Fast microjets from expanding and collapsing cavitation bubbles into a capillary

Cavitation bubbles collapsing near capillary exits can generate fast microjets that penetrate into the channel and promote fluid exchange. Here, we combine high-speed imaging and axisymmetric compressible volume-of-fluid simulations to study laser-induced cavitation bubbles near the exit of a long capillary. By varying the stand-off distance

, the normalised inner radius

upper R Subscript i Superscript asterisk R i ∗ $R_i^*$

and the normalised outer radius

upper R Subscript o Superscript asterisk R o ∗ $R_o^*$

, we identify two distinct jetting mechanisms. Regular jets are driven primarily by the vertical collapse of the bubble surface distal to the capillary, whereas wave-driven jets arise from the propagation and convergence of an interfacial wave initiated at the proximal bubble surface, causing distal-surface splitting and strong local pressure amplification. Wave-driven jets occur for sufficiently small stand-off distances and outer radii, with formation velocities typically above

500 normal m normal s Superscript negative 1 500 m s − 1 $500\,\mathrm{m\,s}^{-1}$

and reaching up to

2000 normal m normal s Superscript negative 1 2000 m s − 1 $2000\,\mathrm{m\,s}^{-1}$

in the simulations. Regular jets dominate at larger stand-off distances or larger outer radii, where interfacial-wave effects weaken. The outer radius

upper R Subscript o Superscript asterisk R o ∗ $R_o^*$

governs the jetting regime by controlling interfacial-wave formation, propagation and convergence, while the inner radius

upper R Subscript i Superscript asterisk R i ∗ $R_i^*$

mainly affects the later jet dynamics, including the impact location and vapour-jet penetration into the capillary. Compared with the needle jet generated by a bubble collapsing near a smooth rigid wall at nearly zero stand-off distance, the wave-driven jet here occurs over a much wider parameter range and its velocity and size can be tuned geometrically. These results establish a physical framework for cavitation-driven jetting near capillary boundaries and suggest strategies for controlling confined microjets in microfluidics and biomedical applications.