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Heterogeneous freezing of single sulfuric acid solution droplets: laboratory experiments utilizing an acoustic levitator

by M. Ettner, S. K. Mitra, S. Borrmann
Atmospheric Chemistry and Physics ()

Abstract

The heterogeneous freezing temperatures of sin- gle binary sulfuric acid solution droplets were measured in dependency of acid concentration down to temperatures as low as −50◦C. In order to avoid influence of supporting sub- strates on the freezing characteristics, a new technique has been developed to suspend the droplet by means of an acous- tic levitator. The droplets contained immersed particles of graphite, kaolin or montmorillonite in order to study the in- fluence of the presence of such contamination on the freez- ing temperature. The radii of the suspended droplets spanned the range between 0.4 and 1.1mm and the concentration of the sulfuric acid solution varied between 5 and 14 weight percent. The presence of the particles in the solution raises the freezing temperature with respect to homogeneous freez- ing of these solution droplets. The pure solution droplets can be supercooled up to 40 degrees below the ice-acid so- lution thermodynamic equilibrium curve. Depending on the concentration of sulfuric acid and the nature of the impu- rity the polluted droplets froze between −11◦C and −35◦C. The new experimental set-up, combining a deep freezer with a movable ultrasonic levitator and suitable optics, proved to be a useful approach for such investigations on individual droplets.

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