Citation:
Abstract:
Motion of particles in optically dense media gives rise to temporal fluctuations in the intensity of multiply scattered light. We show that useful information about the dynamics of the scatterers can be obtained from measurements of the temporal autocorrelation functions of these fluctuations in the multiply scattered light. We develop a phenomenological theory, which models the transport of light as a random walk between scatterers, and obtain explicit expressions for the autocorrelation functions for several experimental geometries. These expressions are compared with experiments probing the dynamics of colloidal suspensions and are shown to be in excellent agreement with the data. The dependence of the autocorrelation functions on the experimental geometry provides a powerful means of exploring the particle dynamics over vastly different length and time scales. Thus, this technique extends the conventional single scattering technique of Dynamic Light Scattering to the multiple scattering regime. We call this new technique Diffusing Wave Spectroscopy (DWS). We illustrate the power of DWS by applying it to measure the particle size in concentrated suspensions and to study the diffusion of particles in porous media and the flow of particles under shear. In addition, we show that DWS can be extended to study the dynamics of interacting colloids by including the consequences of the correlations between the particle positions and velocities. DWS can also be used to study the nature of the transport of light in disordered systems and, in particular, the limitations of using a continuum diffusion approximation. To exploit this, we show that other quantities, such as the angular dependence of the coherent backscattering cone and the absorption dependence of the incoherent backscattering intensity, depend on the distribution of light paths through the sample in the same way as the temporal autocorrelation functions obtained in backscattering.