Measurement of the Local Pressure near a Cavitation Bubble Cloud Formed by the Focused Ultrasound

Abstract

The growth of a cavitation bubble cloud and the corresponding pressure near the tip of the cavitation bubble cloud formed by the backscattering of the focused ultrasound from a laser-induced bubble have been analyzed experimentally. The laser-induced bubble which is generated near the geometrical focus of the focused ultrasound is used as a reflector of the incident wave to yield strong negative pressure which leads to the cavitation inception and following cavitation bubble cloud formation. In the present study, optical observation by a high-speed video camera with frame rate up to 1.25 Mfps and the pressure measurement by the fiber optic prove hydrophone (FOPH) are carried out simultaneously to estimate the cavitation inception pressure. It is shown that the cavitation bubble cloud grows conically along with the propagation axis of the incident wave. The distance between the top interface of the cavitation bubble cloud and the geometrical focus of the focused ultrasound becomes a limited value even though the focused ultrasound keeps on irradiating, which indicates that the cavitation bubble cloud grows within the focal region with sufficient negative pressure due to backscattering. The pressure measurement reveals that the negative pressure amplitude backscattered from a free surface is larger than that of the focused ultrasound itself. Negative peaks in the pressure waveform have a long term fluctuation whose increments correspond the sudden growth of the cavitation bubble cloud. Cavitation inception pressure was measured defining it as the negative pressure at the top interface of the cavitation bubble cloud just before the FOPH was covered with cavitation bubbles. The measured cavitation inception pressure was -24 MPa in the present study.