Leidenfrost effect and wettability

The Leidenfrost effect is a case of thin-film boiling where a drop of liquid levitates on a surface heated to temperatures significantly higher than the liquid’s boiling point. When the drop contacts this superheated surface, a thin film of vapor (typically around 100 microns) forms instantaneously between the surface and the drop. The vapor layer supports the weight of the drop and thermally shields it from immediate evaporation. Due to the absence of direct contact between the drop and the surface, the Leidenfrost effect represents the case of a perfectly hydrophobic surface. In this chapter, we discuss the effect of surface wettability on the onset of this thin-film boiling state. We discuss passive methods, such as surface texturing, and active methods, such as using external fields to alter and control the transition to the Leidenfrost effect. The absence of a contact line provides extremely high mobility to these levitating drops and virtually eliminates friction. We discuss how this reduced friction can, in one case, reduce viscous drag on solid objects and, in another case, by introducing an asymmetry in the vapor flow, induce self-propulsion of levitating drops.