Focused jets from a bursting bubble induced by another cavitation bubble

When a driving pressure accelerates a concave air–liquid interface, the fluid converges centrally, inducing flow focusing and subsequent jet formation. This study demonstrates that a near-surface bursting bubble creates an open cavity with a concave air–liquid interface, which interacts with a submerged cavitation bubble to generate a driving pressure, resulting in a focused jet. By focusing dual laser beams near the free surface, two axisymmetric in-phase cavitation bubbles were generated, with one a bursting bubble responsible for the open cavity and the other a control bubble positioned below for jet production. Numerical simulations were conducted utilizing the compressibleInterIsoFoam solver within the open-source platform OpenFOAM, employing a geometric volume-of-fluid method to capture interfaces dynamics. Experimental results and numerical simulations demonstrate that the process of focused jet generation can be delineated into two distinct stages: cavity elongation and subsequent jet formation. During cavity elongation, the concave interfaces of the bursting bubble migrate downward until reaching its peak velocity, followed by a pronounced deceleration leading to maximum elongation, resulting in a high stagnation pressure. The curvature reversal during elongation produces a smaller concave interface, leading to a thinner and longer focusing jet accompanied by elevated velocity. A theoretical model is proposed and verified numerically for jet formation to derive a scaling law between the jet velocity and the maximum elongation, and a critical elongation length between a regular jet and a thin jet. The findings of this study provide new insights into the field of needle-free injection and inkjet printing.