High-speed measurements of Leidenfrost vapor layer oscillations and collapse

The thin vapor layer generated between a very hot surface and an evaporating liquid is subject to complex, hydrodynamic fluctuations. These fluctuations can excite spontaneous oscillations of Leidenfrost drops, and are the eventual progenitor of the collapse of the vapor layer and explosive boiling. Using a new electrical technique, we have characterized these fluctuations on sub-microsecond time scales. The vapor layer is treated as the dielectric of a capacitor where one conductor is a heated metallic surface and the other is a conducting liquid. The liquid is pumped with a high-frequency carrier signal so that the capacitive reactance of the vapor layer is low. The amplitude of the carrier wave is monitored in time. Collapse of the vapor layer is marked by a sharp increase in the real part of the complex electrical impedance. With this technique, coupled with high-speed video, we characterized numerous oscillations of Leidenfrost drops, including well-known star-shaped oscillations, as well and the eventual collapse the of vapor layer near a hot titanium finger immersed in a salt water bath.