"Two-phase" thermodynamics of the Frenkel line

TL;DR

This paper proposes a two-phase thermodynamics model to clarify the Frenkel line as a dynamic boundary between rigid and nonrigid fluid states, resolving previous theoretical debates.

Contribution

It introduces a new thermodynamic framework that unifies existing definitions of the Frenkel line and clarifies its role as a dynamic fluid boundary.

Findings

The model provides new criteria for defining the Frenkel line.

It locates the rigid-nonrigid transition in soft- and hard-sphere models.

The Frenkel line is proposed as a dynamic boundary, independent of gas-liquid transition.

Abstract

The Frenkel line, a crossover line between rigid and nonrigid dynamics of fluid particles, has recently been the subject of intense debate regarding its relevance as a partitioning line of supercritical phase, where the main criticism comes from the theoretical treatment of collective particle dynamics. From an independent point of view, this Letter suggests that the two-phase thermodynamics model may alleviate this contentious situation. The model offers new criteria for defining the Frenkel line in the supercritical region and builds a robust connection among the preexisting, seemingly inconsistent definitions. In addition, one of the dynamic criteria locates the rigid-nonrigid transition of the soft-sphere and the hard-sphere models. Hence, we suggest the Frenkel line be considered as a dynamic rigid-nonrigid fluid boundary, without any relation to gas-liquid transition. These findings provide an integrative viewpoint combining fragmentized definitions of the Frenkel line, allowing future studies to be carried out in a more reliable manner.