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

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 crystallize 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 crystallization. Furthermore, this suggests that transient force balance is what distinguishes glasses from supercooled liquids. Glass is characterized by stochastic and slow structural relaxation dynamics, whose details remain elusive due to its complicated kinetic processes. Here, the authors show that avalanche-like dynamics in both ageing and devitrifying glasses are governed by thermodynamic initiation and a transient loss in mechanical stability.