Abstract:
A three-dimensional standing wave in a resonant liquid-filled cavity allows the trapping of multiple millimeter-size air bubbles. The coupling of the radiation pressure stresses to the bubble interfaces induces a net downward force counteracting gravity, a steady-state shape deformation, and it also drives natural shape oscillations of large amplitude under certain circumstances. The parameter space for the shape stability of trapped bubbles in ultrasonic fields having a frequency between 45 and 65 kHz has been experimentally determined in pure distilled water as well as in aqueous solutions containing surfactants. The variable parameters are the sound pressure, the sound wavelength, and the bubble equilibrium radius. The effects of the sound field intensity on the interaction of contacting and noncontacting neighboring bubbles are also experimentally investigated. In particular, a preliminary study of the impact of the sound intensity on the coalescence efficiency of contacting bubbles is conducted in pure water and in aqueous surfactant solutions. [Work funded by NASA.]