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Cavitation cloud formation by the backscattering of focused ultrasound from a bubble interface in gelatin has been studied experimentally. A 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. The maximum size of the laser-induced bubble is about 2 mm in radius. After the laser-induced bubble collapses, a small bubble whose radius is about 0.3 mm remains around the laser focus. In the present experiment, this residual bubble is also used as a reflector bubble, and the effect of reflector bubble size on cavitation cloud formation has been evaluated. Cavitation inception and cavitation cloud formation near the reflector bubble has been observed with a high-speed camera. It is shown that the cavitation cloud grows along with the propagation axis of the incident wave. The larger the reflector bubble size becomes, the earlier the first cavitation inception near the reflector occurs. On the other hand, regardless of the reflector bubble size, the distance between the top interface of the cavitation cloud and the geometrical focus of the focused ultrasound at the maximum expansion of the cloud is almost constant. The results also show that when a residual bubble is used as a reflector, the growth rate of a cavitation cloud in the direction of the propagation axis becomes higher than that when a laser-induced bubble is used. This may be due to the difference of the interface shape of the tip of developing cavitation cloud. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The direct numerical simulations are conducted for the backscattering of incident pressure waves from a bubble with the ghost fluid method in order to understand the formation of secondary cavitation in the high intensity focused ultrasound (HIFU). It is shown that the ratio of a bubble collapse time t0 to a characteristic time of wave propagation ts, η = t0/ts, is an important determinant for generating negative pressure region by backscattering. When η is of the order of 0.1, no negative pressures are generated around the bubble and the bubble collapses by the pressure increase due to the incident pressure wave. When η is of the order of 1, negative pressures are generated and an impulse per unit area with negative values takes the minimum although the positive pressure wave by the bubble collapse, which arrives at the negative pressure region, would reduce the growth of incepted cavitation. Also, the collapse time of a bubble takes the minimum when η = 0.733. When η is of the order of 10, high negative pressures are generated in a relatively smaller volume during shorter duration, which may cause the inception of the secondary cavitation. We also evaluate a negative pressure volume Vth and a duration of negative pressure τth associated with cavitation inception. The results also show that the product of Vth and τth, which is an indicator of probability of cavitation inception in the classical homogeneous nucleation theory, takes the local maximum in terms of η: A larger Vthτth suggests a higher probability of cavitation inception.