Diffusing acoustic wave spectroscopy of fluidized suspensions

The velocity fluctuations and local strain rate in a fluidized suspension o f particles are investigated using the new ultrasonic technique of diffusin g acoustic wave spectroscopy. DAWS probes the relative motion of the partic les at very short length scales down to the inter-particle separation, and allows the spatial correlations of the velocity fluctuations to be probed b y varying the transport mean free path of the diffusing ultrasonic waves. O ur results demonstrate the power of this ultrasonic technique to probe the dynamics of sedimenting particles at larger length scales and Reynolds numb ers than can be achieved by light scattering methods. (C) 2000 Elsevier Sci ence B.V. All rights reserved.