A study of ultrasonically induced pulsations of gas-filled channels in Elodea

The ultrasonically stimulated pulsation of pockets of in elodea has been studied using optical microscopy. The intensity dependence of the microstreaming associated with these pulsations has been measured by observing the rotation of chloroplasts and other organelles in the microstreaming vortices. The threshold energy density to produce microstreaming in a standing wave field was found to be 0 . 03 J m-3 at 700 kHz, equivalent to an average intensity of 5 mW cm-2 in the standing wave field, and 0 . 01 J m-3 at 3 MHz, equivalent to an intensity of 15 mW cm-2. The channels had resonant frequencies in the low megahertz range and the relationship between the channel diameter and the resonant frequency was found to be similar to that for a gas bubble in a liquid.