Time-dependent collective diffusion of colloidal particles.

We use a combination of numerical simulations and laboratory experiments to study the timedependent collective diffusion coefficient Dsq, td in concentrated colloidal suspensions. At short times, the particle configuration is frozen, and Dsq, td probes the temporal and spatial evolution of hydrodynamic interactions, via their effects on the particle velocities. We find that Dsq, td exhibits a surprising scaling behavior, with a single, q-dependent relaxation time, suggesting that the suspension behaves as an effective medium for hydrodynamic interactions over a wide range of length scales and time scales.