Supersonic jetting from cavitation bubble pair interaction

Cavitation bubbles in a heterogeneous environment are known to develop jets that penetrate through the bubble. Jets with supersonic velocities with respect to the bubble content have been found for bubbles collapsing very close to elastic and rigid boundaries. Here, we demonstrate that two bubbles that are identical in volume but are created out of phase at a proper distance can accelerate a jet to supersonic velocities. Compared to a rigid boundary, the high-speed jet formed by bubble pairs does not atomize while penetrating the body of the bubble. The fluid dynamics leading to this ultrafast needle-shaped jet are studied with high-speed imaging at two million frames per second. We present evidence that this ultrafast needle jet is induced by a singularity created by the concave collapse of the elongated neck of the second bubble that is further amplified through shock wave emission. Compressible viscous flow simulations reveal a favorable comparison with the experiments and explain the narrow parameter range where the supersonic jetting is found.