Vapor growth, molecular dynamics

Recent developments in the application of molecular dynamics promise a resolution of many questions in nucleation theory. A review of simulation methods in nucleation theory and of their results is given in [2.39]. Consequently, a survey will not be undertaken here. However, some of the results of SCHIEVE and co-workers [2.40-42] will be discussed here as they throw light on several questions of importance in terms of modelling aerosol growth from vapor condensation. 

Figure 7. Time dependence of the wavelength of the fastest growing density fluctuation (Affiax) during a molecular dynamics simulation of isothermal liquid-vapor spinodal decomposition in the three-dimensional Lennard-Jones fluid kT/e = 0.8 p

Recently Rao, Berne, and Kalos have reported Monte Carlo and molecular dynamics studies of the homogeneous nucleation of a Lennard-Jones fluid (argon). Several features of this study are noteworthy. Perhaps most importantly, the authors conclude from their dynamical study that growth of the nucleation microclusters takes place under essentially adiabatic conditions. That is, collisions between the microcluster and the vapor are not sufficiently frequent to maintain the droplet at a constant temperature. This result suggests that the isothermal assumption common in conventional nucleation theories... 

Y. Shibuta and J. A. Elliott. A molecular dynamics study of the graphitization ability of transition metals for catalysis of carbon nanotube growth via chemical vapor deposition. Chem. Phys. Lett. 472, 2009, 200-206. 

The production and growth of particles in the presence of condensable vapors is a major dynamic process. A considerable body of literature has accumulated on the subject, beginning with the thermodynamics of phase transition and continuing with the kinetic theory of molecular cluster behavior. 

The VBS provides a convenient framework for organic dynamics in addition to equilibrium partitioning because equilibrium is a balance between condensation (the molecular flux from the gas to the particle phase) and evaporation (the molecular flux from the particle phase to the gas). The difference between the vapor concentrations at the particle surface and far away from it serves as a driving force for net condensation or evaporation. Because the particle surface is usually assumed to be in equilibrium with the gas phase adjacent to it, evaporation depends explicitly on volatility. Condensation on the other hand depends only on the collision rate of molecules with the surface and so it is first order independent of volatility. The volatility of organic compounds thus affects the aerosol growth dynamics specifically through its influence on the evaporation term in the driving force for mass transport. 

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