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T. G. Owe Berg and Thomas A. Gaukler

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T. G. Owe Berg and Thomas A. Gaukler

Abstract

The charge on an AgI particle, freely suspended in an air current of terminal velocity, has been measured at various temperatures, humidities and cooling rates. The wall of the flow pipe serves as a source or sinkfor water vapor, simulating neighbor droplets and ice particles in a cloud. The data show that the AgI particle sorbs water below the dew point and thereby acquires negative charge, that the sorbed water freezes at 0C, and that the electrification is reverisble. There are sudden changes in charge at the onset of sorption at the dew point and at freezing and melting, but the largest changes take place in prolonged and extensive sorption or desorption. The electrification follows an exponential rate law, indicating an autocatalytic process. Ice formation at 0C was also obtained with a dense cloud of AgI and correspondingly small condensate droplets in the expansion chamber. It is concluded that nucleation occurs at 0C, and that super-saturation pertains to the growth of the ice to detacable size. The data are compared with data in the literature. It is concluded that contradictions among published data are the results of differences in experimentalconditions, especially substrate effects in experiments with supported drops. The mechanisms of nucleation,growth and electrification are discussed.

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T. G. Owe Berg, Thomas A. Gaukler, and Urte Vaughan

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Collision efficiencies E have been determined from particle trajectories for the case of a 1-mm glass sphere and 6–20 μ spherical glass particles falling in still air. An empirical formula for the dependence of E upon scavenger size, scavenger velocity, and particle terminal velocity has been derived.

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T. G. Owe Berg, Thomas A. Gaukler, and Urte Vaughan

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The collision of a falling drop With a small particle has been studied by high-speed photography. The trajectory of the particle relative to the center of the drop and relative to a fixed point has been determined under various conditions. The effect of electrostatic charges on drop and particle has been studied.

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T. G. Owe Berg, Joyce E. Clutter, Thomas A. Gaukler, and Robert L. McDonald

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Experiments were conducted with sublimate AgI or AgI smoke in an expansion chamber at various temperatures. A condensate cloud of liquid droplets was formed at all temperatures. Below −5C ice crystals appeared in the cloud a few seconds after its formation. Photographs were taken of liquid droplets and of partially frozen droplets. The droplet diameter was 7 μ above 0C, but at lower temperatures it was larger, reaching 15 μ at −7C. The results show that the water vapor is converted to ice by condensation followed by freezing of the liquid droplets under these conditions. Impurities in the AgI and the air had no effect upon these observations.

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