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 gamma γ γ , the normalised inner radius upper R Subscript i Superscript asterisk R i ∗ Rᵢ^* and the normalised outer radius upper R Subscript o Superscript asterisk R o ∗ Rₒ^* , 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\,m\,s⁻¹ and reaching up to 2000 normal m normal s Superscript negative 1 2000 m s − 1 2000\,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 ∗ Rₒ^* governs the jetting regime by controlling interfacial-wave formation, propagation and convergence, while the inner radius upper R Subscript i Superscript asterisk