When micro bubbles flow into an ultrasonic wave field, the microbubbles often produce bubble aggregation. The aggregated bubbles create an inherent aggregate pattern depending on the bubbles, ultrasonic wave frequency and sound pressure. To evaluate the microbubble aggregation, which is produced by ultrasonic waves, the Bjerknes force which is produced by aggregated bubbles is derived. We assume that 2n þ 1 bubbles are aligned with a separation shorter than the wavelength and that they oscillate independently producing secondary ultrasonic waves around the bubbles. The relative phase between the secondary wave radiation of the aggregated bubbles and the radial oscillation of a single neighboring bubble is an important parameter for characterizing the bubble aggregation. Based on this discussion, a novel method of measuring the relative phase between the secondary wave radiation and the radial oscillation is proposed. This method is applied to micro bubbles with polyvinyl chloride shells.
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