Abstract
Understanding of the cavitation inception and the following bubble cloud formation by the high intensity focused ultrasound (HIFU) is important in both physical and medical aspects. The backscattering from a primary cavitation is a key of these phenomena. In the present study, the cavitation inception pressure and the mechanism of bubble cloud formation by the backscattering of HIFU are investigated experimentally and numerically. In the experiment, a laser-induced bubble generated near the HIFU focus is utilized as a primary cavitation, and the optical observation with a high-speed video camera and the pressure measurement with a fiber optic prove hydrophone (FOPH) are conducted simultaneously. It is shown that the bubble cloud grows with forming multiple layers of tiny bubbles, and finally becomes a cone-shaped bubble cloud. The pressure distribution by the backscattering from bubbles simulated with the ghost fluid method reveals the forming process of bubble clouds in the experiment. The pressure measurement shows that the measured pressure has a stronger negative pressure value by the superposition of the incident wave and the backscattered wave from bubble interfaces. The pressure before the cavitation inception occurring at the tip of the cone-shaped bubble cloud is found to be around -26 MPa.
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