Search arXiv⌕ Search
Home/Archive/arXiv

arXiv · 2511.05588

Non-Equilibrium Stratification in Supercritical CO2

Paul Fruton·Emma Lisoir·Happiness Imuetinyan·Cédric Giraudet·Fabrizio Croccolo
View original source ↗Read source document ↗Export metadata ↓

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.

Explore related subjects

physics.chem-phphysics.flu-dyn

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Paul Fruton, Emma Lisoir, Happiness Imuetinyan, Cédric Giraudet, Fabrizio Croccolo. 2026-06-04. Non-Equilibrium Stratification in Supercritical CO2. https://arxiv.org/abs/2511.05588

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Charge Transfer with a Spin. I: A Variational Constrained-CASSCF Framework for Investigating Charge Transfer in the Presence of Spin-Orbit Coupling

Charge transfer in open-shell molecular systems can involve a delicate interplay between charge localization, orbital relaxation, and spin-orbit coupling (SOC), particularly near ground-excited state crossings. Here, we introduce a very inexpensive variational framework for treating these effects in odd-electron systems by extending the electron/hole-transfer Dynamically-weighted State-Averaged Constrained CASSCF (eDSC/hDSC) method to include SOC. Our method incorporates the SOC Hamiltonian directly into the variational orbital optimization through complex-valued spinor orbitals, allowing orbital and spin degrees of freedom to relax self-consistently while preserving the time-reversal symmetry of doublet states. The method achieves smooth potential energy surfaces and rapid self-consistent field (SCF) convergence; moreover, from physically small to artificially large SOC strengths, the calculation can be converged to very tight thresholds across the whole potential energy surface. While the results presented here are for systems with two charge centers, as shown in the appendix, the theory is quite general and can be adapted to either a system of $N$ charge centers or a metal-molecule surface with a continuum of states, so that charge transport (and not just charge transfer) can also be studied. The approach therefore provides a route towards ab initio studies of spin-dependent charge transfer and charge transport in molecular systems with nontrivial spin degrees of freedom.

physics.chem-ph↗
arXiv2026

Charge Transfer with a Spin. II: A Framework for Diabatization which Localizes Charge and Spin

We investigate a diabatization procedure that localizes charges (in real space) and localizes spins (in spin space) for open-shell systems that exhibit charge transfer in the presence of spin-orbit coupling. The procedure is applied to a two-state crossing between pairs of Kramers-restricted doublet states (which can also be considered effectively a four-state crossing). To generate the relevant diabatic states, we employ a two-step optimization over complex-unitary rotations that sequentially maximizes dipole and spin moments through iterative Jacobi sweeps; the resulting update rules are effectively equivalent to those of approximate joint diagonalization (AJD) applied to charge and spin. The method converges rapidly and yields smooth diabatic potential energy surfaces that preserve dipole and spin properties (e.g., a smoothly varying spin quantization axis) along the reaction coordinate while maintaining time-reversal symmetry.

physics.chem-ph↗
arXiv2026

Fixed-Dimensional Latent Flow for Generating Variable-Size 3D Molecules

Molecular size is coupled to composition, structure, and function, yet most 3D molecular generators require a predefined atom count. We introduce Equivariant-Free Transformer-Autoencoded Latent Flow Matching, a two-stage framework that samples a fixed-dimensional latent vector using flow matching and uses an autoregressive Transformer to determine molecular size, atom types, coordinates, and chemical attributes. Canonical atom ordering and rigid-pose alignment enable Transformers without equivariant layers, while decoded attributes guide bond reconstruction. On PCQM4Mv2, unconditional generation yields 87.9\% unique, novel molecules passing sanitization and PoseBusters checks, exceeding baselines with lower end-to-end training and sampling time and higher end-to-end throughput. Across ten target HOMO-LUMO gaps, internal ranking retains 30\% of screened candidates and increases the density functional theory-verified hit rate within 0.1 eV from 25.0\% to 52.4\%, while largely preserving novelty and diversity. These results demonstrate fixed-dimensional latent generation with autoregressive decoding as a practical approach to molecular design without prespecifying size.

physics.chem-ph↗