TEMPORAL AND SPATIAL DEPENDENCE OF HYDRODYNAMIC CORRELATIONS - SIMULATION AND EXPERIMENT

Time-dependent hydrodynamic interactions in a colloidal suspension of hard spheres are studied, both experimentally and through computer sim ulation. The focus is on the behavior at small wave vectors, which dir ectly probes the temporal evolution of hydrodynamic interactions betwe en nearby particles. The computer simulations show that the time-depen dent diffusion coefficient has the same functional form for all wave v ectors, with a single characteristic scaling time for each length scal e and for each volume fraction. Wave-vector-averaged effective diffusi on coefficients, measured experimentally using diffusing wave spectros copy, also scale to the same functional form. In this case, the scalin g time is dependent on both volume fraction and particle size; it decr eases sharply with decreasing particle radius, reflecting the greater contribution from smaller wave vectors that is contained in the scatte ring from the smaller particles. For a direct comparison of simulation and experiment, we simulate the experimentally observed correlation f unctions, by averaging the wavevector-dependent computer-simulation da ta with the weighting appropriate to the experimental technique. Altho ugh the overall scaling is similar, there are quantitative differences in the simulated and measured relaxation times. We suggest these diff erences are due to the compressibility of the suspension, and that the resultant pressure waves make an unexpectedly significant contributio n to the hydrodynamic interactions.