Dynamics of Highly Supercooled Liquids

TL;DR

This study uses molecular dynamics simulations to reveal the heterogeneous and time-dependent nature of particle diffusion in highly supercooled liquids, highlighting deviations from classical diffusion behavior.

Contribution

It demonstrates the heterogeneity of particle diffusion on short time scales and characterizes the transition to homogeneous diffusion in supercooled liquids.

Findings

Heterogeneous diffusion occurs on time scales comparable to the structural relaxation time.

The van Hove function exhibits a large tail indicating active regions with faster diffusion.

Diffusion becomes homogeneous beyond approximately three times the stress relaxation time.

Abstract

The diffusivity of tagged particles is demonstrated to be heterogeneous on time scales comparable to or less than the structural relaxation time %taking place at the interparticle distance in a highly supercooled liquid via 3D molecular dynamics simulation. The particle motions in the relatively active regions dominantly contribute to the mean square displacement, giving rise to a diffusion constant systematically larger than the Stokes-Einstein value. The van Hove self-correlation function $G_s(r,t)$ is shown to have a large $r$ tail which can be scaled in terms of $r/t^{1/2}$ for $t \ls 3 \tau_\alpha$, where $\tau_\alpha \cong$ the stress relaxation time. Its presence indicates heterogeneous diffusion in the active regions. However, the diffusion process eventually becomes homogeneous on time scales longer than the life time of the heterogeneity structure ($\sim 3 \tau_{\alpha}$).