Kinetic model for the sublimation of a solid and evaporation of colloidal particles from a solid substrate

A kinetic approach, based on a first passage time analysis, is proposed to study sublimation. A surface located molecule/particle is considered to perform a chaotic thermal motion in a potential field created by its pairwise interactions with all other molecules of the solid. For the normal (to the surface) motion of the surface molecule this field represents a potential well from which the molecule escapes during sublimation. The mean time necessary for this escape can be determined by solving the Smoluchowski equation for the single molecule distribution function in a potential well wherein the molecule performs its chaotic motion. Knowing this time, one can determine the rate of evaporation of molecules/particles from the surface. The method is applied to two kinds of sublimation. One is the "molecular" sublimation of a solid during which its molecules evaporate from its free surface. The other is the emission of nanoparticles from a solid substrate on which they are adsorbed. Both the substrate and nanoparticles can be coated with thin films which allow one to control the rate of particle emission at a given temperature. The effect of various parameters of the system on the process is studied numerically.