Abhishek Kumar Sharma

Nucleus-size pinning. todetermine nucleation free energy barriers and nucleus geometry

First order phase transitions from metastable to stable phases often take place through the mechanism of nucleation and growth. In computational studies at small to moderate degrees of supersaturation, nucleation is a rare event, and simulating it requires special techniques that can access such low probability states. In this talk I will present a new method called nucleus size pinning (NSP) that improves on the conventional seeding approach to explore nucleation processes. This method is particularly effective at small supersaturations where nucleation barriers are large and other methods like umbrella sampling are inefficient. In NSP, a seed of the incipient phase is pinned to a preselected size to iteratively drive the system toward the conditions where the seed becomes a critical nucleus. Classical nucleation theory (CNT) can then be used to estimate the free energy barriers and orientationally-averaged interfacial free energy. We can also use CNT to obtain quick estimates for interfacial free energy within a single simulation, which match those estimated by standard techniques. NSP is first validated by estimating the critical nucleation conditions for the disorder-to-order transition in hard spheres, and then applied to simulate and characterize the highly non-trivial (prolate) morphology of the critical crystal nucleus in hard gyrobifastigia. We corroborate these results through measurement of interfacial free energy through cleaving walls method and predicting a faceted nucleus shape via Wulff construction. We believe NSP can be easily adapted to elucidate nucleation and growth processes in other contexts to improve our understanding of such first-order phase transitions.