Collisional charging of water drops

Abstract

Results are presented for the first phase of an experimental program studying the collisional charging of water drops. Observations were made of the noncoalescing collisions of uncharged, 1-mm-radius water drops supported in a vertical wind tunnel in a region of vertical electric field. The individual collision products were collected and their charges measured directly with an electrometer. Over the range of field studied (10–120 v/cm), the charging became appreciable, increasing rapidly at a field of about 50 v/cm. At 30 v/cm the average charge separated was 1×10−13 coul; at 80 v/cm it was 1×10−12 coul. At the higher fields the dominant mechanism was the temporary fusion and subsequent separation of the drops. Satellite droplets (diameters from 50 to 200 μ, charges typically 10% of those on the large drops) were frequently formed in these encounters, particularly at the higher fields. The probability of permanent coalescence of the large drops used here was found to be about 50% and essentially independent of field. The collisional charging mechanism is considered as a possible source of the charge found on rain. The direction of charging was in almost all cases such as to dissipate rather than to intensify the existing electric field.