Revealing the Mechanism of the Viscous-to-Elastic Crossover in Liquids

Abstract

In this work we report on inelastic X-ray scattering experiments combined with the molecular dynamics simulations on deeply supercritical Ar. The presented results unveil the mechanism and regimes of sound propagation in the liquid matter, and provide a compelling evidence for the adiabatic-to-isothermal longitudinal sound propagation transition. We introduce a Hamiltonian predicting low-frequency transverse sound propagation gaps which is confirmed by experimental findings and molecular dynamics calculations. As a result, a universal link is established between the positive sound dispersion (PSD) phenomenon and the origin of transverse sound propagation revealing the viscous-to-elastic crossover in liquids. The PSD and transverse phononic excitations evolve consistently with theoretical predictions. Both can be considered as a universal fingerprint of the dynamic response of a liquid which is also observable in a sub-domain of supercritical phase. The simultaneous disappearance of both these effects at elevated temperatures is a manifestation of the Frenkel line. We expect that these findings will advance the current understanding of fluids under extreme thermodynamic conditions.

Continuum mechanics and thermodynamics in the Hamilton and the Godunov-type formulations

“…We emphasize that the viscoelastic and even solid like properties of simple liquids at small time scales is an experimental fact, see e.g. [15,10,12,11,13] and was predicted by Frenkel [26].…”

Onset of transverse (shear) waves in strongly-coupled Yukawa fluids

“…This phenomenon, referred to as the q-gap in the transverse mode, is a well known property of the fluid state. [27][28][29][30] It applies to conventional neutral fluids, but also to the charged plasma-related systems at strong coupling. Not surprisingly, it received considerable attention in the context of complex (dusty) plasmas within the YOCP concept.…”

The Spectrum of Density Fluctuations of Noble Gases Probed by THz Neutron and X-ray Spectroscopy

“…More recently, a joint IXS and MD simulation work on deeply supercritical argon [108] evidenced a close connection between the ability of a fluid system to support shear mode propagation and the presence of a sizeable PSD. In this work IXS spectra were measured along a nearly isobaric path (with P « 1 GPa) with T spanning the 300 K-436 K temperature range, while MD data spanned the same isobar within a larger T-interval (up to 800 K).…”

“…Furthermore, in the work of Bolmatov and collaborators [108] the Frenkel line [110] was identified as the crossover line between the two thermodynamic regions. The interpretation of the Frenkel line as a crossover line demarcating the presence/absence of transverse acoustic propagation was originally discussed in [111].…”

“…The corresponding hydrodynamic dispersion is also reported as a dashed line for comparison. Adapted with permission from [108]. Copyright (2016) of American Chemical Society.…”