Measurements of homogeneous nucleation rates for n-nonane vapor using a two-piston expansion chamber

Abstract

The homogeneous nucleation rate J in supersaturated n-nonane vapor was measured as a function of supersaturation S and temperature T using a newly developed expansion chamber. Homogeneous nucleation is observed only during a short supercritical nucleation pulse so that nucleation and droplet growth are decoupled under the considered experimental conditions. Reproducible nucleation pulses with a duration of about 1 ms are achieved by means of two spring-operated pistons moving in a well-defined time sequence. The number concentration of the droplets growing in the expansion chamber is determined immediately after the end of the nucleation pulse by means of constant angle Mie scattering. Taking into account the duration of the nucleation pulse we measured homogeneous nucleation rates ranging from about 5×105 to 5×109 cm−3 s−1. Homogeneous nucleation in n-nonane vapor was observed in the temperature range from 199 to 241 K. Argon was used as the carrier gas. The measured nucleation rates were compared with the classical theory of nucleation. The slopes of the J–S curves were found to be in good agreement and thus for n-nonane the dependence of the nucleation rate on the supersaturation appears to be correctly described by the classical nucleation theory. However, the actual values of the experimental nucleation rates were found to be several orders of magnitude higher than the theoretical prediction and show a significantly different temperature dependence. A qualitative explanation of the obtained experimental results can probably be given by means of Burton’s recent microscopic calculations of the cluster free energy.