8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Tue 611:23-14:00P2.85Kinetics of thermostatted ice growth from supercooledwater in simulationsVolker C. Weiss, 1 Markus Rullich, 1 Christof Köhler, 1 and Thomas Frauenheim 11 Universität Bremen, Bremen Center for Computational Materials Science, Am Fallturm 128359, Bremen, GermanyIn experiments, the growth rate of ice from supercooled water has been seen to increase with thedegree of supercooling, i.e., the lower the temperature, the faster the crystallization [1]. Recently,it has become feasible to study the growth of preformed ice crystals in contact with supercooledwater at the molecular level by molecular dynamics simulations [2, 3]. In these simulations, thetemperature is usually kept constant by means of a thermostat that removes the heat releasedduring the crystallization process by scaling the velocities of the particles. This artificial removalof energy is believed to be responsible for the curious observation that the thermostatted icegrowth proceeds fastest near the melting point and more slowly at lower temperatures, thus exactlyopposite to the experimental findings [2]! This trend is explained by the diffusion of moleculesin the liquid becoming the rate-determining step, which is slower at low temperatures [2, 3]. Inextended simulations of the TIP4P/Ice model, we show that the actual temperature dependence ofthe thermostatted ice growth is more complex: The crystallization process is very slow close to themelting point where the thermodynamic driving force is weak. The growth rate initially increaseson lowering the temperature and displays a maximum near 260 K. At even lower temperatures,the freezing process slows down again due to the lower diffusivity.[1] H. R. Pruppacher, Pure Appl. Geophys. 68, 186 (1967).[2] M. A. Carignano, P. B. Shepson, I. Szleifer, Mol. Phys. 103, 2957 (2005).[3] R. G. Fernandez, J. L. F. Abascal, C. Vega, J. Chem. Phys. 124, 144506 (2006).85