Homogeneous nucleation of carbon dioxide in supersonic nozzles I: experiments and classical theories

Abstract

We studied the homogeneous nucleation of carbon dioxide in the carrier gas argon for concentrations of CO₂ ranging from 2 to 39 mole percent using three experimental methods. Position-resolved pressure trace measurements (PTM) determined that the onset of nucleation occurred at temperatures between 75 and 92 K with corresponding CO₂ partial pressures of 39 to 793 Pa. Small angle X-ray scattering (SAXS) measurements provided particle size distributions and aerosol number densities. Number densities of approximately 10¹² cm⁻³, and characteristic times ranging from 6 to 13 μs, resulted in measured nucleation rates on the order of 5 × 10¹⁷ cm⁻³ s⁻¹, values that are consistent with other nucleation rate measurements in supersonic nozzles. Finally, we used Fourier transform infrared (FTIR) spectroscopy to identify that the condensed CO₂ particles were crystalline cubic solids with either sharp or rounded corners. Molecular dynamics simulations, however, suggest that CO₂ forms liquid-like critical clusters before transitioning to the solid phase. Furthermore, the critical clusters are not in thermal equilibrium with the carrier gas. Comparisons with nucleation theories were therefore made assuming liquid-like critical clusters and incorporating non-isothermal correction factors.