Thirty milliseconds in the life of a supercooled liquid

Abstract

We combine the swap Monte Carlo algorithm to long multi-CPU molecular dynamics simulations to analyse the equilibrium relaxation dynamics of model supercooled liquids over a time window covering ten orders of magnitude for temperatures down to the experimental glass transition temperature Tg. The analysis of time correlation functions coupled to spatio-temporal resolution of particle motion allow us to elucidate the nature of the equilibrium dynamics in deeply supercooled liquids. We find that structural relaxation starts at early times in rare localised regions characterised by a waiting time distribution that develops a power law near Tg. At longer times, relaxation events accumulate with increasing probability in these regions as Tg is approached. This accumulation leads to a power-law growth of the linear extension of relaxed domains with time, with a large dynamic exponent. Past the average relaxation time, unrelaxed domains slowly shrink with time due to relaxation events happening at their boundaries. Our results provide a complete microscopic description of the particle motion responsible for key experimental signatures of glassy dynamics, from the shape and temperature evolution of relaxation spectra to the core features of dynamic heterogeneity. They also provide a microscopic basis to understand the emergence of dynamic facilitation in deeply supercooled liquids and allow us to critically reassess theoretical descriptions of the glass transition.

Elasticity, Facilitation and Dynamic Heterogeneity in Glass-Forming liquids

“…The authors of Refs. [16,17] have shown that dynamic facilitation is enhanced at lower temperatures and very likely plays an important role in the asymmetric relaxation spectra found in molecular liquid experiments [18]. Note that the mean-field facilitated trap models introduced in [19,20] shares important conceptual similarities with the one studied in this work.…”

“…The first one encompasses the numerical studies which have demonstrated the existence and the importance of dynamic facilitation in the slow dynamics of atomistic models of glass-forming liquids. These results were first obtained at temperatures close to the Mode-Coupling-Theory cross-over [7,15] and then recently at lower temperatures (closer to the glass transition) [11,16,17]. The authors of Refs.…”

“…Determining from atomistic simulations of glass-forming liquids which one of these scenarios holds is a very interesting open challenge, see e.g., Refs. [7,15,17].…”

Predicting dynamic heterogeneity in glass-forming liquids by physics-informed machine learning

“…3a. The MD results show how marginally rearranged active clusters at small times (white and red) coarsen with time and become both larger and more strongly contrasted to the unrelaxed background (blue) [35]. GlassMLP is able to predict remarkably well the location and the geometry of the relaxing clusters from the sole knowledge of the initial structure.…”

“…Neither proposal is consistent with our data. Because dynamic heterogeneity appears increasingly contrasted with more compact boundaries at lower temperatures [35], we introduce a cubic term q 3 by analogy with Porod's law describing two-phase systems with sharp interfaces [47]: S 4 (q, t) = χ4 (t)/ 1 + (ξq) 2 + A(ξq) 3 . This expression contains the minimal ingredients to describe both the evolution of the characteristic length scale ξ (Fig.…”