Kinetic analysis of homogeneous droplet nucleation using large-scale molecular dynamics simulations

Abstract

Studies on homogeneous nucleation have been conducted for decades, but a large gap between experiment and theory persists when evaluating the nucleation rate because the classical nucleation theory (CNT) with all its modifications still cannot fully incorporate the kinetics of homogeneous nucleation. Recent large-scale molecular dynamics (MD) simulations on homogeneous nucleation estimated a nucleation rate around the same order of magnitude as that obtained in experiments. This immensely improved agreement between experiment and theory is exciting because MD can provide detailed information on molecular trajectories. Therefore, a better understanding of the kinetics of homogeneous nucleation can now be obtained. In this study, large-scale MD simulations on homogeneous nucleation were performed. Through kinetic analysis of the simulation results, the nucleation rate, free energy barrier, and critical cluster size were found. Although the nucleation rates directly obtained from the simulations differed from those calculated from the CNT by 8-13 orders of magnitude, when the parameters calculated from the molecular trajectories were substituted into the classical theory, the discrepancy between the nucleation rates decreased to within an order of magnitude. This proves that the fundamental formulation of the theoretical equation is physically sound. We also calculated the cluster formation free energy and confirmed that the free energy barrier decreases with increasing supersaturation ratio. The estimated barrier height was twice that determined by theory, whereas the critical cluster size showed very good agreement between simulation and theory.

Systematically extending classical nucleation theory

“…The rates observed for water are in better agreement at low temperatures but deviates as temperature increases [8]. Recent large-scale simulations of droplet nucleation in a Lennard-Jones system show smaller deviations in the nucleation rate compared to CNT, 8-13 orders of magnitude, but that the agreement was better for the size of the critical nucleus [9]. This is largely in agreement with earlier studies such as [10].…”

“…Another distinction is that sometimes the quantities required in CNT, such as the rate at which molecules attach to a cluster, are taken directly from simulation and in this case, good agreement with CNT is often found (as discussed, e.g. in [9] above and in the case of crystallization, in e.g. [11]).…”

Homogeneous nucleation of carbon dioxide in supersonic nozzles II: molecular dynamics simulations and properties of nucleating clusters

“…It has been shown that the best way to avoid this artefact is to introduce explicit carrier gas molecules in the simulation, and apply a thermostat only to the latter, even though this increases the computational cost significantly. 4,5,[19][20][21] Here, we have applied a Nosé-Hoover thermostat with time constant 0.1 ps to the Ar atoms. This approach prevents heating of the carrier gas due to latent heat release.…”

“…Recently, the standard kinetic scheme behind the majority of nucleation theories has been validated by atomistic simulations. 4,5 In classical nucleation theory (CNT), the thermodynamics related to nucleation, i.e. the clusters' formation free energies, are treated in a simple fashion using the bulk properties of the nucleating substance.…”

“…While many studies on the homogeneous nucleation of the Lennard-Jones fluid and H 2 O have been carried out using MD simulations, 5,[19][20][21][22][23] direct nucleation simulations of carbon dioxide remain rare. To our knowledge, only Horsch et al 24,25 have used MD simulations to study homogeneous CO 2 nucleation from the vapour phase.…”