Main Question or Discussion Point
A new form of ice has been discovered dubbed ice VII:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.155701Kurt Vonnegut's 1963 novel Cat's Cradle introduced the world to so-called "Ice Nine," a fictional form of water that freezes at room temperature. If it so much as touches a drop of regular water, that will freeze, too, and so on, spreading so rapidly that it freezes everything that comes into contact with it.
Fortunately for Earth, Ice-Nine doesn't exist. But there is an exotic form of ice dubbed "ice VII" that physicists can create in the laboratory. It's harmless in terrestrial conditions. But on an ocean world like Jupiter's moon, Europa, it could behave just like Ice-Nine under the right conditions, freezing an entire world within hours—with some key implications for the possibility of finding life on distant exoplanets. Now we know more about just how that special freezing process occurs, according to a recent paper in Physical Review Letters.
The fundamental study of phase transition kinetics has motivated experimental methods toward achieving the largest degree of undercooling possible, more recently culminating in the technique of rapid, quasi-isentropic compression. This approach has been demonstrated to freeze water into the high-pressure ice VII phase on nanosecond timescales, with some experiments undergoing heterogeneous nucleation while others, in apparent contradiction, suggest a homogeneous nucleation mode. In this study, we show through a combination of theory, simulation, and analysis of experiments that these seemingly contradictory results are in agreement when viewed from the perspective of classical nucleation theory. We find that, perhaps surprisingly, classical nucleation theory is capable of accurately predicting the solidification kinetics of ice VII formation under an extremely high driving force (|Δμ/kBT|≈1) but only if amended by two important considerations: (i) transient nucleation and (ii) separate liquid and solid temperatures. This is the first demonstration of a model that is able to reproduce the experimentally observed rapid freezing kinetics.