Heterogeneous diffusion, viscosity and the Stokes Einstein relation in binary liquids

TL;DR

This study explores the breakdown of the Stokes-Einstein relation in binary liquids, revealing that dynamic heterogeneity and particle mobility variations cause deviations, especially at lower temperatures.

Contribution

It demonstrates how dynamic heterogeneity affects diffusivity and viscosity in binary liquids, introducing a 'slow' viscosity concept to recover the SER.

Findings

Weak SER breakdown at high temperatures due to collectivized motion

Strong breakdown at low temperatures linked to increased heterogeneity

Identification of slow particles with diffusivity less than SER predictions

Abstract

We investigate the origin of the breakdown of the Stokes-Einstein relation (SER) between diffusivity and viscosity in undercooled melts. A binary Lennard-Jones system, as a model for a metallic melt, is studied by molecular dynamics. A weak breakdown at high temperatures can be understood from the collectivization of motion, seen in the isotope effect. The strong breakdown at lower temperatures is connected to an increase in dynamic heterogeneity. On relevant timescales some particles diffuse much faster than the average or than predicted by the SER. The van-Hove self correlation function allows to unambiguously identify slow particles. Their diffusivity is even less than predicted by the SER. The time-span of these particles being slow particles, before their first conversion to be a fast one, is larger than the decay time of the stress correlation. The contribution of the slow particles to the viscosity rises rapidly upon cooling. Not only the diffusion but also the viscosity shows a dynamically heterogeneous scenario. We can define a "slow" viscosity. The SER is recovered as relation between slow diffusivity and slow viscosity.