Stable, streamlined and helical cavity formation by the impact of Leidenfrost spheres

This work reports results from an experimental study on the formation of stable-streamlined and helical cavity wakes following the free-surface impact of Leidenfrost spheres. The Leidenfrost effect encapsulates the sphere by a vapor layer to prevent any physical contact with the surrounding liquid. This phenomenon is essential for the pacification of acoustic rippling along the cavity interface to result in a stable-streamlined cavity wake. Such a streamlined configuration experiences drag coefficients an order of magnitude lower than those acting on room temperature spheres. A striking observation is the formation of helical cavities which occur for impact Reynolds numbers ${Re}_{0}\ge 1.4\times {10}^{5}$ and are characterized by multiple interfacial ridges, stemming from and rotating synchronously about an evident contact line around the sphere equator. This helical configuration has $40-55\thinspace \% $ smaller overall force coefficients than those obtained in the formation of stable cavity wakes.