Nanosecond Freezing of Water at the Limits of Metastability and Classical Nucleation Theory

The importance and ubiquity of water is universally recognized. The phase diagram of water, which encapsulates its equilibrium behavior, is still not firmly established. Understanding its non-equilibrium behavior presents an even greater challenge. One of the most familiar non-equilibrium processes is freezing, and of particular interest is freezing at pressures above 2 GPa. These are conditions found in the icy sheets of some Jovian moons, as well as extrasolar super-Earths. Several studies have performed dynamic compression experiments to drive water well into the ice VII region of the phase diagram. The studies report a number of findings, including an apparent limit of metastability at around 7 GPa, where water undergoes homogeneous nucleation and freezes to ice VII on a nanosecond timescale. Here we show how a theoretical/computational modeling framework based on classical nucleation theory can provide both a qualitative and quantitative explanation for many of these observations. This is the first demonstration that such a framework can accurately capture nanosecond freezing kinetics under high-pressure dynamic compression.