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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]

A cavitation inception theory of a vapor bubble in flowing liquid is presented by taking account of a set of fluid dynamics boundary conditions for nonequilibrium evaporation at the bubble wall. The boundary conditions for the vapor temperature, pressure and density at the wall are a set of solutions of the polyatomic type of ES-BGK Boltzmann equation. The evaporation rate and the vapor-liquid temperature discontinuity at the bubble wall are treated in a physically correct way at a molecular level. The theory also considers the translational motion of the bubble in a flowing liquid with arbitrary relative velocity between the bubble and the surrounding liquid. The translational motion largely reduces the liquid pressure at the bubble wall, and the inception process of cavitation is thereby influenced greatly. Furthermore, the numerical calculation of expansion process of the vapor bubble is performed for the case where the bubble is instantaneously exposed to the low liquid pressure, and its results are compared with those of the equilibrium evaporation. A critical liquid pressure considered as a criterion of expansion and contraction of the bubble is numerically estimated for the various relative velocities between the bubble and the liquid and the various initial bubble radii, and it is compared with the pressure of the classical theory. Numerical calculation reveals that the critical pressure for the relative velocity of zero in the case of nonequilibrium evaporation or condensation is lower than the classical one in any initial bubble radius.