Bubble entrainment from droplet impact on a liquid film

<p indent="0mm">The impact of droplets on liquid films is a common physical phenomenon observed in both natural and industrial settings. We conducted both experimental and numerical studies on the transient process of bubble entrainment during droplet impact, focusing particularly on the mechanisms behind the closure of the splashing crown and the formation of large bubbles (significantly larger than the droplet itself). Using high-speed photography, we captured the entire process of bubble entrainment, while an incompressible two-phase flow model was developed employing the finite volume method and the volume of fluid (VoF) method. Our findings indicate that bubble formation is influenced by the Weber number and the dimensionless liquid film thickness. We established a critical condition for bubble formation and quantified the probability of bubble generation across a broad range of parameters. Numerical simulations revealed the continuous generation of vortex rings within the splashing crown, which gradually move toward the cavity bottom. A notable pressure drop was observed at the vortex core, which facilitates crown closure and is largely underestimated by the traditional Bernoulli equation. Additionally, we identified the gas-liquid density ratio, <italic>α</italic>, as another key factor affecting bubble formation. A higher <italic>α</italic> accelerates crown closure, thereby promoting bubble formation.