THERMAL RELAXATION OF SUPERCRITICAL FLUIDS BY EQUILIBRIUM MOLECULAR-DYNAMICS

Two-dimensional molecular dynamics (MD) simulations are performed in the NVT ensemble (constant number of particles, volume, and temperature) with a truncated and shifted Lennard-Jones potential in order to model highly compressible fluids near the critical point. The thermodynamic and transport properties are computed for three different supercritical states (same critical density and different temperatures) and one ''normal'' fluid with a different density. The static properties are obtained from the relevant equilibrium fluctuations while the time-dependent properties are computed here in terms of the equilibrium time-correlation functions (TCF's). The TCF's of local density, current, as well as temperature are calculated. The thermal diffusivity and the sound velocity decrease when the temperature is approaching the critical temperature, thus showing the expected behavior. However, this behavior is only a nearly critical one, since this study has been performed in a temperature range where the hydrodynamic description still appears to be valid. Indeed, good agreement has been found between the equilibrium fluctuations computed by MD and hydrodynamics. The time scales for the normal and the supercritical fluids are quite different: a considerably slower decay of the fluctuation relaxations is observed in supercritical fluids. © 1995 The American Physical Society.