Thermodynamic $R$-diagrams reveal solid-like fluid states

TL;DR

This study uses thermodynamic curvature $R$-diagrams to identify solid-like and complex fluid states in common fluids, revealing regimes of solid-like behavior at high pressures and near the triple point, with implications for understanding fluid phase behavior.

Contribution

The paper introduces the use of thermodynamic $R$-diagrams to detect solid-like properties in fluids, highlighting regimes of positive $R$ at high pressures and near the triple point.

Findings

Positive $R$ indicates solid-like fluid states.

Regimes of positive $R$ occur at high pressures in all four fluids.

Water shows a decrease in $R$ on cooling, suggesting a possible second critical point.

Abstract

We evaluate the thermodynamic curvature $R$ for fluid argon, hydrogen, carbon dioxide, and water. For these fluids, $R$ is mostly negative, but we also find significant regimes of positive $R$, which we interpret as indicating solid-like fluid properties. Regimes of positive $R$ are present in all four fluids at very high pressure. Water has, in addition, a narrow slab of positive $R$ in the stable liquid phase near its triple point. Also, water is the only fluid we found having $R$ decrease on cooling into the metastable liquid phase, consistent with a possible second critical point.