarsen.