Numerical study on laser-induced cavitation bubble dynamics inside a millimetric droplet

In this study, complicated nonlinear interactions of a single laser-induced cavitation bubble inside a millimetric water droplet were numerically investigated using a fully compressible three-phase homogeneous model. A general condensation phase-change model and high-resolution interface-capturing schemes were adopted to accurately predict the bubble collapsing and rebound stages as well as strongly deformable droplet interface evolutions. The numerical model was validated using experimental data in terms of the equivalent bubble radius until the second collapse stage, and good quantitative agreement was achieved. The variation in the droplet surface velocity was detected and could better reveal the mechanism underlying the complicated bubbles and droplet interactions, particularly in droplet surface splash dynamics. Subsequently, the complex bubble–droplet interaction phenomena were studied by investigating the ratio of the maximum bubble radius to the initial droplet radius. The numerical results show that the bubble collapsing time decreases monotonically with an increase in the bubble–droplet radius ratio. The droplet surface instabilities became more dominant as the radius ratio increased. In addition, four distinct patterns of droplet motion, namely, stable, multi-spike, ventilating jet, and splashing phenomena, were captured. Finally, the specific mechanisms leading to droplet surface jetting were identified.