How boundary layer flows influence cavitation bubble-driven particle jumps from rigid walls

The interaction between cavitation bubbles and particles near rigid boundaries plays a crucial role in applications from surface cleaning to cavitation erosion. We present a combined experimental, numerical and theoretical investigation of how boundary layer flows affect particle motion during the growth and collapse of the cavitation bubble. Using laser-induced cavitation bubbles and particles of varying radius ratios and stand-off distances, we observe that increasing the bubble-to-particle size ratio suppresses particle displacement. Through one-way coupled simulations and theoretical modelling, we demonstrate that this suppression arises from a shift in the dominant forces acting on the particle: for small radius ratios, the pressure gradient force governs particle motion, while for large ratios, the interplay between added mass, lubrication, and pressure gradient forces becomes significant due to boundary layer growth in the bubble-induced stagnation flow. Based on a theoretical framework combining potential flow theory and axisymmetric viscous stagnation flow analysis, we identify the inviscid- and viscous-flow dominated regimes characterised by the combination of the stand-off distance, the bubble-to-particle radius ratio, and the bubble Reynolds number. Finally, we derive scaling laws for particle displacement consistent with experiments and simulations. These findings advance our understanding of unsteady boundary layer effects in cavitation bubble-particle interactions, offering new insights for applications in microparticle manipulation and flow measurements.