Anomalous Behavior of the Homogeneous Ice Nucleation Rate in “No-Man’s Land”

We present an analysis of ice nucleation
kinetics from near-ambient
pressure water as temperature decreases below the homogeneous limit TH by cooling micrometer-sized droplets (microdroplets)
evaporatively at 103–104 K/s and probing
the structure ultrafast using femtosecond pulses from the Linac Coherent
Light Source (LCLS) free-electron X-ray laser. Below 232 K, we observed
a slower nucleation rate increase with decreasing temperature than
anticipated from previous measurements, which we suggest is due to
the rapid decrease in water’s diffusivity. This is consistent
with earlier findings that microdroplets do not crystallize at <227
K, but vitrify at cooling rates of 106–107 K/s. We also hypothesize that the slower increase in the nucleation
rate is connected with the proposed “fragile-to-strong”
transition anomaly in water.