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Siegel der Universität
Department für Chemie - Arbeitsgruppe Prof. Strey

Keimbildung und Tröpfchenwachstum in übersättigtem Argon-Dampf

Alexander Fladerer, June 2002

In general, measurements of homogeneous nucleation rates of liquid droplets as function of the supersatuation from a vapor phase reveal a weaker temperature dependence than predicted by classical nucleation theory. Recent formulations based on density funktional theory are limited to simple systems like e.g. noble gases. Quantitative experiments on vapor phase condensation of noble gases did not exist. The physico-chemical properties, though, like density, surface tension, vapor pressure etc. are well-known. Therefore, a cryogenic nucleation pulse chamber for measuring homogeneous nucleation rates of argon was constructed. As the measurements show, the growth rate of argon droplets at nucleation conditions is rather high so that nucleation and growth could not be decoupled. Nevertheless, the experiments permit extracting an estimate of the nucleation rates to J=107cm-3s-1 at temperatures 52 < T / K < 59 and supersatuations around S=10. Accordingly, these experimants indicate - despite their preliminary nature - a complete failure of the classical nucleation theory, which predicts for the quoted conditions nucleation rates on the order of 10-35 to 10-20cm-3s-1. Interestingly, density functional theory can only partially explain the discrepancy. In the course of the experimants, growth curves for argon droplets were obtained from the measured MIE-scattering curves. The good agreement of the experimental growth curves with model calculations permit a near-quantitative description of the experimental light-scattering signal. The procedure provides an estimate for the number of density of the droplets along with a mesure of their polydispersity.