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T. G. Owe Berg and Thomas A. Gaukler

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T. G. OWE BERG and D. C. GEORGE

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Charged water drops were suspended in a nonuniform a.c. field and observed through the microscope visually and by high-speed photography. The initial drop size was approximately 100µ. The drop shrank by evaporation and became unstable at approximately 40µ. The evaporation of stable drops and the behavior of unstable drops were studied. The rates of changes of mass and charge were determined at various temperatures, and the activation energy for the evaporation was determined.

The evidence indicates that ions are ejected from an unstable drop, and that a burst of ions occurs in the evaporation of a stable drop under these conditions. The removal of water molecules from the evaporating drop is not rate-determining under these conditions. The drop evaporated rapidly at −26°C. and became unstable repeatedly without freezing.

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T. G. Owe Berg and Thomas A. Gaukler

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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, G. C. Fernish, and T. A. Gaukler

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The coalescence of two liquid drops, pressed against each other while a voltage is applied across the drops, has been studied with high-speed photography. The delay between contact and coalescence is of the order of milliseconds for distilled water, alcohols, and aqueous solutions of hydrochloric acid. The inverted value of this time, the rate of coalescence, is proportional to the voltage between the drops at low voltages and to the square of the voltage at high voltages. In both cases, the plot of rate against voltage extrapolates to zero rate at zero voltage. In the linear case the rate is proportional to (ε–1)½, in the parabolic case to ε, where ε is the dielectric constant.

The following interpretation of the data is offered: Coalescence is effected by the formation of bonds across the interface between the drops. This may occur in two ways, by breaking of bonds and formation of new bonds, or by gradual rearrangement of bonds. In the former case, the rate is proportional to the energy in the drops and thereby to ε and the square of the voltage. In the latter case, the rate is proportional to (ε–1)½ and the voltage.

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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, Robert J. Trainor Jr., and Urte Vaughan

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Experiments with charged water droplets show the existence of the metastable states predicted by Cahn as well as the well-known unstable state predicted by Lord Rayleigh. The charge lost at metastability is completely recovered with time, whereas the charge lost at instability is only partially recovered. The recovery of charge may be a space-charge effect, or it may be a result of electrification that accompanies the exchange of water vapor.

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T. G. Owe Berg, T. A. Gaukler, and R. J. Trainor Jr.

Abstract

The temperature of strongly charged 100μ water droplets, suspended in a non-uniform ac field, was estimated from measurements of the rate of loss of mass at various temperatures and from freezing experiments with water containing colloidal AgI. The observations were compared with information in the literature. It is concluded that such droplets exchange heat with the surrounding air much more slowly than do unchanged or weakly charged droplets.

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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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