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

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.

Nucleation in Sheared Granular Matter

“…Our results are qualitatively different from the crystallization dynamics observed in constant volume molecular dynamics simulations of supercooled hard spheres: the existence of a nucleation barrier in our system disagrees with the autocatalytic growth of crystalline regions ob- served in [23]. Further, we do not observe burst-like growth events as seen in mature glasses [24,25]. These differences indicate that nucleation in our sheared granular system is governed by its own dynamics.…”

“…A bed of frictional hard spheres increases in density if subjected to repeated small disturbances that are introduced, for example, by shearing, shaking, or fluidization. The repeated shear cycles in our experiment break some of the force chains that form a skeleton supporting the bed, and this leads to compaction [24]. However, this simple picture does not address the homogeneous nucleation seen in our experiment ( Fig.…”

Crystallization Instability in Glass-Forming Mixtures

“…These regions can be larger than the critical crystal nucleus size of the one-component system under similar conditions. Nucleation in these regions is fast on the timescale of this deeply supercooled liquid, apparently requiring little rearrangement of the particles, as is known to be the case for hard spheres at deep supercooling [47,48]. Our findings are important, as the results we reveal here pose a fundamental challenge for the development of glass-forming materials: mixtures whose components crystallize easily are themselves inherently unstable to crystallization and thus ultimately compromised as glass formers.…”

Two-time correlations for probing the aging dynamics of glassy colloids

“…As has been observed previously [8][9][10][11], those distributions are characterized by broadened, non-Fickian tails that get broader with time ∆t, but each in a manner that is characteristic of its age. Those tails clearly emphasize the fact that anomalously large fluctuations in the displacement of particles drive the structural relaxation, which proves intimately related [12,13] to the spatial dynamic heterogeneity as well as the temporal intermittency that have been observed in many experiments [8,11,[14][15][16][17][18][19][20][21][22]. The dynamic events of interest are therefore rare and nonself-averaging, requiring many independent simulations of large systems to yield clear results.…”