MULTIPLE LIGHT-SCATTERING IN TAYLOR-COUETTE FLOW

We report dynamic light scattering experiments on turbid colloidal suspension under stationary and laminar flow, as well as in the regime of flow instabilities. It is shown that the time autocorrelation function C1(t) = [E(0)E*(t)]/[\ E(0)\ 2] of the scattered light field E(t) is not sensitive to the mean velocity flow but rather to the root mean square of velocity gradient. C1(t) is characterised on the level of each scattering event by the correlation time required by a pair of scatterers initially separated by a transport mean free path to move a relative distance of optical wavelength due to the velocity gradient. We verified this theoretical analysis using planar Couette flow as an example for homogeneous velocity gradients, and planar Poiseuille flow for inhomogeneous velocity gradients. Agreement between experiment and theory is excellent. Finally, this technique is applied to spatially varying velocity gradient fields for measuring the threshold and wave number of the Taylor-Couette instability. This illustrates the possibility of studying hydrodynamic instabilities and quasi-local velocity gradients even under conditions of strong multiple scattering.