"Liquid-gas" transition in the supercritical region: Fundamental changes in the particle dynamics

TL;DR

This paper identifies a fundamental dynamic transition line in the supercritical phase of fluids, marking a shift from liquid-like to gas-like behavior, independent of the critical point, based on particle dynamics and sound dispersion.

Contribution

It introduces a new dynamic line in the supercritical phase diagram and links it to changes in particle dynamics and sound properties, distinct from the critical point.

Findings

Dynamic transition line coincides with changes in velocity autocorrelation and mean-square displacement.

Positive sound dispersion disappears near the dynamic transition line.

The liquid-like dynamic region shrinks with increasing repulsive potential exponent.

Abstract

Recently, we have proposed a new dynamic line on the phase diagram in the supercritical region. Crossing this line corresponds to the radical changes of the fluid properties. Here, we focus on the dynamics of model Lennard-Jones and Soft-Sphere fluids. We show that the change of the dynamics from the liquid-like to gas-like can be established on the basis of the velocity autocorrelation function and mean-square displacement. Using the rigorous criterion, we show that the crossover of particle dynamics and key liquid properties occurs at the same line. We further show that positive sound dispersion disappears in the vicinity of this line in both kinds of systems. The dynamic line bears no relationship to the existence of the critical point. We find that the region of existence of liquid-like dynamics narrows with the increase of the exponent of the repulsive part of inter-particle potential.