Simulations Of Homogeneous Nucleation Research Articles

Molecular Dynamics Simulation of Homogeneous Nucleation and Growth of Supercooled Al Liquid

Molecular dynamics simulation is used to study the spontaneous nucleation and solidification of Al liquid. According to the mean first-passage time (MFPT), the critical crystal nucleus size at 31.6% undercooling is 152 atoms, the nucleation rate is [Formula: see text][Formula: see text]m[Formula: see text][Formula: see text]s[Formula: see text]. The nucleation rate obtained by the survival probability (SP) is [Formula: see text][Formula: see text]m[Formula: see text][Formula: see text]s[Formula: see text], which is very consistent with the result obtained by MFPT. Using Johnson–Mehl–Avrami (JMA) law to analyze the growth of the two extreme conditions in the experiment, the system with the smallest average atomic volume (Run38) grows faster than the system with the largest volume (Run73). In terms of microstructure, Run38 is a lamellar (LAM) structure, and Run73 is a complex polycrystalline structure accompanied by five-fold twinning (FFT). The shapes of clusters in a given range (5–10,000 atoms) during solidification in 100 experiments were counted. The results show that no clusters are perfectly spherical, but ellipsoids are of different shapes, and the larger the ellipsoid size, the closer to a spherical shape.

Monte Carlo simulations of homogeneous nucleation and particle growth in the presence of background particles

The application of the Monte Carlo (MC) simulation technique for the modelling of nucleation processes with an existing background particle concentration is presented in this paper. Next to the nucleation of novel particles, the coagulation of an existing particle population as well as the condensational growth and evaporation of unstable particles (whose diameter is smaller than the critical Kelvin diameter) are included into the simulation. The usage of statistically weighted MC particles allows the description of particle size distribution (PSD), whose concentrations differ in several orders of magnitude. It is shown, that this approach allows to model the complex interplay between freshly nucleated particles and an existing background particle population. In this work, the nucleation of novel particles is modelled by three different nucleation theories discussed by [Girshick, S. L. and C.-P. Chiu (1990), The Journal of Chemical Physics 93], which comprise of (1) the classical nucleation theory, (2) a mathematical correction to (1) and (3) a self-consistency correction of (2). For the chosen simulation conditions, the resulting PSDs are independent of the used nucleation theory for longer simulation times, in which the simulations are described by the coagulation mechanism only. The time-frame is identified for which relevant discrepancies of the PSDs have to be taken into account.

Nucleation and droplet growth from supersaturated vapor at temperatures below the triple point temperature.

In 1897 Ostwald formulated his step rule for formation of the most stable crystal state for a system with crystal polymorphism. The rule describes the irreversible way a system converts to the crystal with lowest free energy. But in fact the irreversible way a supercooled gas below the triple point temperature Ttr.p. crystallizes via a liquid droplet is an example of Ostwald's step rule. The homogeneous nucleation in the supersaturated gas is not to a crystal, but to a liquid-like critical nucleus. We have for the first time performed constant energy (NVE) Molecular Dynamics (MD) of homogeneous nucleation without the use of a thermostat. The simulations of homogeneous nucleation in a Lennard-Jones system from supersaturated vapor at temperatures below Ttr.p. reveal that the nucleation to a liquid-like critical nucleus is initiated by a small cold cluster [S. Toxvaerd, J. Chem. Phys. 143, 154705 (2015)]. The release of latent heat at the subsequent droplet growth increases the temperature in the liquid-like droplet, which for not deep supercooling and/or low supersaturation, can exceed Ttr.p.. The temperature of the liquid-like droplet increases less for a low supersaturation and remains below Ttr.p., but without a crystallization of the droplet for long times. The dissipation of the latent heat into the surrounding gas is affected by a traditional MD thermostat, with the consequence that droplet growth is different for (NVE) MD and constant temperature (NVT) MD.

Modification of the classical nucleation theory based on molecular simulation data for surface tension, critical nucleus size, and nucleation rate

Nucleation in supersaturated vapor is investigated with two series of molecular-dynamics simulations in the canonical ensemble. The applied methods are (a) analysis of critical nuclei at moderate supersaturations by simulating equilibria of single droplets with surrounding vapors in small systems; (b) simulation of homogeneous nucleation during condensation with large systems containing 10(5)-10(6) particles for calculating the nucleation rate of vapors at high supersaturations. For the Lennard-Jones fluid, truncated and shifted at 2.5 times the size parameter, it is shown that the classical nucleation theory underestimates both the nucleation rate and the size of the critical nucleus. A surface property corrected modification of this theory is proposed to consistently cover data on the surface tension of the curved interface, the critical nucleus size, and the nucleation rate.

Method for enhanced sampling in the simulations of dynamical systems

An algorithm for enhanced configurational sampling in molecular dynamics simulations based on stochastic scaling of the intermolecular potential is described. It is implemented in simulations of homogeneous nucleation of a supercooled molecular liquid, carbon dioxide, resulting in significant savings of computer time. This method preserves a canonical form for the potential energy distribution and does not alter average dynamical properties for equilibrium systems.

Computer simulation of homogeneous nucleation in gaseous medium

Computer simulation of homogeneous nucleation in gaseous medium