Multiple Light Scattering from Isotropic and Anosotropic Turbid Media

The Brownian motion of particles in a fluid is a problem with spectacular historical roots [1, 2] and continues to provide us with intriguing new physics. Recently there has been resurging interest in the first steps of these motions [3, 4, 5, 6, 7, 8]. This is largely a result of new experimental probes, such as Diffusing-Wave Spectroscopy (DWS) [7, 9, 10, 11], that enable us to measure particle displacements down to 1Å or even smaller, and thereby offer the possibility to quantitatively test hydrodynamic theories [4, 6] of nondiffusive particle motion, especially in the first random walk step. In the first experiment we present here, we use Diffusing-Wave Interferometry (DWI) [12, 13] to resolve the mean square displacement, (∆r2(τ)), of particles in hard-sphere suspensions during the first twenty nanoseconds of their motion. These are the shortest time scales ever probed in the study of Brownian dynamics.