Analyzing multistep homogeneous nucleation in vapor-to-solid transitions using molecular dynamics simulations

Abstract

In this paper, we present multistep homogeneous nucleations in vapor-to-solid transitions as revealed by molecular dynamics simulations on Lennard-Jones molecules, where liquidlike clusters are created and crystallized. During a long, direct NV E (constant volume, energy, and number of molecules) involving the integration of (1.9-15) × 10 6 molecules in up to 200 million steps (=4.3 μs), crystallization in many large, supercooled nanoclusters is observed once the liquid clusters grow to a certain size (∼800 molecules for the case of T 0.5ε/k). In the simulations, we discovered an interesting process associated with crystallization: the solid clusters lost 2-5 % of their mass during crystallization at low temperatures below their melting temperatures. Although the crystallized clusters were heated by latent heat, they were stabilized by cooling due to evaporation. The clusters crystallized quickly and completely except at surface layers. However, they did not have stable crystal structures, rather they had metastable structures such as icosahedral, decahedral, face-centered-cubic-rich (fcc-rich), and hexagonal-close-packed-rich (hcp-rich). Several kinds of cluster structures coexisted in the same size range of ∼1000-5000 molecules. Our results imply that multistep nucleation is a common first stage of condensation from vapor to solid.

Homogeneous nucleation of carbon dioxide in supersonic nozzles I: experiments and classical theories

“…Of these three possibilities, the formation of a crystalline cluster directly from the vapor phase is highly unlikely since the cost of forming an organized cluster directly from the vapor is generally higher than forming a disorganized liquid cluster. Furthermore, in recent large scale MD simulations of Lennard-Jones fluid, 83 nucleation from the vapor phase always occurred via the supercooled liquid, even at temperatures 23 where Tc is the critical temperature. Post-critical clusters only froze when they reached sizes greater than approximately 800 molecules.…”

Theoretical analysis of crystallization by homogeneous nucleation of water droplets

“…The large pressure may cause the deviation of the self-diffusion coefficient D in nanometer-sized water droplets, but the effect would not be large because the pressure dependence of D in liquid is generally small. However, there is another possibility that the self-diffusion in nanometer-droplets is larger than the bulk value because of the surface effect, i.e., the molecules in small droplets were more diffusive than in the bulk liquid due to the surface effect of the finite boundary 31 . This effect increases the nucleation rate, although the dependency is not fully understood yet.…”

Dust Formation from Vapor through Multistep Nucleation in Astrophysical Environments

“…In the simulations, the nuclei of supercooled liquid appear and growth. After the growth of nuclei, the crystallizations of supercooled nano-clusters are observed and the crystallized nano clusters have various structures of metastable phase (Tanaka et al 2017). Our simulations indicate that the vapor-to-solid transition occurs through multistep nucleation which is vapor-to-liquid nucleation (first step nucleation) and crystallization in the supercooled liquid droplets (second step nucleation), even though the temperature is much lower than the triple temperature (Fig.…”

“…Overview of vapor-to-solid transition obtained by our molecular dynamics simulation(Tanaka et al 2017).…”