Free Surface Enhancement of Droplet Rupture by Cavitation Bubble Collapse

The interaction between cavitation bubbles and surrounding droplets governs a wide range of processes such as surface cleaning, ultrasonic emulsification, and drug delivery. These applications rely on mechanisms such as bubble-induced microjets and shear flows that drive the deformation and breakup of the droplet interfaces, all of which are significantly influenced by geometric confinements. Here we investigate the hydrodynamic interaction between cavitation bubbles and oil droplets within a thin water layer, considering the coupling confinements of a free surface and a rigid wall. We reveal two distinct regimes of droplet response to cavitation bubble collapse: the rupture regime, where oil droplets fragment into smaller droplets, and the no rupture regime, where the droplet remains intact. We derive the expression for the non-dimensional Kelvin impulse induced by the cavitation bubble. We further obtain a scaling law using an energy-based argument which relates the critical Kelvin impulse to a characteristic Weber number and bubble-to-droplet size ratio, which predicts the experiments well. Our findings provide important insights into the mechanisms driving cavitation-bubble-induced droplet rupture and establish quantitative criteria for predicting rupture behavior.