# Universality, scaling and collapse in supercritical fluids

## TL;DR

This paper reveals universal scaling laws in supercritical fluids, showing that their thermodynamic and dynamic properties can be described by a single master curve across different substances and conditions.

## Contribution

It introduces a machine learning-based order parameter that uncovers universal scaling relations and collapse phenomena in supercritical fluids, suggesting a common underlying physics.

## Key findings

- All isotherms collapse onto a single master curve via a scaling relation.
- Phase diagrams of different fluids collapse onto their master curves with the same scaling exponent.
- Results support a model of supercritical fluids as a mixture of two interchangeable microstates.

## Abstract

The Supercritical Fluid (SCF) is known to exhibit salient dynamic and thermodynamic crossovers and inhomogeneous molecular distribution. But the question as to what basic physics underlies these microscopic and macroscopic anomalies remains open. Here, using an order parameter extracted by machine learning, the fraction of gas-like (or liquid-like) molecules, we find simplicity and universality in SCF: First, all isotherms of a given fluid collapse onto a single master curve described by a scaling relation. The observed power law holds from the high-temperature and pressure regime down to the critical point where it diverges. Second, phase diagrams of different compounds collapse onto their master curves by the same scaling exponent, thereby demonstrating a putative law of corresponding supercritical states in simple fluids. The reported results support a model of the SCF as a mixture of two interchangeable microstates, whose spatiotemporal dynamics gives rise to unique macroscopic properties.