Jets from interacting cavitation bubbles*

Ultrafast microjet generation has sparked interest due to its potential application in needle-free injections to enable mass vaccination. Cavitation bubbles collapsing near surfaces can produce high-speed jets, but their direction is difficult to control near biological interfaces due to their elasticity. Introducing cavitation bubble pairs to generate jets, however, can overcome this difficulty. Here we show, experimentally and numerically, that a variety of dimensionless parameters such as the bubble size ratio, the distance between the bubbles, and the time difference in the bubble generation govern the dynamics of a pair of interacting cavitation bubbles and the resulting jets. These parameters can manoeuvre the jets' direction and velocity, reaching values of up to hundreds of metres per second. The mechanism behind ultrafast jets is also revealed, suggesting that a rebounding bubble imparts momentum to the liquid contained between bubbles and focuses the flow onto the neighbouring bubble, resulting in powerful jets. Finally, we also demonstrate that these jets can penetrate soft materials. The results highlight their potential in controlled, precise, and directional use in therapeutic (e.g., cell perforation, cell transfection) and needle-free drug-delivery applications.