Two liquid states of matter: A new dynamic line on a phase diagram (1104.3414v2)

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.

• The paper introduces a new dynamic line based on the condition τ ≈ τ₀ that distinguishes rigid and non-rigid liquid states.
• It employs simulations with Lennard-Jones and soft-sphere models, identifying an isochoric crossover in diffusion and viscosity behaviors.
• The findings challenge traditional phase diagrams and offer insights for industrial applications of supercritical fluids.

Overview of "Two Liquid States of Matter: A New Dynamic Line on a Phase Diagram"

The paper by Brazhkin et al. proposes a hypothesis challenging the traditional understanding of the supercritical state of matter. Rather than viewing the supercritical region as a uniform phase, the authors delineate two distinct liquid states: "rigid" and "non-rigid" liquids, defined by their shear resistance properties. This analysis represents a significant advancement in the comprehension of liquid states beyond conventional thermodynamic transitions by introducing a new dynamic line in phase diagrams.

Key Findings and Theoretical Framework

The delineation between "rigid" and "non-rigid" liquid states is primarily governed by the relationship between the liquid relaxation time ($\tau$) and the minimal period of transverse quasi-harmonic waves ($\tau_0$). When $\tau \approx \tau_0$, this denotes a transition point where liquids lose their shear stiffness entirely and adopt properties resembling those of gases. This identification results in a new dynamic line that transcends the classical boundaries defined by the presence of a liquid-gas critical point or transition, effectively existing at any pressure or temperature.

Significantly, the researchers assert that this dynamic line does not correlate with the Widom line. While the Widom line is constrained by proximity to the critical point, the dynamic line described here persists in systems without a liquid-gas transition.

Observational and Simulation Evidence

The research provides evidence for the existence of the proposed dynamic line through analysis of Lennard-Jones and soft-sphere potential model systems and real substances such as Ar, Ne, and N$_2$. The authors reveal that the conditions corresponding to $\tau = \tau_0$ result in noticeable changes in the mechanical and thermal properties of liquids, such as the vanishing of high-frequency sound and a transformation in the temperature dependence of diffusion and viscosity.

The numerical investigations highlight that this dynamic line consistently demonstrates an isochoric crossover from exponential to power-law temperature dependencies for both diffusion and viscosity. Their soft-sphere models further illustrate that no critical anomalies deviate the dynamic behavior, lending credence to the universality of this dynamic line.

Practical and Theoretical Implications

From a practical standpoint, this newfound understanding could inform the development of industrial processes involving supercritical fluids. The applications might encompass chemical engineering fields where precise manipulation of liquid states under extreme conditions is required.

Theoretically, the paper enhances foundational knowledge on phase transitions, integrating kinetic and dynamic factors into the traditionally thermodynamic-centric view. This work extends the framework initially proposed by J. Frenkel regarding liquid dynamics, solidifying the notable role of relaxation phenomena.

Furthermore, the research specifies methodologies to experimentally identify this dynamic line by focusing on parameters such as changes in specific heat and the absence of positive sound dispersion. Such identification can lead to precise mapping of phase behaviors in materials science.

In conclusion, Brazhkin et al. present a compelling case for reconsidering the properties of supercritical fluids, integrating dynamic lines into the conventional phase diagram. This advance prompts a revision of liquid phase analysis and presages future exploration into states of matter beyond classical models.