Dynamical Behavior Near a Liquid-Liquid Phase Transition in Simulations of Supercooled Water

TL;DR

This study uses molecular dynamics simulations to explore how the diffusion coefficient of supercooled water near a liquid-liquid transition behaves, revealing a fragile-to-strong crossover linked to network formation.

Contribution

It demonstrates the connection between diffusion behavior and network defects in the low-density liquid phase of the ST2 water model, clarifying the nature of the liquid-liquid transition.

Findings

Low-density liquid shows a fragile-to-strong transition.

Diffusion coefficient relates to defect concentration in the network.

The low-density phase is a well-defined metastable liquid.

Abstract

We examine the behavior of the diffusion coefficient of the ST2 model of water over a broad region of the phase diagram via molecular dynamics simulations. The ST2 model has an accessible liquid-liquid transition between low-density and high-density phases, making the model an ideal candidate to explore the impacts of the liquid-liquid transition on dynamics. We locate characteristic dynamical loci in the phase diagram and compare them with the previously investigated thermodynamic loci. The low-density liquid phase shows a crossover from non-Arrhenius to Arrhenius behavior, signaling the onset of a crossover from fragile-to-strong behavior. We explain this crossover in terms of the asymptotic approach of the low-density liquid to a random tetrahedral network, and show that the temperature dependence of the diffusion coefficient over a wide temperature range can be simply related to the concentration of defects in the network. Our findings thus confirm that the low-density phase of ST2 water is a well-defined metastable liquid.