Fast microjets from expanding and collapsing cavitation bubbles into a capillary

Abstract

Abstract Content of image described in text. 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 $\gamma$ , the normalised inner radius upper R Subscript i Superscript asterisk Ri∗ $R_i^*$ and the normalised outer radius upper R Subscript o Superscript asterisk Ro∗ $R_o^*$ , 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 500ms−1 $500\,\mathrm{m\,s}^{-1}$ and reaching up to 2000 normal m normal s Superscript negative 1 2000ms−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 Ro∗ $R_o^*$ governs the jetting regime by controlling interfacial-wave formation, propagation and convergence, while the inner radius upper R Subscript i Superscript asterisk Ri∗ $R_i^*$ 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.