Liquid-like behavior of supercritical fluids

TL;DR

This study reveals that supercritical fluids exhibit liquid-like dynamics with sound velocities exceeding hydrodynamic predictions, and links these behaviors to the extrapolated liquid-vapor boundary and metal-metal transitions.

Contribution

It demonstrates that supercritical fluids maintain liquid-like dynamics beyond the critical point and connects these behaviors to structural and electronic transitions.

Findings

Supercritical oxygen shows liquid-like sound velocity exceeding hydrodynamic values.

The liquid-vapor coexistence line extrapolation marks the transition between liquid- and gas-like dynamics.

Connection between structural modifications and transport properties in dense fluids.

Abstract

The high frequency dynamics of fluid oxygen have been investigated by Inelastic X-ray Scattering. In spite of the markedly supercritical conditions ($T\approx 2 T_c$, $P>10^2 P_c$), the sound velocity exceeds the hydrodynamic value of about 20%, a feature which is the fingerprint of liquid-like dynamics. The comparison of the present results with literature data obtained in several fluids allow us to identify the extrapolation of the liquid vapor-coexistence line in the ($P/P_c$, $T/T_c$) plane as the relevant edge between liquid- and gas-like dynamics. More interestingly, this extrapolation is very close to the non metal-metal transition in hot dense fluids, at pressure and temperature values as obtained by shock wave experiments. This result points to the existence of a connection between structural modifications and transport properties in dense fluids.