Viscosity and self-diffusion of supercooled and stretched water from molecular dynamics simulations

TL;DR

This study uses molecular dynamics simulations to explore the anomalous viscosity and self-diffusion behaviors of supercooled and stretched water, revealing new anomalies at negative pressures and analyzing these with a dynamic two-state model.

Contribution

It extends the dynamic two-state model to simulate water's anomalies over a broad pressure range, including negative pressures, and compares simulation results with experimental data.

Findings

Reproduces experimental viscosity and diffusion anomalies in water.

Identifies additional anomalies at negative pressure.

Shows decoupling of viscosity and diffusion at low temperatures.

Abstract

Among the numerous anomalies of water, the acceleration of dynamics under pressure is particularly puzzling. Whereas the diffusivity anomaly observed in experiments has been reproduced in several computer studies, the parallel viscosity anomaly has received less attention. Here we simulate viscosity and self-diffusion coefficient of the TIP4P/2005 water model over a broad temperature and pressure range. We reproduce the experimental behavior, and find additional anomalies at negative pressure. The anomalous effect of pressure on dynamic properties becomes more pronounced upon cooling, reaching two orders of magnitude for viscosity at 220 K. We analyze our results with a dynamic extension of a thermodynamic two-state model, an approach which has proved successful in describing experimental data. Water is regarded as a mixture of interconverting species with contrasting dynamic behaviors, one being strong (Arrhenius), and the other fragile (non-Arrhenius). The dynamic parameters of the two-state models are remarkably close between experiment and simulations. The larger pressure range accessible to simulations suggests a modification of the dynamic two-state model, which in turn also improves the agreement with experimental data. Furthermore, our simulations demonstrate the decoupling between viscosity $\eta$ and self-diffusion coefficient $D$ as a function of temperature $T$. The Stokes-Einstein relation, which predicts a constant $D \eta/T$, is violated when $T$ is lowered, in connection with the Widom line defined by an equal fraction of the two interconverting species. These results provide a unifying picture of thermodynamics and dynamics in water, and call for experiments at negative pressure.