Delayed Leidenfrost States on Hydrophobic Surfaces

The Leidenfrost phenomenon refers to the state when a liquid droplet placed on a superheated surface starts levitating due to the build-up of vapor pressure underneath. Such a scenario can play a crucial role in droplet manipulation, spray cooling, and frictional drag reduction. This state manifests after the formation of a continuous vapor film beneath the droplet. In this context, rough hydrophobic surfaces are believed to accelerate vapor film formation due to their low adhesion energy and reduced liquid-solid contact area. Contrary to this logic, we have found that hydrophobic surfaces realized by carving arrays of doubly reentrant cavities (DRCs) actually significantly delay the emergence of the Leidenfrost phenomenon. High-speed imaging experiments reveal that water droplets remain pinned onto superheated silica surfaces with arrays of DRCs much longer than common hydrophobic surfaces where levitation is achieved immediately. The contributions of microstructure and surface chemistry on this counterintuitive phenomenon were investigated thoughtfully. These findings advance our notions of the Leidenfrost phenomenon and may open a door for applying hydrophobic surfaces to reduce fluid drags without compromising heat transfer.