(694a) Homogeneous Nucleation of Ice in Droplets and Free-Standing Films of Supercooled Water

The problem of homogeneous ice nucleation is of immense importance to fields as diverse as biology and chemistry, to geology and atmospheric sciences. Of particular interest is the formation of ice in confined geometries. One such example is the nucleation of ice in droplets and free-standing thin films, systems that are especially important in geological and atmospheric sciences. In this work, we use molecular simulations to compute the rates and unravel the mechanism of ice formation in droplets and thin films of supercooled water. The water model used in this study is the monoatomic potential proposed by Molinero et al. [1]. This model has no explicit hydrogen atoms, and the local tetrahedrality of water molecules that arises from the formation of hydrogen bonds is enforced by including a three-body interaction term in the inter-molecular potential. The lack of electrostatic interactions in the Molinero water leads to faster dynamics and increased computational efficiency. 

Since the formation of critical nuclei within a supercooled liquid is a rare event, it cannot be properly sampled from a single long Molecular Dynamics (MD) trajectory of the system even at relatively large supercoolings. This makes conventional MD unsuitable for studying nucleation. We therefore combine conventional molecular dynamics with Forward-Flux Sampling (FFS) which is an advanced sampling technique developed for computing the rates of rare events such as protein folding, evaporation and crystallization [2]. We calculate crystallization rates for temperatures 220-235 K, which are then used for computing activation energies and critical nucleus sizes. We also compare our findings with nucleation rates in the bulk to understand how geometric confinement affects the crystallization process. Our findings will help us understand how a free interface affects ice formation in water, which will be an invaluable knowledge for both statistical physicists and atmospheric scientists. 

1- Molinero V, Moore EB, J. Phys. Chem. B 113(13): 4008-16 (2008).

2- Allen RJ, Frenkel D, ten Wolde PR, J. Chem. Phys. 124: 194111 (2006).