Investigation of cavitations dynamics induced by tandem shock waves in water

Summary form only given. We have developed a shock wave generator where two cylindrical pressure waves are focused to a common focal region by a metallic parabolic reflector, and the waves can be switched on with a different time delay. Interaction of two successive shock waves (tandem shocks) in water focused to a common focal point was investigated. Amplitude of the of the pressure wave reaches up to 100 MPa at the focus, while the amplitude of the rarefaction wave falls down to -25 MPa (well above the cavitation threshold), producing thus numerous cavitations. Schlieren photography of the focal region demonstrated creation of a very complex pressure field with many secondary spherical short wavelength shocks that originate in collapsing cavitations. These secondary shocks can interact with cell scale structures and they are considered to play the main role in cell membranes damage when organic tissue is exposed to the shock waves. Therefore, we are interested in the waveform and the pressure amplitude of these secondary shocks with an ultimate goal in enhancement of cancer treatment efficiency by activation of sonosensitizers based on the effects of the collapsing cavitations. In this work cavitations dynamics induced by the tandem shock waves in water is investigated in more detail. It was demonstrated that the acoustical non homogeneity created in water by the first wave strongly modifies propagation of the second one. Measurements of the pressure waveforms by PVDF shock gauges at the focus demonstrated that at some time delays between shocks the second wave reaches the focus as a rarefaction wave producing thus a large number of cavitations. To determine a gas pressure in the cavitations, strong electric field in the focal area will be applied to generate electrical discharges inside the cavitations. Breakdown voltage with known diameter of the cavitations can provide information about the gas density in the fully developed cavitations. From observation of a speed of the cavitations collapse with the knowledge of the gas density a temperature inside collapsing cavitations and the pressure of the secondary shock waves can be determined.