Time scale for the onset of Fickian diffusion in supercooled liquids

TL;DR

This paper introduces a quantitative measure for the onset of Fickian diffusion in supercooled liquids, using simulations to analyze how this onset time varies with temperature in a Lennard-Jones mixture.

Contribution

It provides a new quantitative measure for the Fickian onset time and explores its temperature dependence in supercooled liquids through simulations.

Findings

Fickian onset time ranges from 6.5 to 31 times the alpha relaxation time.

Onset time increases faster than the alpha relaxation time as temperature decreases.

Mean squared displacement at onset increases with decreasing temperature.

Abstract

We propose a quantitative measure of a time scale on which Fickian diffusion sets in for supercooled liquids and use Brownian Dynamics computer simulations to determine the temperature dependence of this onset time in a Lennard-Jones binary mixture. The time for the onset of Fickian diffusion ranges between 6.5 and 31 times the $\alpha$ relaxation time (the $\alpha$ relaxation time is the characteristic relaxation time of the incoherent intermediate scattering function). The onset time increases faster with decreasing temperature than the $\alpha$ relaxation time. Mean squared displacement at the onset time increases with decreasing temperature.