The kinetic fragility of liquids as manifestation of the elastic softening

TL;DR

This paper demonstrates a universal relationship between liquid fragility and elastic softening, linking thermodynamics and kinetics, supported by simulations and experimental data, and introduces a predictive elastic master curve for viscosity and relaxation times.

Contribution

It reveals a universal elastic softening effect on fragility and introduces an elastic master curve that predicts viscosity and relaxation times across diverse glassformers.

Findings

Fragility increases with elastic softening in a universal manner.

A fragility-independent elastic master curve extends over 18 decades.

Viscosity of supercooled silica is predicted over 15 decades without adjustable parameters.

Abstract

We show that the fragility $m$, the steepness of the viscosity and relaxation time close to the vitrification, increases with the degree of elastic softening, i.e. the decrease of the elastic modulus with increasing temperature, in universal way. This provides a novel connection between the thermodynamics, via the modulus, and the kinetics. The finding is evidenced by numerical simulations and comparison with the experimental data of glassformers with widely different fragilities ($33 \le m \le 115$), leading to a fragility-independent elastic master curve extending over eighteen decades in viscosity and relaxation time. The master curve is accounted for by a cavity model pointing out the roles of both the available free volume and the cage softness. A major implication of our findings is that ultraslow relaxations, hardly characterised experimentally, become predictable by linear elasticity. As an example, the viscosity of supercooled silica is derived over about fifteen decades with no adjustable parameters.