Common mechanism of thermodynamic and mechanical origin for ageing and crystallisation of glasses

TL;DR

This paper reveals a unified mechanism involving thermodynamic and mechanical factors that drive both ageing and crystallisation in glasses, highlighting the role of particle rearrangements and transient force balance.

Contribution

It uncovers a common kinetic pathway for ageing and crystallisation in glasses driven by particle rearrangements and transient force imbalance.

Findings

Particle rearrangements occur in low-density regions with specific structural order.

Avalanche-like collective displacements facilitate structural transitions.

Transient force imbalance distinguishes glasses from supercooled liquids.

Abstract

The glassy state is known to undergo slow structural relaxation, where the system progressively explores lower free-energy minima which are either amorphous (ageing) or crystalline (devitrification). Recently, there is growing interest in the unusual intermittent collective displacements of a large number of particles known as "avalanches". However, their structural origin and dynamics are yet to be fully addressed. Here, we study hard-sphere glasses which either crystallise or age depending on the degree of size polydispersity, and show that a small number of particles are thermodynamically driven to rearrange in regions of low density and bond orientational order. This causes a transient loss of mechanical equilibrium which facilitates a large cascade of motion. Combined with previously identified phenomenology, we have a complete kinetic pathway for structural change which is common to both ageing and crystallisation. Furthermore, this suggests that transient force balance is what distinguishes glasses from supercooled liquids.