The Frenkel Line: a direct experimental evidence for the new thermodynamic boundary

TL;DR

This paper provides the first experimental evidence of structural crossovers in supercritical fluids, revealing a new thermodynamic boundary that challenges the traditional view of supercritical states as uniform and monotonic.

Contribution

It reports the discovery of structural crossovers in supercritical fluids through diffraction experiments and explains them using phonon theory and molecular dynamics simulations.

Findings

First experimental observation of structural crossovers in supercritical fluids

Identification of a new thermodynamic boundary in the pressure-temperature diagram

Implications for understanding supercritical phase and potential applications

Abstract

Supercritical fluids play a significant role in elucidating fundamental aspects of liquid matter under extreme conditions. They have been extensively studied at pressures and temperatures relevant to various industrial applications. However, much less is known about the structural behaviour of supercritical fluids and no structural crossovers have been observed in static compression experiments in any temperature and pressure ranges beyond the critical point. The structure of supercritical state is currently perceived to be uniform everywhere on the pressure-temperature phase diagram, and to change only in a monotonic way even moving around the critical point, not only along isotherms or isobars. Conversely, we observe structural crossovers for the first time in a deeply supercritical sample through diffraction measurements in a diamond anvil cell and discover a new thermodynamic boundary on the pressure-temperature diagram. We explain the existence of these crossovers in the framework of the phonon theory of liquids using molecular dynamics simulations. The obtained results are of prime importance since it implies a global reconsideration of the mere essence of the supercritical phase. Furthermore, this discovery may pave the way to new unexpected applications and to the exploration of exotic behaviour of confined fluids relevant to geo- and planetary sciences.