On the structure of the air disk surrounding the central air bubble : Droplet impact on immiscible liquid pool*

In this work, we probe the structure and fluid dynamics of the peripheral air disk surrounding the central air dimple beneath an impacting water droplet on an immiscible liquid pool of silicone oil. For the first time, we report the detection of hydrodynamic instability signature in the air disk. We studied low impact energies with Weber-number in the range of 2 - 20. The instability patterns in the air disk were similar in signature to thin-film and Saffman Taylor instability. The air disk undergoes rupture due to thin-film instability and entraps the central air bubble. We provide a detailed mechanistic view of the rupture process experimentally using high-speed reflection interferometric imaging and theoretically using scaling analysis. The central bubble and Messler entrainment found during drop impact on liquid pools result from the air-disk rupture.