de la Matière Condensée

Persistent correlations in colloidal suspension

Thomas Franosch (Univ. Innsbruck)

Transport properties  of a hard-sphere colloidal fluid are investigated by Brownian dynamics simulations. We implement a novel algorithm for the time-dependent velocity-autocorrelation function (VACF) essentially eliminating the noise of the bare random motion. The measured VACF reveals  persistent  anti-correlations manifested by a negative algebraic power-law tail $t^{-5/2}$ at all densities. At small packing fractions the simulations fully agree with the analytic low-density prediction, yet  the amplitude of the tail becomes dramatically suppressed as the  packing fraction is increased. The mode-coupling theory of the glass transition provides a qualitative explanation for the strong variation   in terms of the static compressibility as well as the  slowing down of the structural relaxation.

In the second part of the presentation, I will discuss a microrheological set-up where a single probe particle is immersed in a complex fluid and exposed to a strong external force driving the system out of equilibrium. The time-dependent response of a probe particlein a dilute suspension of Brownian particles to a large step-force is derived analytically, exact in first order of the density of the bath particles. The time-dependent drift velocity approaches its stationary state value exponentially fast for arbitrarily small driving in striking contrast to the power-law prediction of linear response encoded in the long-time tails of the velocity autocorrelation function. We show that the stationary-state behavior depends nonanalytically on the driving force and connect this behavior to the persistent correlations in the equilibrium state.