The elastic Leidenfrost effect: An interplay between vaporization rate, gas flow rate, and shape.

When a liquid droplet comes near 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 floating liquid case, vapor pressure and surface tension balance create an inversion of curvature on the droplet underbelly. Naively, one might expect that in the case of a floating soft solid, vapor pressure and elasticity would create a similar equilibrium with a similar curvature inversion, and indeed theoretical work predicts this is the case. We use high-speed interferometric imaging to measure the 2D height profile underneath a floating hydrogel sphere, and to our surprise, we find that there is no curvature inversion. Instead, a downward-facing conical shape is created underneath the sphere. We speculate that the interplay between vaporization rate, gas flow rate, and shape is essential for this behavior.