Insights on bubble encapsulation after drop impact on thin liquid films

The accurate understanding of the phenomenology of drop impact onto dry/ wetted and cold/heated surfaces is increasingly relevant to implement biofuels in civil aviation. The outcome of drop impact depends on the pre-impact conditions and a seldom researched event is the encapsulation of a bubble when this impact occurs on thin liquid films. Therefore, the goal of the experimental work reported is to investigate the mechanism of this bubble encapsulation. Results show that the mechanism leading to a bubble formation has two stages. In the first stage, after the drop impacts a steady liquid film, a prompt splash occurs followed by a crown splash. The uprising sheet propagates in an almost normal direction relative to the liquid film, but its radius at the base continues to expand, eventually leading to the inward collapse of the crown-bounding rim encapsulating air inside the dome. In the second stage, three different phenomenologies of bubble encapsulation can occur. At the top of the closed crown, one jet (phenomenology 1) or two jets are formed (phenomenologies 2 and 3). For phenomenology 2, the upward jet eventually collapses due to gravitational influence, while the downward jet continues to grow until it reaches the liquid film, attaching to it, stretching and detaching from the top at the hemispheric thin sheet, forming a bubble. In phenomenology 3, the upward jet is high enough to allow its breakup and ejection of one large droplet before the collapse of the upward jet. Many secondary droplets fall on the bubble and one of them will eventually break the dome, leading to more secondary atomization. Additionally, the first perturbation imposed on the liquid film by the droplet impact is studied and an empirical correlation is proposed for its propagation velocity. Finally, bubble geometry is investigated.

Automated summary

The goal of this study is to investigate the mechanism of bubble encapsulation when a drop impacts a thin liquid film. The results show that the mechanism consists of two stages. In the first stage, the drop impacts the liquid film, causing a prompt splash and a crown splash. The collapsing crown-bounding rim encapsulates air, forming a bubble. In the second stage, three different phenomena of bubble encapsulation can occur. The study also examines the first perturbation imposed by the drop impact and proposes an empirical correlation for its propagation velocity. Additionally, the geometry of the bubble is investigated.