The behavior of water drops at terminal velocity in air

An insight into the behavior of large raindrops has been obtained by studying drops of water freely suspended in a vertical air stream. The wind tunnel employed is described. The stroboscopic pictures taken of the waterdrops show that extreme periodic deformations occur on drops of more than five millimeters diameter. The behavior of drops before and after collision was observed. Drops which come into a region within six centimeters above another drop usually begin to fall in an ever tightening spiral until collision takes place. The bombardment of large drops with a spray of small droplets showed that not all the small droplets coalesced with the large drops. Some of the smaller droplets would roll across the under surface of the large drop exhibiting a “bounce-off” effect. The breakup of a large waterdrop is easily brought about by imparting a sudden upward acceleration to the air through which it falls. On the other hand, a downward acceleration of the air is relatively ineffectual in causing breakup. The influence of the periodic deformation in causing breakup can be demonstrated by actually pumping air into a drop until the air bubble constitutes as much as 30 per cent of the drop. Such airbubble drops show little or no deformation and will thus remain intact even when subjected to extremely turbulent air.