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Researchers experimentally investigated supercritical carbon dioxide under non-equilibrium conditions with a temperature gradient and discovered that the fluid spontaneously stratifies into distinct layers rather than remaining homogeneous. Using shadowgraphy techniques, they observed dramatic variations in thermodynamic properties at transition regions, particularly when crossing Widom lines where response functions reach extrema. The study detected Brunt-Vaisala oscillations at multiple frequencies, indicating a layered structure that challenges the classical view of supercritical fluids as continuous, homogeneous phases.
Why it matters
These findings have significant implications for industrial applications involving supercritical CO2, including carbon capture and storage, enhanced oil recovery, and power generation systems, where understanding non-equilibrium behavior is crucial for system design and safety. The research suggests that classical thermodynamic assumptions may not adequately describe supercritical fluids in real-world conditions where temperature gradients are common.
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arXiv:2511.05588v3 Announce Type: replace
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.
Source: Looking Inside the Widom Region: Non-Equilibrium Stratification in Supercritical CO2