Hydrodynamic interactions in concentrated suspensions

We study the effects of hydrodynamic interactions on the diffusion of hard spheres in concentrated suspensions. Using a multiple-light-scattering technique that measures the early-time behavior, we find ${\mathit{D}}_{\mathrm{eff}}$/${\mathit{D}}_0$=1-(1.86±0.07)φ, where φ is the volume fraction of spheres, ${\mathit{D}}_{\mathrm{eff}}$ is the effective diffusion coefficient, and ${\mathit{D}}_0$ is the free-particle diffusion coefficient. This agrees with the linear φ term calculated theoretically for short-time self-diffusion. The short-time diffusion coefficient is also found to be continuous across the freezing transition.