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Measurements of the low-shear viscosity eta(o) with a Zimm-Crothers viscometer for dispersions of colloidal hard spheres are reported as a function of volume fraction phi up to 0.56. Nonequilibrium theories based on solutions to the two-particle Smoluchoski equation or ideal mode coupling approximations do not capture the divergence. However, the nonhydrodynamic contribution to the relative viscosity Deltaeta(o) is correlated over a wide range of volume fractions by the Doolittle and Adam-Gibbs equations, indicating an exponential divergence at phi(m)=0.625+/-0.015. The data extend the previously proposed master curve, providing a test for improved theories for the many-body thermodynamic and hydrodynamic interactions that determine the viscosity of hard-sphere dispersions.

Dynamic light scattering is extended to optically thick (opaque) media which exhibit a very high degree of multiple scattering. This new technique, called diffusing-wave spectroscopy (DWS), exploits the diffusive nature of the transport of light in strongly scattering media to relate the temporal fluctuations of the multiply scattered light to the motion of the scatterers. A simple theory of DWS, based on the diffusion approximation for the transport of light, is developed to calculate the temporal electric field autocorrelation functions of the multiply scattered light. Two important scattering geometries are treated : transmission and backscattering. The theory is compared to experimental measurements of Brownian motion of submicron-diameter polystyrene spheres in aqueous suspension. The agreement between theory and experiment is excellent. The limitations of the photon diffusion approximation and the polarization dependence of the autocorrelation functions are discussed for the backscattering measurements. The effects of absorption of light and particle polydispersity are also incorporated into the theory and verified experimentally. It is also shown how DWS can be used to obtain information about the mean size of the particles which scatter light

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Light backscattered from an optically dense random medium is shown to exhibit a pronounced polarization dependence. An unexpected memory of the incident circular polarization of multiply scattered light arises because the wave's helicity is randomized less rapidly than is its direction.A simple model is developed to account for the observed polarization dependence of the intensity and temporal correlations of the intensity Auctuations of backscattered light.The propagation of light in optically dense random media is characterized by multiple scattering which randomizes the direction, phase, and polarization of the incident wave. This randomization accounts for the remarkable success of scalar difII'usion theory in describing the transport properties of multiply scattered light. Recent experimentsshave demonstrated the power of this approach by extending traditional quasielastic light scattering to the multiple scattering regime, thereby allowing one to probe the structure and dynamics of optically dense media. The diffusion approximation, however, fails to fully describe backscattered light, because such light is comprised of a signi6cant contribution of rrtultip/y scattered modes whose path lengths are comparable to the transport mean free path. In this Rapid Communication, we demonstrate that these modes lead to a remarkable and unexpected persistence of polarization of multiply scattered light. Single Rayleigh scattering is known to result in the polarization of scattered light in a cloudless blue sky. What is surprising is that for rirctdarly polarized light, randomization of the polarization requires many more scattering events than are required for the complete randomization of the wave's direction. This polarization memory has important consequences both for the average scattered intensity and for the temporal correlation of the intensity fluctuations of light backscattered from a time varying medium. In particular, we show that contrary to previous reports, the form of the autocorrelation functions is not universal, but instead depends on both particle size and polarization.To demonstrate the polarization memory in a multiply scattering medium, we consider a system composed of uncorrelated and noninteracting spherical particles of radius a, suspended in a liquid. The multiply scattered light results in a random speckle pattern, which fluctuates as the particles undergo Brownian motion. The backscattered intensity is comprised of the contributions of light following many different scattering paths. Each path leads to a decay of the temporal correlations of the scattered 6eld, which depends on the number of scattering events, and consequently the path length. The 6eld autocorrelation function is ' Here, P(s) is the number of scattering paths of length s and io (A/2tr) /Dg, where X is the optical wavelength in the liquid and Dg is the self-di6'usion coe%cient of the suspended particles. The time averaged intensity is given by G~(0). The transport mean free path 1 signi6es the distance the light must travel b...

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We discuss the entension of dynamic light scattering to very strongly scattering media, where the propagation of light is described by the diffusion approximation, allowing the distribution of the light paths to be determined. The temporal evolution of the length of each of these paths, due to the dynamics of the scattering medium, is calculated, and an expression for the temporal autocorrelation function of the intensity fluctuations of the scattered light is obtained. This relates the measured decay of the autocorrelation function to the dynamics of the medium. This technique is called diffusing wave spectroscopy (DWS). To extend its utility, we consider the consequences of interactions between 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 hydrodynamic interactions between a particle and the surrounding fluid. We are able to probe the decay of the velocity correlation function of the particles, and we demonstrate its algebraic decay, with a t-3 / 2 time dependence. We also show that the time-dependent self diffusion coefficient exhibits an unexpected scaling behavior, whereby all the data, from 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 rearrangement of the bubbles within the foam as they coarsen.