Spectral Study of the Luminosity Produced during Coalescence of Oppositely Charged Falling Water Drops

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Abstract
An experimental study was made of the luminosity that accompanies the coalescence of oppositely charged falling water drops in order to establish which of several possible mechanisms was responsible for the luminosity. Spectrograms which show excitation of the N2 2P and N2+ 1N systems when the drops fall through air, and which fail to show these systems when the drops fall through CO2, establish that for the range of parameters studied, dielectric breakdown of the gas through which the drops fall is the process responsible for the luminosity. Oscillographic studies showed that the production of luminosity and the transfer of charge occurred simultaneously and lasted less than 14 nsec. These observations, showing the importance of dielectric breakdown in effecting charge transfer, contrast with much that appears in the literature that stresses the importance of the bulk dielectric relaxation time and of surface tension as parameters that govern the rate at which charge is lost by water surfaces under slightly different circumstances. Typical conditions resulting in luminosity of the type studied are: drops radius ∼500 μ, drop charge ∼50 × 10−12 C, relative approach velocity of main body of drops ∼10 cm/sec, and bulk dielectric relaxation time for water of the quality used 10−5 sec.

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