Criterion for bubble encapsulation on drop impact onto a liquid film

Abstract

The phenomenon of bubble encapsulation results from droplet impact on a liquid film for specific impact conditions, but there is no established criterion for predicting its onset. Phenomenon visualization from two perspectives, the common lateral perspective and a bottom perspective, provided insights into the dynamics and formation mechanisms. Namely, the bottom shadowgraphs show capillary wavy patterns and perturbations imposed on the steady liquid film, which suggests a greater role of the liquid film in the onset of bubble encapsulation. Also, some considerations about the cavity development underneath the bubble limited by the solid wall allow concluding that the cavity shape is independent of the bubble encapsulation phenomenon. Additionally, using the bottom shadowgraphs, the crown closure time shows a systematic decrease in the dimensionless film thickness of 0.5<δf<0.6, which will be subject of future work. Finally, while most drop impact correlations focus on using the droplets' characteristics and thermophysical properties, the experimental results point in a different direction. Considering correlations relating the Ohnesorge and Reynolds numbers, the new criterion for the onset of bubble encapsulation uses drop characteristics and properties in the Reynolds number, while the liquid film thickness and thermophysical properties are used in the Ohnesorge number because most of the crown material comes from the liquid film. Therefore, the criterion based on 100% occurrence of bubble encapsulation is not a threshold, but a range: kbe=ln(34.5/ReD)/ln(Ohf), with kbe∈[1.022,1.142]. Other authors observed this phenomenon and despite being outside the validation range of this correlation, the values are close to their boundaries.