Universal two-component dynamics in supercritical fluids

TL;DR

This study reveals a universal two-component microscopic dynamic behavior in supercritical fluids, linking liquidlike bonding and gaslike free-particle motions, which governs their transition properties.

Contribution

It demonstrates a universal two-component dynamic pattern in supercritical fluids and connects the ratio of these components to key fluid properties, providing new insights into their microscopic behavior.

Findings

Universal two-component behavior observed across different supercritical fluids.

The ratio of components correlates with the fluid's self-diffusion coefficient.

Crossover from liquidlike to gaslike dynamics occurs around the Widom line.

Abstract

Despite the technological importance of supercritical fluids, controversy remains about the details of their microscopic dynamics. In this work, we study four supercritical fluid systems -- water, Si, Te, and Lennard-Jones fluid -- \emph{via} classical molecular dynamics simulations. A universal two-component behavior is observed in the intermolecular dynamics of these systems, and the changing ratio between the two components leads to a crossover from liquidlike to gaslike dynamics, most rapidly around the Widom line. We find evidence to connect the liquidlike component dominating at lower temperatures with intermolecular bonding, and the component prominent at higher temperatures with free-particle, gaslike dynamics. The ratio between the components can be used to describe important properties of the fluid, such as its self-diffusion coefficient, in the transition region. Our results provide insight into the fundamental mechanism controlling the dynamics of supercritical fluids, and highlight the role of spatiotemporally inhomogenous dynamics even in thermodynamic states where no large-scale fluctuations exist in the fluid.