Two liquid states of matter: A new dynamic line on a phase diagram

TL;DR

This paper reveals that supercritical liquids can exist in two distinct states, 'rigid' and 'non-rigid', separated by a new dynamic line on the phase diagram, which influences many physical properties.

Contribution

It introduces a new universal dynamic line on the phase diagram that distinguishes two liquid states beyond the critical point, supported by theoretical and empirical analysis.

Findings

Transition marked by loss of shear stiffness at all frequencies

Disappearance of high-frequency sound in the non-rigid state

Specific heat reduces to 2kB per particle in the non-rigid state

Abstract

It is generally agreed that the supercritical region of a liquid consists of one single state (supercritical fluid). On the other hand, we show here that liquids in this region exist in two qualitatively different states: "rigid" and "non-rigid" liquid. Rigid to non-rigid transition corresponds to the condition {\tau} ~ {\tau}0, where {\tau}is liquid relaxation time and {\tau}0 is the minimal period of transverse quasi-harmonic waves. This condition defines a new dynamic line on the phase diagram, and corresponds to the loss of shear stiffness of a liquid at all available frequencies, and consequently to the qualitative change of many important liquid properties. We analyze the dynamic line theoretically as well as in real and model liquids, and show that the transition corresponds to the disappearance of high-frequency sound, qualitative changes of diffusion and viscous flow, increase of particle thermal speed to half of the speed of sound and reduction of the constant volume specific heat to 2kB per particle. In contrast to the Widom line that exists near the critical point only, the new dynamic line is universal: it separates two liquid states at arbitrarily high pressure and temperature, and exists in systems where liquid - gas transition and the critical point are absent overall.