Shear and bulk viscosities of water up to 1.6 GPa and anomaly in the structural relaxation time

TL;DR

This study measures water's shear and bulk viscosities up to 1.6 GPa, revealing a unique pressure-induced anomaly in its structural relaxation time linked to hydrogen bond dynamics and structural changes.

Contribution

It provides the first combined experimental and simulation analysis of water's viscosities and relaxation time under high pressure, uncovering a structural anomaly in water's dynamics.

Findings

Shear viscosity increases faster than bulk viscosity with pressure.

The ratio of shear to bulk viscosity decreases two-fold up to 1.6 GPa.

Structural relaxation time reaches a minimum near 0.5 GPa, indicating a dynamic anomaly.

Abstract

Deep in the Earth's crust, pressure exceeds one thousand times the atmospheric pressure. Water still flows under these conditions, but experiences dramatic changes in structure and fluidity. Using combined dynamic and inelastic light scattering techniques, we simultaneously measure the shear and bulk viscosities of water as a function of pressure. The former increases faster than the latter, so that their ratio shows a two-fold decrease from 0 to 1.6 GPa; we confirm this trend with simulations. We analyze our results in terms of the structural relaxation time $\tau$. Contrary to other liquids, pressure initially accelerates relaxation in water. Our measurements reveal that $\tau$ reaches a minimum close to 1 ps around 0.5 GPa. We interpret $\tau$ as a the equilibration time of hydrogen bonds, and propose that the minimum in $\tau$ arises from a structural anomaly which allows fastest interconversion between local structures in water, and generates a cascade of thermodynamic and dynamic anomalies.