Complex morphology on the underside of a Leidenfrost-levitated hydrogel sphere.

When a liquid droplet approaches a hot surface, vaporization can become sufficient to cause the drop to levitate—this is the Leidenfrost effect. Vaporizable soft solids, e.g., hydrogels, can also exhibit levitation or, additionally, a sustained bouncing effect. In the case of floating liquids, vapor pressure and surface tension balance create an inversion of curvature on the droplet underbelly. Naively, one might expect that with a levitating soft solid, vapor pressure and elasticity should create a similar equilibrium with a similar curvature inversion. We use high-speed interferometric imaging to measure the 2D height profile underneath a floating hydrogel sphere and discover that the presence of such curvature inversion critically depends on the approach speed and total duration in the levitation state. Poking the system from a variety of experimental angles, we find that this curious behavior is due to permanent morphological changes caused by mass loss during vaporization.