Experimental Studies of Droplet Evaporation Kinetics: Validation of Models for Binary and Ternary Aqueous Solutions

Abstract

Experiments were conducted with an electrodynamic levitation system to study the kinetics of droplet evaporation under chemically rich conditions. Single solution droplets of known composition (HNO₃/H₂O or H₂SO₄/HNO₃/H₂O) were introduced into an environmentally controlled cubic levitation cell. The gaseous environment was set intentionally out of equilibrium with the droplet properties, thus permitting the HNO₃ mass accommodation coefficient to be determined. Measurements were performed at room temperature and various pressures (200–1000 hPa). Droplet sizes (initial radii in the range 12–26 μm) were measured versus time to high precision (±0.03 μm) via Mie scattering and compared with sizes computed by different models for mass and heat transfer in the transition regime. The best agreement between the theoretical calculations and experimental results was obtained for an HNO₃ mass accommodation coefficient of 0.11 ± 0.03 at atmospheric pressure, 0.17 ± 0.05 at 500 hPa, and 0.33 ± 0.08 at 200 hPa. The determination of the mass accommodation coefficient was not sensitive to the transport model used. The results show that droplet evaporation is strongly limited by HNO₃ and occurs in two stages, one characterized by rapid H₂O mass transfer and the other by HNO₃ mass transfer. The presence of a nonvolatile solute (SO²⁻₄) affects the activities of the volatile components (HNO₃ and H₂O) and prevents complete evaporation of the solution droplets. These findings validate recent attempts to include the effects of soluble trace gases in cloud models, as long as suitable model parameters are used.