Viscoelastic effect on the dynamics of high-speed needle jets generated during bubble collapse in proximity to a solid boundary

When a cavitation bubble collapses near a solid boundary within a small stand-off distance, a needle jet could be generated and directed toward the solid wall in Newtonian fluids. However, cavitation bubbles in viscoelastic fluids also widely exist, including in interstitial fluid, blood, and chemical materials, and related needle jetting dynamics remain largely unknown. This Letter presents a comprehensive numerical investigation of the jetting dynamics during bubble collapse near a rigid boundary in a viscoelastic liquid. Here, we mainly focus on the small dimensionless stand-off distances γ and adopt the FENE-P model characterized by the dimensionless polymer-extension parameter ζl and the Deborah number De to describe the viscoelastic effect of the fluid. Several distinct features are observed. First, for both the Newtonian and viscoelastic fluids, there are four distinct jetting patterns as γ increases, including the radially converging needle jet, the inclined converging needle jet, the concentric annular jet, and the central regular jet. Second, when the viscoelastic effect is considered, the maximum γ that could form the needle jet decreases compared to the Newtonian fluid. Third, the simulation results show that the viscoelastic effect has a significant influence on the quantitative jetting dynamics, including the peak jet velocity and wall pressure, whereas the polymer relaxation time shows a weak influence on the jetting dynamics. Overall, the main findings of this study are believed to provide a theoretical foundation for multiple fields, such as shock wave lithotripsy, needle-free injection, and cavitation erosion mitigation or utilization.