Molecular Dynamic Simulations of Liquid Water on Cooling

Owing to a small volume of droplets, a considerable fraction of water aerosols may remain in liquid state (supercooled liquid) under the thermodynamic conditions of the upper troposphere and lower stratosphere for long periods of time. In addition to fundamental importance of the problem of metastable liquids, there are  important practical aspects as the usual way of treating the IR spectra of supercooled water droplets in remote sensing is to use a mixed phase, which is the sum of the spectral contributions of hexagonal ice and “normal” liquid water. Although this approach seems to be adequate in many cases, it may lead to substantial uncertainty in the retrieving of the spectra of supercooled water samples.

Small (micron-sized) water droplets in a cryogenic flow tube were probed by IR spectroscopy. The analysis of the IR spectra suggests that there is considerable "ice-like" fraction of molecules in supercooled water over the temperature range between 300K and 240K.

The molecular dynamics simulations for systems of 1000 and 1728 molecules in NVT ensemble using the SPC/E potential function were carried out in the temperature range between 240 and 300K.

The combined IR spectroscopy and MD simulations study suggests  that a relatively large fraction of molecules in supercooled water tend to assemble into multiple small (30 to 100 molecules, see figures --------->) clusters before freezing. If these are viewed as pre-critical nuclei, the freezing mechanism may then be interpreted as the extension of this process (formation of ordered domains) to a point where several domains have a non-zero chance to overlap thus forming a critical nucleus on which the crystalline ice lattice forms

There is a relative increase of about 30% (see figure -------->) in the fraction of low density domains (clusters) in water on cooling over the temperature range between 300K and 240K

Snapshot of the system of 1000 water molecules simulated at 240K. Only oxygen atoms are shown.  Red circles show the molecules that belong to the low density domain (cluster) in supercooled water

Typical cluster in supercooled water

Fraction of "ice-like" molecules in supercooled water as a function of temperature