  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.
