Coupling the Leidenfrost effect and elastic deformations to power sustained bouncing

The Leidenfrost effect occurs when an object near a hot surface vaporizes rapidly enough to lift itself up and hover(1,2). Although well understood for liquids(1-14) and stiff sublimable solids(15-18), nothing is known about the effect with materials whose stiff ness lies between these extremes. Here we introduce a newphenomenon that occurs with vaporizable soft solids-the elastic Leidenfrost effect. By dropping hydrogel spheres onto hot surfaces we find that, rather than hovering, they energetically bounce several times their diameter for minutes at a time. With high-speed video during a single impact, we uncover highfrequency microscopic gap dynamics at the sphere/substrate interface. We show how these otherwise-hidden agitations constitute work cycles that harvest mechanical energy from the vapour and sustain the bouncing. Our findings suggest a newstrategy for injecting mechanical energy into a widely used class of soft materials, with potential relevance to fields such as active matter, soft robotics and microfluidics.