An experiment on the interaction of a 1 MHz focused ultrasound (US) with Sonazoid microbubbles has been carried out in the presence of an agar gel. The survival number of Sonazoid microbubbles was examined by changing the exposure time Tex, the acoustic energy density E and the Sonazoid concentration Cv. High-speed photography with the maximum framing rate of 300,000 frames/s was conducted to observe the behavior of bubbles near an agar gel. It was found that bubble coalescence frequently occurred during an earlier stage after US irradiation. Subsequently a newly created bubble migrated towards the agar gel surface and away from it, behaving a kind of circulation due to an acoustic streaming. A minimum volume of a bubble was reached within a region very close to the gel surface, presumably emitting pressure waves at each rebound which could impact on the gel surface. In fact, a number of depressions in dozens nanometers were found on the agar gel surface which were detected by using an AFM.
