Characterization of secondary ultrasonic waves radiated from bubbles based on small-bubble trapping pattern analysis

When a microbubble oscillates under an ultrasonic wave, the bubble radiates a secondary ultrasonic wave around it. This wave produces the secondary Bjerknes force between the neighboring bubbles and it assists in the aggregation of bubbles to make bubble clouds if the phases of the vibrations are the same. In this paper, a novel technique to characterize the secondary ultrasonic wave radiated from a bubble is proposed. This method is based on the observation of the interference fringe pattern which is produced by small bubbles trapped around the bubble of interest. A method to estimate the frequency, phase and amplitude of the secondary ultrasonic wave is discussed. Experiments are carried out for bubbles produced by an ultrasonic wave contrast agent. The results show that aggregated bubbles of a few tens of micrometers radiate approximately 7th- to 8th-order harmonic waves at 200 kPa sound pressure.