Nanoscale Dynamics of Phase Flipping in Water near its Hypothesized Liquid-Liquid Critical Point

TL;DR

This study investigates the nanoscale dynamics of water near its hypothesized liquid-liquid critical point, revealing rapid phase flipping and crystallite melting, and accurately locating the critical point through finite-size scaling analysis.

Contribution

It provides the first detailed analysis of phase flipping dynamics and critical point location in the ST2 water model near the hypothesized liquid-liquid transition.

Findings

Water flips between high and low density states at nanoscale.

Crystallites melt within nanoseconds near the critical point.

Finite-size scaling accurately locates the liquid-liquid critical point.

Abstract

Achieving a coherent understanding of the many thermodynamic and dynamic anomalies of water is among the most important unsolved puzzles in physics, chemistry, and biology. One hypothesized explanation imagines the existence of a line of first order phase transitions separating two liquid phases and terminating at a novel "liquid-liquid" critical point in a region of low temperature ($T \approx 250 \rm{K}$) and high pressure ($P \approx 200 \rm{MPa}$). Here we analyze a common model of water, the ST2 model, and find that the entire system flips between liquid states of high and low density. Further, we find that in the critical region crystallites melt on a time scale of nanoseconds. We perform a finite-size scaling analysis that accurately locates both the liquid-liquid coexistence line and its associated liquid-liquid critical point.