High frequency longitudinal and transverse dynamics in water

TL;DR

This study uses high-resolution inelastic x-ray scattering to explore the complex longitudinal and transverse dynamics of liquid water across various thermodynamic conditions, revealing the transition from viscous to elastic behavior and identifying a transverse dynamic signature.

Contribution

It provides new insights into water's dynamic structure by linking the observed excitations to structural relaxation and viscoelastic transitions, highlighting the transverse dynamics in water.

Findings

Confirmation of two inelastic contributions in water's S(Q,)

Observation of the transition from viscous to elastic sound velocity

Identification of a transverse dynamic signature in the scattering data

Abstract

High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S(Q,\omega) of liquid water have been performed for wave vectors Q between 4 and 30 nm^-1 in distinctly different thermodynamic conditions (T= 263 - 420 K ; at, or close to, ambient pressure and at P = 2 kbar). In agreement with previous inelastic x-ray and neutron studies, the presence of two inelastic contributions (one dispersing with Q and the other almost non-dispersive) is confirmed. The study of their temperature- and Q-dependence provides strong support for a dynamics of liquid water controlled by the structural relaxation process. A viscoelastic analysis of the Q-dispersing mode, associated with the longitudinal dynamics, reveals that the sound velocity undergoes the complete transition from the adiabatic sound velocity (c_0) (viscous limit) to the infinite frequency sound velocity (c_\infinity) (elastic limit). On decreasing Q, as the transition regime is approached from the elastic side, we observe a decrease of the intensity of the second, weakly dispersing feature, which completely disappears when the viscous regime is reached. These findings unambiguously identify the second excitation to be a signature of the transverse dynamics with a longitudinal symmetry component, which becomes visible in the S(Q,\omega) as soon as the purely viscous regime is left.