The impact of a solid sphere onto a liquid surface resulting in the formation of an air cavity exemplifies the classical water-entry problem relevant to several naval and military applications such as in air-to-sea ballistic missiles, slamming of ships and seaplane landings. In this video we show the formation of two new kinds of cavities, namely, stable 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. The absence of a contact line in such a non-wetting scenario, entrainment of an elongated cavity (from wall effects) and suppression of the downward facing jet at the cavity apex is shown to result in the formation of a stable-streamlined cavity (impact Reynolds numbers \(Re_0 \lesssim 1.4 \times 10^5\), Froude numbers \(Fr \gtrsim 120\)). For \(Re_0 \gtrsim 1.4 \times 10^5\), the cavity develops evident ridges or streaks which emanate from the sphere’s surface and follow random trajectories about its equator. The ridges become arranged at equidistant circumferential locations after initial cavity collapse and begin rotating synchronously to form a helical cavity wake.