PS-5 Optical Observation of the Collapse of a Single Bubble Adhered to the Quartz Wall in Ultrasound Irradiation with Ultra High Speed Camera

The mechanical action under ultrasound irradiation results from the collapse of bubbles. There are two types for the collapse of a bubble. One is the symmetrical collapse in which shock wave is generated. The other is asymmetrical collapse in which micro-jet is generated. We focus on the asymmetrical collapse. There are few reports about the asymmetrical collapse in the ultrasound field and therefore the mechanism is unknown in detail. In order to clarify the asymmetric collapse of a bubble in ultrasound field, it was attempted to observe the collapse behavior of a single bubble adhered to the rigid wall with highspeed video camera of which maximum recording rate is 1000000 frames/sec. We succeeded in obtaining the continuous images of the asymmetrical collapse behavior which includes the initial stage of micro-jet formulation. In this paper, this observation results are discussed. As a result, it is shown that the temporal oscillation of the surface fluctuation on the bubble wall is important factor for the micro-jet formulation. Also, we measured both the speed and the tip diameter of the observed micro-jet. It is shown that the speed of the observed micro-jet is at least 13 m/s or more and the tip diameter is about 9 mum