Applicability of dynamic facilitation theory to binary hard disk systems

TL;DR

This study numerically tests the applicability of dynamic facilitation theory to binary hard disk systems under pressure, revealing localized excitations and relaxation behaviors consistent with the theory.

Contribution

It demonstrates that dynamic facilitation theory applies to pressure-controlled glass-forming systems, extending its relevance beyond temperature-driven models.

Findings

Relaxation times follow a parabolic law with pressure.

Excitations are spatially localized and decay exponentially with pressure.

Energy scales logarithmically depend on excitation size.

Abstract

We investigate numerically the applicability of dynamic facilitation (DF) theory for glass-forming binary hard disk systems where supercompression is controlled by pressure. By using novel efficient algorithms for hard disks, we are able to generate equilibrium supercompressed states in an additive non-equimolar binary mixture, where micro-crystallization and size segregation do not emerge at high average packing fractions. Above an onset pressure where collective heterogeneous relaxation sets in, we find that relaxation times are well described by a "parabolic law" with pressure. We identify excitations, or soft-spots, that give rise to structural relaxation, and find that they are spatially localized, their average concentration decays exponentially with pressure, and their associated energy scale is logarithmic in the excitation size. These observations are consistent with the predictions of DF generalized to systems controlled by pressure rather than temperature.