Kinetic Heterogeneities at Dynamical Crossovers

TL;DR

This study uses molecular dynamics simulations to analyze kinetic heterogeneities in a glass-forming liquid, revealing two types of dynamical crossovers unrelated to imminent glass or jamming transitions.

Contribution

It introduces a method to measure kinetic heterogeneities across different length and time scales and identifies two distinct dynamical crossovers near the glass transition.

Findings

Two types of maxima in dynamic susceptibility are identified.

Kinetic heterogeneities are not necessarily linked to glass or jamming transitions.

Crossovers in dynamical behavior are associated with these maxima.

Abstract

We perform molecular dynamics simulations of a model glass-forming liquid to measure the size of kinetic heterogeneities, using a dynamic susceptibility $\chi_{\rm ss}(a, t)$ that quantifies the number of particles whose dynamics are correlated on the length scale $a$ and time scale $t$. By measuring $\chi_{\rm ss}(a, t)$ as a function of both $a$ and $t$, we locate local maxima $\chi^\star$ at distances $a^\star$ and times $t^\star$. Near the dynamical glass transition, we find two types of maxima, both correlated with crossovers in the dynamical behavior: a smaller maximum corresponding to the crossover from ballistic to sub-diffusive motion, and a larger maximum corresponding to the crossover from sub-diffusive to diffusive motion. Our results indicate that kinetic heterogeneities are not necessarily signatures of an impending glass or jamming transition.