Effect of a carrier gas on nucleation in diffusion chambers

Abstract

Diffusion cloud chambers (DCC) h ave frequently been employed when investigating homogeneous nucleation. The typical scheme of an experiment on studying nucleation in DCC is as follows (Heist et al., 1994). DCC usually consists of two horizontal plates top cold and bottom hot, but an inverse scheme has also been utilized (for instance, by (Lushnikov, 1997)). 0 ver the bottom plate there is a liquid which vapor condensation is a subject of the research. The space between plates is usually filled in with a background gas. By virtue of the existing distribution of temperature and pressure the vapor evaporating from the bottom surface moves due to diffusion through a chamber then cooling and again condensing. As a result of these processes an appropriate steady-state distribution of supersaturation S over the height of the chamber t ( usually reckoned from the bottom plate) is established. An occurrence of drops of the condensed vapor is detected by some kind of light-scattering or even visually. Since the vapor concentration in the chamber is low as compared with the concentration of the buffer gas, the processes of condensation do not practically influence the distribution of temperature and pressure over the height of the chamber which are determined only by the boundary conditions at the walls and by the carrier gas pressure PO. Moreover, it is possible to show that gradients of the temperature dlnT/dJ M 0.1 cm-’ and density realized in the chamber are small as compared with gradients of the clusters’ concentrations and in the first approximation may be neglected when describing the nucleation kinetics in DCC. Since the experimental investigations of the carrier gas effect on the nucleation rate in DCC became relatively popular it is possible to note from the literature that there is a trend to present the experimental data in coordinates S,(Po) or S,(T,) where S, is the supersaturation corresponding to J = 1 drop/cm3/s. Here we propose a new approach which combines a proper treatment of physical processes in DCC and new kinetic scheme of nucleation in the presence of the background gas (diffusion-limited kinetic of nucleation). Fist we analyze the transport processes in DCC with allowance for brownian diffusion, gravity, a drag and thermophoretic forces, that values depend on the cluster size or the Knudsen number specific for droplets and nucleation and the droplet growth and reveal their role with respect to the measured number of drops in DCC. Than we analyze what is the nucleation rate J actually determined in the experiment. Because usually experimental&s represent their results (in particular, J) as a function of S,, T, and PO where subscript “*” marks the point of the maximum supersaturation or a close point of the maximum nucleation rate, the main goal of this consideration is to express explicitly J through these parameters. However, for this pur