Short-time dynamics of colloidal suspensions

We report numerical simulations of the velocity autocorrelation function (VACF) for tagged particle motion in a colloidal suspension. We find that the asymptotic decay follows the theoretical expression for the VACF of an isolated particle, but with the suspension viscosity replacing the pure fluid viscosity (at long times the suspension behaves, so far as a tagged particle is concerned, like a fluid with the suspension viscosity—as an 'effective fluid"). While physically appealing, this observation is hard to reconcile with a recent theoretical prediction that at long times the VACF in a suspension should be the same as the VACF at infinite dilution. It also differs, in a rather subtle manner, from a scaling rule which has been used in the analysis of experimental and computer simulation results. From the scaling behavior of the VACF we conclude that effective fluid behavior only occurs on a time scale somewhat longer than the time taken for transverse momentum to diffuse a particle radius. This contrasts with the findings of earlier workers who concluded that effective fluid behavior is already observed at much shorter times.