A new look at supercooled liquid water

Liquid water can be cooled below 0° C without freezing, a phenomenon known as supercooling.  However, below -50° C, the properties of supercooled water are difficult to measure due to rapid ice crystallization.  There is a long-standing question of whether there exist two distinct phases of liquid water under deeply supercooled conditions, called "high density liquid" and "low density liquid".  This system is very difficult to study experimentally due to rapid ice crystallization, but also difficult computationally as it requires a highly accurate model potential, large simulation cell sizes, and long simulation times.

In a study recently published in the Proceedings of the National Academy of Sciences, the Wang and Donadio research groups made a breakthrough in demonstrating evidence for two phases of supercooled liquid water.  Using a highly accurate, state-of-the-art water model (iAMOEBA), they found large fluctuations of the density of around 10% at -85° C (188 K) and 1600 atm, close to the hypothetical liquid-liquid critical point.  Next, they constructed a simulation cell containing high-density and low-density liquid side by side, and tracked the time evolution of the interface with an order parameter that assigns the phase of each water molecule based on its local molecular environment.  They found a stable interface between the liquid phases and measured the interfacial tension, thereby showing that a free energy barrier exists between the liquid phases.  The new simulations shed new light on this famous problem, which may have implications for liquid water at non-supercooled conditions, as well as confined water in biological or geological contexts.