Does mesoscopic elasticity control viscous slowing down in glassforming liquids?

TL;DR

This study investigates whether mesoscopic elastic stiffness, related to local excitations, controls the viscous slowdown in glass-forming liquids, proposing a new perspective beyond macroscopic elasticity models.

Contribution

The paper introduces a mesoscopic elastic stiffness $ppa(T)$ as a key factor in glassy dynamics, linking it to energy barriers and highlighting differences based on energy landscape granularity.

Findings

$ppa(T)$ increases more rapidly than $G(T)$ with decreasing temperature.

A proportionality $ppa(T) \u2200 elta E(T)$ holds for some liquids.

Highly granular energy landscapes correlate with the failure of the $ppa(T)$-barrier relation.

Abstract

The dramatic slowing down of relaxation dynamics of liquids approaching the glass transition remains a highly debated problem, where the crux of the puzzle resides in the elusive increase of the activation barrier $\Delta E(T)$ with decreasing temperature $T$. A class of theoretical frameworks -- known as elastic models -- attribute this temperature dependence to the variations of the liquid's macroscopic elasticity, quantified by the high-frequency shear modulus $G_\infty(T)$. While elastic models find some support in a number of experimental studies, these models do not take into account the spatial structures, length scales, and heterogeneity associated with structural relaxation in supercooled liquids. Here, we propose that viscous slowing down is controlled by a mesoscopic elastic stiffness $\kappa(T)$, defined as the characteristic stiffness of response fields to local dipole forces in the liquid's underlying inherent states. First, we show that $\kappa(T)$ -- which is intimately related to the energy and length scales characterizing quasilocalized, nonphononic excitations in glasses -- increases more strongly with decreasing $T$ than the macroscopic inherent state shear modulus $G(T)$ in several computer liquids. Second, we show that the simple relation $\Delta E(T)\propto\kappa(T)$ holds remarkably well for some computer liquids, implying a direct connection between the liquid's underlying mesoscopic elasticity and enthalpic energy barriers. On the other hand, we show that for other computer liquids, the above relation fails. Finally, we provide strong evidence that what distinguishes computer liquids in which the $\Delta E(T) \propto \kappa(T)$ relation holds, from those in which it does not, is that the latter feature highly granular potential energy landscapes, where many sub-basins separated by low activation barriers exist. [Rest of abstract abridged]