Acoustic cavitation, bubble jetting and surface instabilities in a free-falling droplet

We present a study of laser-induced bubble nucleation and dynamics within millimeter-sized liquid droplets in free-fall motion, where the bubbles experience the influence of a free boundary in all directions. The first part of the research focuses on investigating the nucleation of secondary bubbles induced by the rarefaction wave resulting from the reflection of the shock wave emitted by the laser-induced plasma at the droplet's surface. Interestingly, three-dimensional clusters of cavitation bubbles are formed. Direct numerical simulations based on the volume of fluid method allowed us to estimate a cavitation threshold value by comparing the calculated negative pressure distributions with the shape of the clusters.

High-speed recordings of droplet/bubble dynamics are combined with velocity and pressure field simulations under the same initial conditions. The impact of the curved free surface of the drop on the bubble jetting dynamics is qualitatively assessed by classifying cavitation events using a non-dimensional stand-off parameter dependent on the drop size, bubble maximum radius, and relative bubble position inside the droplet. Furthermore, the role of the curvature of the surface is examined through a structural similarity algorithm, comparing bubbles produced near a flat surface to those within the droplet. Remarkably, this quantitative comparison reveals equivalent stand-off distances where bubbles influenced by different boundaries behave similarly. The oscillation of laser-induced bubbles promotes the onset of Rayleigh-Taylor and Rayleigh-Plateau instabilities on the droplet's surface. These are studied by varying the ratio of the maximum radii of the bubble and the drop. Here, specific mechanisms leading to droplet surface destabilization are identified through a detailed analysis of high-speed images and numerical simulations.