Dr. Scott Waitukaitis
University of Leiden,
The Leidenfrost effect occurs when an object near a hot surface vaporizes rapidly enough to lift itself up and hover. Although well-understood for liquids and stiff sublimable solids, little is known about the interaction of vaporizable soft solids with superheated surfaces. In this talk, I will introduce a new phenomenon that occurs with vaporizable soft solids: the elastic Leidenfrost effect. By dropping vaporizable hydrogel spheres onto hot surfaces I will show that, rather than hovering, they energetically bounce several times their diameter for minutes at a time. With high-speed video during a single impact, one sees high-frequency microscopic gap dynamics at the sphere-substrate interface. Solving for the dynamics of a simplified system with a simple numerical model, I will reveal how these otherwise-hidden agitations constitute work cycles that harvest mechanical energy from the vapor and sustain the bouncing. Quite literally, the hydrogel sphere behaves like a soft engine, where nearly all of the components are embedded into a single object made from a single material. These findings suggest a novel strategy for injecting mechanical energy into soft materials, with potential relevance in fields ranging from soft robotics to active matter.