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The early stage of the dynamics of interacting cavitation bubbles in a transmitted spherically focused pulsed wave consisting of compression and rarefaction phases is studied. The cavitation is investigated away from the liquid boundaries for negative pressures up to −42 MPa with a decrease rate of −40 MPa/μs by high-speed microscopic filming (100 million frames per second and a spatial resolution of 5–50μm/pixel) and pressure measurements. It is demonstrated that, according to the kinematics of size variation, the bubbles are separated into two fractions: expanding and collapsing ones. Experimental data provide grounds to assume that the formation of a two-fraction bubble cluster occurs on account of different threshold values of pressure in the rarefaction wave for two characteristic sizes of nuclei, namely, micronuclei (d I < 10μm) and nanonuclei (“bubstons,” d II ∼ 1 nm), which become detectable only when the rarefaction wave amplitude exceeds the critical one. Pulsed compression of small bubbles in the process of transformation of a rarefaction wave into a compression wave occurs under the effect of a cluster’s internal pressure up to 20 MPa and proceeds with the conservation of the spherical shape of the second-fraction bubbles. It is demonstrated that the velocity of the center of mass of a bubble reaches its peak value close to the moment of bubble collapse. The translational and radial dynamics of a bubble are studied in a numerical experiment using the Rayleigh-Plesset equation and taking into account the viscous force and the pressure gradient. The results of measuring the translational shift may be useful for estimating the minimum bubble radius by comparing the numerical results with the experiment.

The determination of the phase of the bubble oscillation at the instant of light emission, which is a key issue for understanding the origin of cavitation luminescence of liquids, is discussed. The observation of luminescence in the course of the nucleation and growth of a bubble up to its collapse is performed in a bipolar wave consisting of a compression phase followed by a rarefaction phase in the regime of a two-fraction bubble cluster formation. The space-time distributions of the luminescence intensity and pressure and the dynamics of the cluster in water and a glycerin solution are investigated at the early stage of cavitation. A correlation between the maximal density of light flashes and the positive pressure pulses in the field of superposition of the initial and secondary cavitation compression waves is revealed. It is shown that the spherical focusing of acoustic pulses both away from the boundaries of the liquid and near its free surface makes it possible to compare the luminescence intensities for different rates of the pressure decrease.

Nonlinear processes accompanying the focusing of a microsecond acoustic pulse produced by an electromagnetic source shaped as a spherical segment are investigated. The processes are considered to be far from the boundaries of a liquid, in the absence of cavitation. Detailed measurements of the pressure field by a fiber-optic sensor and high-speed photography of the shock front are performed. The pressure field is found to be determined by the nonlinear effects that occur in the course of the propagation of the initial converging compression wave and an edge rarefaction wave. The peak pressure amplitudes at the focus are 75 and −42 MPa for the compression and rarefaction waves, respectively, at the maximum voltage of the pulse generator in use. The measured length of the compression wave front is equal to the response time of the sensor (8 ns). The pressure amplitude is shown to be limited by the irregularity of the propagation of a shock wave in the form of Mach’s disk. At the focus, the pressure gradient across the radiator axis reaches 0.5 atm/μm, while the diameter of the focal spot is 2.5±0.2 mm. The focus of the edge rarefaction wave formed due to diffraction is located closer to the radiator than the focus of the compression wave, which may facilitate the study of the biological effect of cavitation independently of the shear motion of the medium.