This study will pioneer the use of a newly emerging light scattering technique, diffusive wave spectroscopy (DWS), to the study of the dynamics of structure formation in colloidal systems. In DWS applied to colloidal suspensions, light scatters off of many colloid particles before leaving the system. The scattered light is measured by a PM tube. For each particular photon path, the displacement of the particles in the path ( ri(t)) gives rise to a change in the scattering intensity. By measuring the scattering intensity as a function of time, the autocorrelation g1(t) of the scattered light can be obtained. It is assumed that (1) the net phase change for the scatterings along a photon path is a Gaussian random variable, (2) the displacement vector of a particle in a photon path ( ri(t)) and the scattering vector qi are uncorrelated, and (3) the direction of propogation becomes uncorrelated over a length l*. With these assumptions, the contribution to the correlation function for one photon path can be written in terms of the mean square displacement < r2>, l* and the path length s. To obtain the contribution of path lengths of length s to the total scattering intensity, the energy density of the light at time t and position r (the photons per unit volume U(t,r)) is first obtained by solving a diffusion equation for the particular sample geometry, and from this function the weighting factor can be obtained. The transport mean free path is obtained by a calibration procedure in which an autocorrelation is measured on particles which are not interacting and the mean square displacement can be estimated from theory. The technique will be used to probe the influence of ionic stregth and concentration on short time particle dynamics in disordered and ordered systems.
