Tagged particle motion in a dense liquid: feedback effects from the collective dynamics

Abstract

The nature of the tagged particle motion in the strongly correlated state of a dense liquid is studied with the self-consistent mode-coupling model. The tagged particle time correlation function ψ _s(q,t) is computed by taking into account the nonlinear feedback effects on its dynamics from the coupling with density fluctuations. We consider the two cases where (a) the short-time dynamics is diffusive resembling colloidal system and (b) the short-time dynamics is Newtonian as in an atomic system. The non-Gaussian parameter α ₂(t) is evaluated using the fourth- and second-order spatial moments of the van Hove self-correlation function G_s(r,t). We observe a two-peaked structure of α₂(t) for both (a) and (b) types of dynamics. We also compare other characteristic aspects of tagged particle dynamics such as the mean square displacement, non-Gaussian nature of G_s(r,t), and fraction of mobile particles. A qualitative comparison is drawn between the theoretical results with the experimental and computer simulation results on colloids.