Time-Dependent Collective Diffusion of Colloidal Particles

We use a combination of numerical simulations and laboratory experiments to study the time-dependent collective diffusion coefficient $D(q, t)$ in concentrated colloidal suspensions. At short times, the particle configuration is frozen, and $D(q, t)$ probes the temporal and spatial evolution of hydrodynamic interactions, via their effects on the particle velocities. We find that $D(q, t)$ 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.