Kinetic structure of a two-dimensional liquid

On an intermediate time scale, a molecular dynamics simulation of a two-dimensional (2D) liquid of soft disks exhibits dramatic structural and kinetic inhomogeneities. We introduce a local relaxation time (defined as the time required for a particle to first travel a distance r from its initial position) in order to present a quantitative analysis of this kinetic structure. The length r is chosen so as to maximize the resolution of the kinetic structure. This analysis produces a natural measure of cooperativity. We demonstrate that caging in the 2D liquid takes place in a subset of particles only, distributed as transient clusters throughout the liquid.