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

TL;DR

This study combines experiments and simulations to elucidate the viscous-to-elastic crossover in liquids, revealing the mechanisms of sound propagation and the role of transverse phononic excitations in supercritical fluids.

Contribution

It introduces a Hamiltonian model predicting transverse sound gaps and links positive sound dispersion to the viscous-elastic transition, supported by experimental and simulation data.

Findings

Identification of adiabatic-to-isothermal sound transition

Confirmation of transverse sound gaps in supercritical Ar

Universal link between PSD and transverse phonons

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 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 subdomain 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.