Looking Inside the Widom Region: Non-Equilibrium Stratification in Supercritical CO2

TL;DR

This study reveals that supercritical CO2 under non-equilibrium conditions exhibits spontaneous stratification and layered structures near Widom lines, challenging the classical view of a homogeneous supercritical phase.

Contribution

It provides experimental evidence of non-equilibrium stratification and dynamic layering in supercritical CO2, highlighting complex behaviors near Widom lines.

Findings

Spontaneous stratification into layers observed in supercritical CO2.

Presence of Brunt-Vaisala oscillations indicating layered structure.

Layered behavior occurs when crossing Widom lines.

Abstract

The supercritical state of matter is usually described as a continuous phase without sharp boundaries between liquid and gas regions. However, under non-equilibrium conditions, this view breaks down. Here we report an experimental investigation of non-equilibrium fluctuations in supercritical carbon dioxide (CO2) subjected to a stabilising temperature gradient. Using shadowgraphy, we reveal spontaneous stratification of the fluid into different layers, separated by transition regions, where thermodynamic properties vary dramatically. These signatures are particularly evident when the system crosses the Widom lines, loci of the extrema of the response function in the supercritical domain. The analysis of the intermediate scattering function of temperature fluctuations highlights the presence of Brunt-Vaisala oscillations within the fluid at multiple frequencies. These oscillations arise from the coupling of thermal and viscous modes under gravity and are a clear signature of the layered structure of the fluid. Our approach enables systematic exploration of a wide range of thermodynamic conditions in a single experiment. These findings suggest that the Widom region cannot be described as a homogeneous phase, but rather as a dynamic assembly of phase-like behaviours, challenging the applicability of classical thermodynamics in non-equilibrium supercritical regimes.