Bubble growth during ``magic carpet breakup`` of a drop on a heated substrate*

Drop impact onto a hot substrate results in a multitude of different outcomes, depending on the substrate temperature and the impact parameters. In our previous study (R. Hatakenaka et al., \textit{Int. J. Heat Mass Transf.}, 2019), a new outcome named magic carpet breakup was identified under reduced ambient pressure (1 - 10 kPa) for water drops impinging onto a superheated smooth substrate with moderate impact velocity (0.46 m/s). Droplets are repelled from the hot surface so violently, that they spread out to a flattened shape in an explosive manner. Here we observe the existence of a growing vapor bubble underneath the drop. Its growth between the drop and the substrate is directly observed via high-speed total internal reflection (TIR) imaging. The bubble emerges at the center of impact point, grows, and finally coalesces with some additional smaller bubbles. Similar observation has been already reported by Yu et al. (\textit{Soft Matter}, 2019), however no quantitative data was presented. We will evaluate the bubble growth rate via image analysis and discuss an applicability of a classical bubble growth model to this problem.