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

Taiki Yanagishima,Hajime Tanaka,John Russo

doi:10.1038/ncomms15954

Taiki Yanagishima, Hajime Tanaka + Show 1 more

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https://doi.org/10.1038/ncomms15954

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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 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.

Highlights

The glassy state is known to undergo slow structural relaxation, where the system progressively explores lower free-energy minima which are either amorphous or crystalline
Sanz et al.[20] found that crystallization occurs through discrete collective events, where groups of particles suddenly undergo large displacements, accompanied by an sudden increase in the proportion of crystalline particles in the system: these events were termed ‘avalanches’ for their intermittent, collective nature. They first found that the randomization of particle velocities in their molecular dynamics simulation averted the incidence of the events, and second, that polydispersity suppressed the growth of crystallinity while maintaining the same intermittent dynamics
We show that this motion leads to a transient loss in the force balance of the system, and it is this that gives rise to the cascade of particle motion which we perceive as an ‘avalanche’

Summary

Introduction

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). 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. What makes the kinetics of the phenomena in glasses special remains unclear, since the relation of ageing and devitrification to corresponding processes in the supercooled liquid state, that is, structural relaxation and crystallization, respectively, are yet to be ascertained We address these fundamental problems by studying structural evolution during avalanches in both ageing and devitrifying hard-sphere systems. We believe this to correspond to the avalanche devitrification identified by Sanz et al, though we go on to show that this happens in ageing systems as well

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