Tagged particle correlations in a dense fluid: Role of microscopic dynamics

Abstract

We study the self diffusion process in a dense fluid in two cases for which the individual fluid particles follow respectively deterministic Newtonian dynamics (ND) and stochastic Brownian dynamics (BD). The role of correlated motion of fluid particles on the single particle diffusion is taken in to account here using mode coupling models. We demonstrate the subtle difference present in the corresponding mode coupling corrections to diffusion constants for fluids with BD and ND by carefully analysing the equations of fluctuating nonlinear hydrodynamics (FNH) in the respective cases. Finally, we use recently developed model for dynamics of binary mixtures to model a BD fluid in terms of a ND mixture whose species have a very large mass ratio. Here the high inertia larger particles are identified as Brownian and a mechanical interpretation of the noise in a Brownian fluid is made in terms of the smaller particles randomly colliding with them.