On the vortex ring dynamics in liquid films upon droplet impact

We present a systematic experimental investigation of the formation, evolution, and breakdown of vortex rings confined in a liquid film generated by droplet impact, with particular emphasis on their emergence and role in governing momentum and mass transport under confinement. Using time-resolved particle image velocimetry and laser-induced fluorescence, we elucidate how film thickness and impact conditions control vortex ring dynamics. Three distinct phases are identified: (i) the formation and radial expansion of a primary vortex ring, (ii) the generation of additional vortex rings driven by boundary-layer separation at the substrate, and (iii) the onset of azimuthal instabilities leading to vortex ring breakdown. We demonstrate that confinement by the liquid film promotes early vortex–wall interaction, with decreasing film thickness accelerating circulation decay and reducing the critical conditions for vortex ring instability. A comprehensive regime map is constructed to delineate stable vortex expansion, multiple vortex ring formation, and vortex ring instability. Moreover, circulation measurements reveal a universal scaling governing the temporal evolution of total vortex ring circulation in liquid films, for which we propose an empirical model that captures the decay process. Together, these results provide a unified and predictive framework for vortex-driven transport in liquid films upon drop impact that extends classical vortex–wall interaction concepts to confined liquid layers.