Bubble dynamics on top of protruding objects observed with x-ray phase-contrast high-speed imaging

This study presents a comprehensive, qualitative investigation of the complex dynamics of laser-induced cavitation bubbles near rigid, protruding objects with various cross-sectional geometries. We employ a threefold approach that combines synchronized optical videography, high-speed x-ray phase-contrast imaging (at MHz frame rates), and three-dimensional compressible multiphase flow simulations. While conventional optical high-speed imaging is often limited by refraction, which obscures the bubble interior, synchrotron x-ray radiation enables unprecedented visualization of internal structures, including secondary cavitation, splashing, and jetting dynamics. Four distinct geometries are investigated: a cylindrical pillar, a cone, a vertical blade, and a three-point star column. We identify several universal qualitative features of bubbles generated above both axisymmetric and anisotropic columns over normalized stand-off distances D∗ in the range [0.047, 0.79] and normalized object widths β in the range [0.251, 0.893], including configurations where the bubble is generated off-center relative to the column. Results show a clear correlation between bubble dynamics and a specific cross-sectional geometry. Furthermore, effects of small-scale features of the object surfaces, such as roughness and sharpness of edges, can occur. These can play a role in apparent pinning of the gas–liquid interface, flow detachment and vortex formation, and secondary cavitation. The combined experimental–numerical framework allows for precise determination of bubble parameters and provides validation of three-dimensional simulations, demonstrating that geometric asymmetries significantly influence jet formation compared with fully axisymmetric configurations.