Greg A. Kimmel, Yuntao Xu, Nikolay G. Petrik, R. Scott Smith and Bruce D. Kay
Pacific Northwest National Laboratory, Richland, WA USA 99352
A detailed understanding of supercooled water is crucial to solving many of the remaining mysteries of liquid water. More experimental data in this temperature range is needed, but progress is impeded by the existence of “no man’s land,” the temperature range from approximately 160 to 230 K in which spontaneous crystallization occurs so fast that most experimental techniques are precluded. We have developed a pulsed laser heating method that allows us to investigate deeply supercooled water. Nanoscale water films adsorbed on metal crystal at ~90 K are transiently-heated by a laser pulse, transforming them into liquids for ~10 ns per laser pulse. Subsequent rapid cooling due to the dissipation of the heat pulse into the metal substrate effectively quenches the liquid dynamics until the next laser heating pulse arrives. The rapid heating and cooling allow us to explore supercooled water at temperatures ranging from the melting point down to ~180 K.
We have used this technique to determine the growth, G(T), of crystalline ice across “no man’s land”. The self-diffusion of supercooled liquid water, D(T), was obtained from G(T) using the Wilson-Frenkel model of crystal growth. For T > 237 K and P ~ 10-8 Pa, G(T) and D(T) have super-Arrhenius (“fragile”) temperature dependences, but both crossover to Arrhenius (“strong”) behavior with a large activation energy in “no man’s land.” The fact that G(T) and D(T) are smoothly varying rules out the hypothesis that liquid water’s properties have a singularity at or near 228 K at ambient pressures. However, the results are consistent with a previous prediction for D(T) that assumed no thermodynamic transitions occur in “no man’s land.” Using the measured growth rates of crystalline ice, the homogeneous nucleation rate of crystalline ice, J(T), has been determined from the observed crystallization kinetics of transiently-heated water films. For thick water films (e.g. 240 nm) where nucleation at the interfaces can be excluded, nucleation rates greater than 1026 m-3s-1 are required to explain the observations, but the large temperature gradients across those films make it difficult to determine the temperature dependence in detail. For thinner water films (e.g. ≤ 30 nm), we find that the nucleation rate is maximized at ~216 K ± 4 K and rapidly decreases at lower temperatures. The maximum nucleation rate observed in these transiently-heated thin films, ~5 ×1028 ± 1, is consistent with recent measurements in nanoscale water drops at comparable temperatures.
This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences.