Effects of liquid and non-condensable gas properties on the dynamics of ultrasonic cavitation

Monodisperse streams of microbubbles are exposed to high-intensity focused ultrasonic beams to investigate the effects of mean pressure (0–4 bar), liquid properties (water, kerosene, cyclohexane), as well as non-condensable gases composition (O₂, N₂, Ar), and concentration on the cavitation dynamics. Ultra-high speed shadowgraphy and holography in a specialized facility enable direct visualization of shockwave emission, plasma generation, and evolution of bubble clouds. Judging based on the frequency of shocks as the acoustic waves propagate through the bubble clouds, the bubble collapse intensifies with increasing mean pressure, i.e. mass of gas in the bubbles, and for argon bubbles, owing to their higher specific heat ratio and stability. With diatomic gases, oxygen bubbles generate significantly fewer shocks than nitrogen bubbles under similar conditions. This trend suggests that chemical reactions reduce the pressure and temperature rise during the final stages of oxygen bubble collapse owing to its lower bond dissociation energy. In fuels, the shocks in kerosene are more intense than those in cyclohexane owing to the higher vapor pressure, hence greater vapor content, in the latter, but both show clear evidence of dissociation. Finally, for all the liquids, gas saturation promotes formation of self-sustained bubble clouds. Yet, the shock counts decrease in comparison to degassed liquids, presumably owing to acoustic shielding.