Collision and Breakup of Water Drops at Terminal Velocity

Abstract

The collision and subsequent breakup of water drops moving essentially vertically and at terminal velocity has been studied for five drop pairs: the diameters Ds of the large drops were 4.8, 3.6 and 3.0 mm; the diameters D3 of the small drops were 1.8 mm and 1.0 mm. 712 collisions were obtained in 25,000 individually recorded attempts. Three distinct types of collision-breakup were found with the following occurrence: necks 27%, sheets 55% and disks 18%. Bag breakups were insignificant with <0.5%. All types are defined and corresponding examples shown. Fragment size and number distributions for the different types and the overall situation give further reasons for the breakdown into the different types. The disk collision has been found to be the major cause for the depletion in number of large drops, hence the cutoff of large drops in rain. The results also form the first data bank for numerically modelling the evolution of raindrop size spectra and the Langmuir chain process.