Propagating rotational jump events drive liquid-liquid transition in supercooled liquid water

TL;DR

This study reveals that propagating rotational jump events, driven by inter-conversion between water molecules of different coordination, underlie the liquid-liquid transition in supercooled water, highlighting a new molecular mechanism for low-temperature anomalies.

Contribution

It introduces a novel mechanism involving propagating jump events and dynamic correlations that explain the liquid-liquid transition in supercooled water.

Findings

Propagating events originate from inter-conversion between 4- and 5-coordinated water molecules.

Dynamical correlations grow and diverge near the transition temperature.

Relaxation times of coordination species slow down dramatically at low temperatures.

Abstract

We observe, at low temperature, the appearance of propagating events that originate from the inter-conversion between adjacent four and five coordinated water molecules in supercooled liquid water, resulting in the migration of the coordination number five in a string-like fashion, creating rotational jumps along the way. The length of the connected events increases with lowering temperature. Each propagation event is terminated by a 3-coordinated species, present in a small number at large supercooling, which interacts cooperatively with 5-coordinated molecules to annihilate both the species. We find that these growing dynamical correlations manifest in a divergent-like growth of a non-linear density response function, \times4(t), which is given by a four point time correlation function (FPTCF). The locus of the maximum of \times4(t), when plotted against the time t* of maximum, exhibits a sharp peak precisely at the temperature where the static response functions (specific heat, isothermal compressibility) also show similar sharp, divergent-like, peak. While the decay of population fluctuation time correlation function of both 4- and 5- coordinated species slows down dramatically and a step-like feature of the relaxation becomes evident, the lifetime itself of 5-corrdinated species remains short. These results suggest a new molecular mechanism of low temperature anomalies and of the liquid-liquid transition in terms of initiation, growth and termination of propagating jump and inter-conversion events.