Visualization of the collective vortex-like motions in liquid argon and water: Molecular dynamics simulation

We propose a new measure of collectivity of molecular motion in the liquid: the average vector of displacement of the particles, , which initially have been localized within a sphere of radius R-sph and then have executed the diffusive motion during a time interval Delta t. The more correlated the motion of the particles is, the longer will be the vector . We visualize the picture of collective motions in molecular dynamics (MD) models of liquids by constructing the vectors and pinning them to the sites of the uniform grid which divides each of the edges of the model box into equal parts. MD models of liquid argon and water have been studied by this method. Qualitatively, the patterns of vectors are similar for these two liquids but differ in minor details. The most important result of our research is the revealing of the aggregates of vectors which have the form of extended flows which sometimes look like the parts of vortices. These vortex-like clusters of vectors have the mesoscopic size (of the order of 10 nm) and persist for tens of picoseconds. Dependence of the vector field on parameters R-sph, Delta t, and on the model size has been investigated. This field in the models of liquids differs essentially from that in a random-walk model. Published by AIP Publishing.