Measurements of inductive charges during drop breakup in horizontal electric fields

Abstract

Waterdrop pairs with initial diameters of 2.1 and 1.7 mm were collided at inclinations < 3° from the vertical after attaining fall speeds of 3.4 and 2.1 m s−1, respectively, and broke up in either negligible or strong electric fields (50 k V m−1). The colliding waterdrops had initial charges < 0.1 pC. In a horizontal electric field of < 100 V m−1 the fragments were not charged measurably by noninductive processes (< 0.05 pC). However, at 50 kV m−1 the waterdrops acquired charges up to 5 pC by induction. All breakups observed between the drop pairs resulted in “disjection”, that is a separation with the largest fragment staying below the other(s). The results, based mainly on filament breakup, over mechanically limited disjection angles from 69° to 111° (given by the connecting line between the centers of the two largest fragments at the moment of separation and the horizontal) provide experimental evidence of a cosine dependence of inductive charging in the presence of horizontal electric fields. They also support application of a theoretical equation developed by Latham and Mason (1962) for rebounding rigid spherical conductors. The results can be generalized to include all raindrop‐raindrop collision/breakups involving drop pairs with diameters > (1.8;0.4 mm) and to all electrical field directions because fields up to 50 kV m−1 do not affect the breakup configurations. A particular conclusion is that breakups of colliding raindrops, in the size and voltage ranges quoted, aid in the dissipation of vertical electric fields in the atmosphere.