Physics (Internal)

Nucleation in Supercooled Liquids

This event is part of the Preliminary Oral Exam.

Examining Committee: William Klein, Tom Keyes, Ophelia Tsui, Kevin Black

Abstract: Nucleation is the process by which a metastable phase decays into a stable phase. It is widely observed in nature, and is responsible for many phenomena like cloud formation and crystal growth. The classical nucleation theory predicts a compact droplet of the stable phase that will initiate the nucleation process. For many systems with long range interactions, however, the droplets are highly ramified and harder to locate due to lack of a well-defined structure. This has significant implications for material properties.

I have been studying nucleation in Lennard-Jones liquids, by performing Molecular Dynamics simulations in a Micro-Canonical ensemble of Argon particles. The system is quenched from liquid temperatures into a metastable solid phase, and allowed to evolve under constant energy dynamics. Once the nucleus appears and starts to grow, the time of nucleation is identified by a method called Intervention. At certain points of the successfully nucleated trajectory, close to the top of the saddle point free energy barrier, several copies of the configuration are made with slightly perturbed velocities. These copies are allowed to evolve independently with the same constant energy dynamics, and the number of successfully nucleated copies are counted to get the success ratio. Ideally, one would expect a monotonic rise in the the success ratio as we approach the energy barrier and cross over it. However, I observed a non-monotonic behavior for deep quenches closer to the spinodal. This non-monotonicity is not a statistical fluctuation, since it persists despite increasing the number of copies. The question we are pursuing is: Could this be an inherent property of spinodal nucleation, and provide us with new insights into the free energy landscape? In addition I am investigating the symmetry of the critical nucleus by pinpointing its exact location and time of appearance and employing spherical harmonics to identify the spatial arrangement of molecules.