Silica through the eyes of colloidal models—when glass is a gel

Abstract

We perform molecular dynamics simulations of ‘floating bond’ (FB) models ofnetwork-forming liquids and compare the structure and dynamics against the BKS modelof silica (van Beest et al 1990 Phys. Rev. Lett. 64 1955), with the aim of gaining a betterunderstanding of glassy silica in terms of the variety of non-ergodic states seen in colloids.At low densities, all the models form tetrahedral networks. At higher densities, tailoring theFB model to allow a higher number of bonds does not capture the structure seen inBKS. Upon rescaling the time and length in order to compare mean squareddisplacements between models, we find that there are significant differences in thetemperature dependence of the diffusion coefficient at high density. Additionally, the FBmodels show a greater range in variability in the behavior of the non-ergodicityparameter and caging length, quantities used to distinguish colloidal gels and glasses.Hence, we find that the glassy behavior of BKS silica can be interpreted as a ‘gel’at low densities, with only a marginal gel-to-glass crossover at higher densities.