Streamlined air-cavities formed by non-superhydrophobic spheres impacting water*

The formation of a stable and streamlined gas cavity following the impact of a sphere on a deep pool, is a recently observed phenomenon$^{\mathrm{1}}$, which shows motion at near-zero drag. This was shown for both Leidenfrost as well as superhydrophobic spheres. We now extend these results by demonstrating that both metallic and non-metallic spheres, with contact angles between \textgreater 30\textdegree and 120\textdegree , can also form stable streamlined cavities, when they are dropped from sufficient height above the pool surface, ranging from 2 to 4 m. The stable streamlined cavity is attached to the sphere surface just above the equator, instead of being wrapped completely around it. This sphere-with-attached-cavity exhibits slightly narrower shape, but retains the near-zero drag and the free-fall velocity is in compliance with the Bernoulli Law of potential flow$^{\mathrm{2}}$. $^{\mathrm{1}}$Vakarelski et al., \textit{Science Advances, 3: e1701558 (2017).} $^{\mathrm{2}}$Vakarelski*, Jetly {\&} Thoroddsen, \textit{Soft Matter}, \textit{10.1039/C9SM01025D (2019).}