(253e) Kinetic Analysis for Molecular Simulations of Nucleation Processes with Low Barriers

Nicholson, D. A., Massachusetts Institute of Technology
Rutledge, G. C., Massachusetts Institute of Technology
A nucleation-mediated phase change occurs through the spontaneous formation of clusters of the new phase within the old phase. Due to the competing contributions from the interfacial free energy penalty and the bulk driving force for crystallization, there is a kinetic barrier with respect to the size of the cluster formed. Once a large enough cluster is formed to pass over the kinetic barrier, rapid growth of the new phase occurs. The relative timescales associated with nucleation and growth have important implications for the analysis of nucleation data obtained from molecular dynamics (MD) simulation. We have developed a method for the analysis of data from MD studies of nucleation processes with small separation between timescales of nucleation and growth. The method is well-suited for nucleation events with low kinetic barriers, in addition to large systems since the separation between timescales becomes smaller as the size of the system increases. In our method, we use the mean first-passage time of the largest cluster, obtained from several nucleation runs, in order to parameterize a theoretical mean first-passage time (MFPT) curve based on the Becker-Döring model. In order the make the method computationally practical, we developed an accurate approximation for the first-passage time distribution based on its cumulant expansion. Application of the method to MD data yields estimates for important kinetic parameters including the critical nucleus size, free-energy barrier, and critical nucleus size, as well as the nucleation rate and a characteristic growth rate. The method was applied in a case study on crystal nucleation of n-eicosane in the presence of flow fields of varying intensity. We find that the method is able to capture the decrease in the free-energy barrier as the intensity of the flow field is increased.