Shape Oscillation of Bubble(s) in Acoustic Field

Abstract

We focus on dynamics of multiple air bubbles exposed to acoustic pressure field while ascending in water. The bubbles are injected into the pool filled with water from a vertical capillary tube, and then the acoustic wave of designated frequency is applied toward the bubbles. The frequency of the acoustic wave is varied from 0.5 to 20 kHz. Volume and shape oscillations of the bubbles are captured by a high-speed camera at frame rates up to 40000 fps with a back-lighting system. Through this system, we succeed in capturing the dynamics of the axisymmetric shape oscillation with a distinct mode number; the bubble exhibits the volume oscillation first with a fundamental frequency f0, and then the gradual transition to the shape oscillation with a fundamental frequency fnm takes place. We evaluate the correlation through the careful observations between the f0 and fnm as f0 ∼ 2.1fnm, which brings almost perfectly confirmation of the prediction through the preceding theoretical works. We also indicate the criterion of the excitation of the shape oscillation by varying the frequencies of the adding pressure field.