Fast X-ray imaging of the onset of ultrasonic horn cavitation*

High-power ultrasonic horns operating in liquids at low frequency are known to generate a cone-shaped cavitation bubble cloud, for which the early stages of the cavitation activity remain elusive. In this presentation, the cavitation inception and the mechanisms resulting in the cone formation are addressed using high-speed synchrotron X-ray radiography. Radiographs reveal a 3-step process. Few microseconds after the sonication starts, several single cavitation bubbles are produced on the horn surface and oscillate. These bubbles then generate individual clouds under a splitting process combined with a lens effect induced by the interface curvature of each initial bubble and the vapor-to-liquid sound speed ratio. Finally, the clouds merge to form a larger cone-shaped bubble cloud. While experiments are conclusive to solely elucidate steps 1 and 3, we complement the description of step 2 using geometrical acoustics where the bubble is modeled as a plano-convex lens. Qualitative descriptions are complemented with quantitative measurements, such as the nuclei relaxation time and the bubble size distribution closely beneath to the horn tip, which are usually penalized by the integration along the line-path when using conventional shadow-based imaging.