SHAPE OSCILLATIONS OF BUBBLES IN WATER DRIVEN BY MODULATED ULTRASONIC RADIATION PRESSURE - OBSERVATIONS AND DETECTION WITH SCATTERED LASER-LIGHT

Steady-state quadrupole shape oscillations of air bubbles trapped in water were excited by amplitude modulation of the acoustic radiation pressure used for levitation. This method of exciting controlled shape oscillations may make possible noncontact dynamical measurements of the rheological properties of bubbles. Bubble sizes ranged from 1.6- to 12-mm diameter corresponding to observed quadrupole mode frequencies of 190 to 17 Hz. Small-amplitude oscillations were detected by interference of scattered laser light. Some larger amplitude oscillations were detected by the unaided eye or with a television camera. The structure of the acoustic field in the levitator needed for the levitation of large bubbles is discussed. In the absence of modulation the levitated bubbles had an oblate shape.