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Formation of nanoparticles in deposition from a gas phase is of interest both in different kinds of atmospheric phenomena and processes of modern technology (e.g. formation of water drops and smog particles in the atmosphere, manufacture of different kinds of nanoparticles). Incorporation of gas molecules into the condensed phase can take place in chemical and physical deposition. Here we consider the case of physical vapour deposition. The vapour-to-particle conversion depends on the transport of vapour molecules to the particle surface and their attachment to the particle. These factors in turn depend on the particle size and the presence in the system of a foreign gas that can be adsorbed on the particle surface.
The equation for the density of the resulting flux of vapour molecules into the nanoscale aerosol particle in view of the joint influence of the dependence of the condensation coefficient of vapour molecules on the particle size and the presence in the system of an adsorbable gas is given. It is shown that, depending on the saturation ratio, the increase of the adsorbable gas pressure can both increase and decrease the resulting flux density of vapour molecules into the particle. This is due to two opposite effects related to adsorption of foreign gas molecules on the nanoparticle surface. On the one hand, these gas molecules hinder the incorporation of vapour molecules into the particle due to the blocking effect of adsorbed molecules in relation to phase transitions on the particle surface. This leads to the decrease of the deposition rate. On the other hand, the adsorbed molecules can reduce surface tension. It leads to the decrease of the evaporation rate of small particle and hence to the increase of the resulting flux density of vapour molecules into the particle. The decrease of the condensation coefficient with the reduction in the particle size leads to the decrease of the resulting flux density of vapour molecules into the particle. The obtained results allow estimating the influence of the above-mentioned factors on the condensational growth of nanoscale aerosol particles.
This work was supported in part by GACR projects 101/05/2214, 101/05/2524 and 104/07/1093.
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