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The Influence of Charge on the Coalescence of Water Drops in Free Fall

Robert R. CzysClimate and Meteorology Section, Illinois State Water Survey, Champaign, Illinois

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Harry T. Ochs IIIClimate and Meteorology Section, Illinois State Water Survey, Champaign, Illinois

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Abstract

The influence of charge on coalescence was determined in the laboratory for isolated pairs of 340 and 190 μm water drops failing freely at terminal velocity. A microcomputer-controlled apparatus was used to produce collisions. Drop charges were independently controlled and collisions occurred in a humidified chamber at laboratory temperature and pressure. Two cameras were used to record the interactions from orthogonal directions and the photographs used to determine the impact angle for each collision. A single charge-independent critical impact angle of 43° ±1° was observed that distinguished an inner collision cross section for coalescence from an outer annular cross section for noncoalescence. Collisions occurring for impact angles less than the critical value always resulted in coalescence regardless of charge. Collisions having impact angles greater than the critical value resulted in either bounce, temporary coalescence or coalescence depending on charge. A satellite drop was produced with temporary coalescence at the intermediate charges of this experiment. Electrohydrodynamic theory in plane geometry applied to the conditions for deformable spheres gave an electric field strength between the approaching drop surfaces sufficient to cause a surface instability that may aid in drop coalescence.

Abstract

The influence of charge on coalescence was determined in the laboratory for isolated pairs of 340 and 190 μm water drops failing freely at terminal velocity. A microcomputer-controlled apparatus was used to produce collisions. Drop charges were independently controlled and collisions occurred in a humidified chamber at laboratory temperature and pressure. Two cameras were used to record the interactions from orthogonal directions and the photographs used to determine the impact angle for each collision. A single charge-independent critical impact angle of 43° ±1° was observed that distinguished an inner collision cross section for coalescence from an outer annular cross section for noncoalescence. Collisions occurring for impact angles less than the critical value always resulted in coalescence regardless of charge. Collisions having impact angles greater than the critical value resulted in either bounce, temporary coalescence or coalescence depending on charge. A satellite drop was produced with temporary coalescence at the intermediate charges of this experiment. Electrohydrodynamic theory in plane geometry applied to the conditions for deformable spheres gave an electric field strength between the approaching drop surfaces sufficient to cause a surface instability that may aid in drop coalescence.

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