Reverse Waterbells Formed During Drop Impact onto Immiscible Oil Layers on Water

The formation of a reverse waterbell (also known as a drop-impact canopy) is a fascinating splashing phenomenon, which is easily observed during heavy rain on puddles, but is yet to be completely understood. This research primarily deals with the impact of an oil droplet with a two-layered liquid consisting of a thin oil layer on top of a deep pool of water. This configuration is important in understanding the process of oil emulsification by rain after an oil-spill occurs on a lake or ocean. High-speed impacts give rise to the generation of a large crater along with an axisymmetric thin liquid sheet which rises out of the pool, initially expands, then retracts and finally collapses onto the axis of symmetry entrapping a hemisphere of air. The reverse waterbell or canopy is observed upon the completion of this whole process, by a bubble sitting on the pool surface. In our investigation, we varied the thickness and viscosity of the oil layer on the pool as well as the impact velocity of the oil droplet to explore where canopies are formed. Image analysis, obtained using two high-speed cameras, provides insights into how the controlling factors of fluid properties and film thickness affect these dynamics. We characterize the canopy size in Reynolds and Weber numbers space.