Homogeneous nucleation of liquid from the vapor phase in an expansion cloud chamber
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I.
INTRODUCTION
H O M O G E N E O U S vapor-to-liquid nucleation is the process by which a vapor undergoes condensation into a drop, in a system of only its own molecules and in the absence of surfaces, ions, or impurities. Nucleation from the vapor phase is the simplest case of nucleation and, therefore, should be the easiest to understand. One anticipates that a good understanding of homogeneous nucleation and the more complex binary homogeneous nucleation will lead ultimately to an understanding of multicomponent and heterogeneous nucleation. Homogeneous nucleation has been studied by several techniques, including the piston cloud chamber, r~l the diffusion cloud chamber, t2-5] supersonic nozzles, I6] and shock tubes, tT] Each of these techniques has its own particular domain of applicability range in the variables temperature, supersaturation ratio, and nucleation rate. The techniques also vary widely in practicality, complexity, and ease of interpretation of the results. II.
EXPERIMENTAL PROCEDURE
The experiments reported here have been performed with expansion cloud chambers which, in design, are direct descendants of Wilson's cloud chambers, r~ Our chamber has been described in great detail elsewhere, I8,91 so only an outline of its operation will be presented here. One creates a supersaturation in a piston cloud chamber by expanding a (uniform) volume containing a (carder) gas and a condensable vapor. One proceeds as follows. Initially, an enclosed volume is allowed to come to thermal equilibrium and is saturated with the condensable vapor. The volume is then expanded, and the carrier gas and vapor decrease in temperature. This process is illustrated in Figure 1 for toluene vapor in an argon carrier gas. One observes that while the expansion slightly decreases the pressure (and density) of the vapor, JOHN L. SCHMITT, Associate Professor and Senior Investigator, is with the Department of Physics and Graduate Center for Cloud Physics Research, University of Missouri-Rolla, Rolla, MO 65401. This paper is based on a presentation made in the "G. Marshall Pound Memorial Symposium on the Kinetics of Phase Transformations" presented as part of the 1990 fall meeting of TMS, October 8-12, 1990, in Detroit, MI, under the auspices of the ASM/MSD Phase Transformations Committee. METALLURGICAL TRANSACTIONS A
the pressure is still much greater than the equilibrium vapor pressure at the lower temperatures. The vapor is thus supersaturated with respect to the equilibrium vapor pressure. In Figure 1, the ratio of the pressure from the expansion curve to the pressure on the equilibrium curve at a given temperature is the supersaturation ratio. One observes that quite large supersaturation ratios can be achieved with the expansion technique. The expansion chamber used for these experiments is illustrated in Figure 2 (schematic cross section). Our device is constructed from stainless steel, glass, and fluorocarbon plastic and thus is compatible with a wide variety of vapors and liquids. The piston is sealed with a very fle
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