Temperature dependence of spatially heterogeneous dynamics in a model of viscous silica

TL;DR

This study uses molecular dynamics simulations to explore how spatially heterogeneous dynamics in viscous silica evolve with temperature, revealing that cluster growth persists even when transport follows Arrhenius behavior, challenging bond-breaking relaxation models.

Contribution

It demonstrates that spatial heterogeneity and cluster growth in silica persist across temperature regimes, contrasting with fragile liquids and highlighting the role of covalent bonds in dynamics.

Findings

Cluster size peaks at intermediate times and increases with decreasing temperature.

Heterogeneous dynamics are similar qualitatively in strong and fragile glass formers.

String-like motion is minimal in silica due to covalent bonds.

Abstract

Molecular dynamics simulations are performed to study spatially heterogeneous dynamics in a model of viscous silica above and below the critical temperature of the mode coupling theory, $T_{MCT}$. Specifically, we follow the evolution of the dynamic heterogeneity as the temperature dependence of the transport coefficients shows a crossover from non-Arrhenius to Arrhenius behavior when the melt is cooled. It is demonstrated that, on intermediate time scales, a small fraction of oxygen and silicon atoms are more mobile than expected from a Gaussian approximation. These highly mobile particles form transient clusters larger than that resulting from random statistics, indicating that dynamics are spatially heterogeneous. An analysis of the clusters reveals that the mean cluster size is maximum at times intermediate between ballistic and diffusive motion, and the maximum size increases with decreasing temperature. In particular, the growth of the clusters continues when the transport coefficients follow an Arrhenius law. These findings imply that the structural relaxation in silica cannot be understood as a statistical bond breaking process. Though the mean cluster sizes for silica are at the lower end of the spectrum of values reported in the literature, we find that spatially heterogeneous dynamics in strong and fragile glass formers are similar on a qualitative level. However, different from results for fragile liquids, we show that correlated particle motion along quasi one-dimensional, string-like paths is of little importance for the structural relaxation in this model of silica, suggesting that string-like motion is suppressed by the presence of covalent bonds.