Dynamic sound scattering investigation of the dynamics of sheared particulate suspensions

We show how Dynamic Sound Scattering (the acoustical analog of Dynamic Light Scattering) can be used to measure the average velocity profile and the particle velocity variance of neutrally buoyant suspensions in the Couette geometry. The motion of the particles was determined from the temporal autocorrelation function of the scattered field fluctuations in a single speckle spot. In contrast to theories that assume isotropic particle velocity fluctuations, it is shown that "suspension temperature" (defined as the average kinetic energy contained in the particle velocity fluctuations) is a tensor, whose components have different magnitudes. For particle volume fractions up to 0.4, components of the fluctuations parallel to the flow and in the vertical direction are much larger than in the radial direction. For higher concentrations the fluctuation anisotropy decreases.