Transient pulsations of small gas bubbles in water

Transient behavior of small gas bubbles in a liquid set into violent motion by ultrasonic pressure waves is of interest because of widespread use of microsecond pulses in diagnostic ultrasound. Such pulses contain only a few pressure cycles and the transient pulsations of bubbles set in motion by such pulses would determine the bubble–ultrasound interaction. A computer study has been made to obtain a global representation of the pulsation amplitudes R(t) of small gas bubbles (nuclei) in water during the first few cycles of a cw ultrasonic pressure. One objective was to obtain a better understanding of cavitation phenomena where many nuclei with initial radii Rn from 0.1–20 μm are set in motion at pressures ranging from 0.5–5 bars and at frequencies from 0.1–10 MHz. Results allowed construction of surfaces showing the relative bubble amplitude R/Rn as a function of Rn and of the time t/TA, where TA is the acoustic period. One finding is that, in the range of peak pressures found in diagnostic pulses, transient cavities would be generated during the first pressure cycle from nuclei with initial radii as small as a few microns (μm). Nuclei that grow into transient cavities in the first pressure cycle are here called ‘‘prompt’’ nuclei. At a specified pressure, the size range of radii Rn in which they occur decreases with increasing frequency. At 5 bars, the range of Rn for prompt nuclei is 0.166–11.35 μm at 0.5 MHz and vanishes at 10 MHz.