Cooperative motions in a finite size model of liquid silica: an anomalous behavior

Abstract

Finite size effects on dynamical heterogeneity are studied in liquid silica with Molecular Dynamics simulations using the BKS potential model. When the system size decreases relaxation times are found to increase in accordance with previous results in finite-size simulations and confined liquids. It has been suggested that this increase may be related to a modification of the cooperative motions in confined liquids. In agreement with this hypothesis we observe a decrease of the dynamical heterogeneities associated to the most and the least mobile atoms when the size L decreases. However we find that the decrease of the dynamical aggregation associated to the least mobile atoms is much more important than the decrease associated to the most mobile atoms. This result is surprising as the liquid is slowed down. The decrease of the heterogeneous behavior is also in contradiction with the increase of the heterogeneities observed in liquids confined in nanopores. However an increase of the non-Gaussian parameter appears both in nanopores and in the finite size simulations. As the non-Gaussian parameter is usually associated with dynamical heterogeneities, the increase of the non-Gaussian parameter together with a decrease of dynamical heterogeneity is also surprising.