ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

3aPAb5. Evolution of surface properties of acoustically levitated bubbles in aqueous solutions and seawater and sources of anomalous damping.

Thomas J. Asaki

Philip L. Marston

Washington State Univ., Pullman, WA 99164-2814

The complex free-decay frequency was measured for quadrupole oscillations of acoustically levitated bubbles. Single bubbles were stably levitated for periods of up to 3 h. During this time the natural frequency and damping were measured at approximately 2-min intervals. Bubble oscillations were detected by means of an optical extinction technique. Each free-decay record was taken by abruptly terminating the amplitude modulation used to induce shape oscillations. The method is remarkably sensitive to the presence of impurities which adsorb onto the surface from solution. Damping associated with ``clean'' small bubbles agrees with predictions associated with viscous and boundary layer considerations for oscillations about a spherical shape. The complex frequency obtained for controlled bulk concentrations of surfactant (Triton X-100) agrees qualitatively with planar surface capillary-wave studies. Salt water solutions and Puget Sound seawater are also examined. Anomalous additional damping was found in association with larger bubbles (radius (greater than or approximately equal to)750 (mu)m) and possible sources will be considered. [Research supported by ONR.]