Spatiotemporal heterogeneity of local free volumes in highly supercooled liquid

TL;DR

This study investigates the spatiotemporal behavior of local free volumes in supercooled liquids, revealing that while dynamical heterogeneity grows with decreasing temperature, free volume correlations do not, and long-range vibrations may influence large-scale dynamics.

Contribution

It introduces a novel analysis linking local free volumes, soft modes, and dynamical heterogeneity in supercooled liquids using molecular dynamics simulations.

Findings

Dynamical heterogeneity correlation length increases as temperature decreases.

No growth observed in correlation length of local free volumes.

Soft modes extend over free volumes and correlate with broken bonds.

Abstract

We discuss the spatiotemporal behavior of local density and its relation to dynamical heterogeneity in a highly supercooled liquid by using molecular dynamics simulations of a binary mixture with different particle sizes in two dimensions. To trace voids heterogeneously existing with lower local densities, which move along with the structural relaxation, we employ the minimum local density for each particle in a time window whose width is set along with the structural relaxation time. Particles subject to free volumes correspond well to the configuration rearranging region of dynamical heterogeneity. While the correlation length for dynamical heterogeneity grows with temperature decrease, no growth in the correlation length of heterogeneity in the minimum local density distribution takes place. A comparison of these results with those of normal mode analysis reveals that superpositions of lower-frequency soft modes extending over the free volumes exhibit spatial correlation with the broken bonds. This observation suggests a possibility that long-ranged vibration modes facilitate the interactions between fragile regions represented by free volumes, to induce dynamical correlations at a large scale.