Bubble production by capillary waves

Abstract

Bubble‐producing capillary waves can be easily generated in a laboratory tank (8 cm × 22 cm × 150 cm) by blowing air over the water. The lowest wind speed required for the occurrence of this process is measured. The generation of the capillary waves depends solely on the surface tension, which can be changed by adding surfactants and other chemicals to the tank water. The effect of surface tension on the windspeed threshold for bubble production is discussed. Using a coincidence detector, the bubble production rates per unit area can be measured. Lowering the surface tension, surprisingly, increases the rate of bubble production. The dependence of the bubble production rate on wind speed and wind fetch is also discussed. The underwater acoustic emissions from these bubbles are measured. Average power spectrum for several wind speeds are obtained that exhibit a broad range of radiated frequencies (i.e., bubble sizes) and a weak dependence on the wind speed. [Work supported by ONR, ONT, and AEAS.]