Oscillatory Wetting under Drops Impacting on a Hot Plate

The Leidenfrost phenomenon, where an evaporating drop levitates above a layer of its vapour on sufficiently hot plates is well-known for gently deposited drops. For impacting drops, the additional impact pressure can cause much thinner vapour layers in the nanometer range, and conventional side or bottom view imaging is incapable of detecting substrate contact. Using frustrated total internal reflection (FTIR), three main regimes were distinguished: contact, nucleate boiling at low temperatures (drop spreads in contact with substrate), Leidenfrost (film) boiling without contact and a broad transition regime. Then, the outer parts of the spreading lamella levitate, while the central region of the drop touches the substrate.
However, the wetted locations and the drop's partially levitated bottom surface increasingly fluctuate with increasing temperature. Most striking are periodic waves travelling from the lamella tips toward the centre of the wetted region. We analyze and discuss this currently unresolved phenomenon.