Fragility in Glassy Liquids: A Structural Approach Based on Machine Learning

TL;DR

This paper investigates the structural origins of fragility in glassy liquids using machine learning to identify a structural order parameter called softness, revealing factors that control the transition from strong to fragile behavior.

Contribution

It introduces a machine learning-based structural parameter, softness, that is consistent across different glassy liquids and links it to fragility.

Findings

Softness correlates strongly with dynamical rearrangements.

Support vector machine identifies a universal structural descriptor.

Factors controlling fragility are linked to softness variations.

Abstract

The rapid rise of viscosity or relaxation time upon supercooling is universal hallmark of glassy liquids. The temperature dependence of the viscosity, however, is quite non universal for glassy liquids and is characterized by the system's "fragility," with liquids with nearly Arrhenius temperature-dependent relaxation times referred to as strong liquids and those with super-Arrhenius behavior referred to as fragile liquids. What makes some liquids strong and others fragile is still not well understood. Here we explore this question in a family of glassy liquids that range from extremely strong to extremely fragile, using "softness," a structural order parameter identified by machine learning to be highly correlated with dynamical rearrangements. We use a support vector machine to identify softness as the same linear combination of structural quantities across the entire family of liquids studied. We then use softness to identify the factors controlling fragility.