A team of researchers at the University of Leiden in the Netherlands led by Scott Waitukaitis, was inspired by a YouTube video showing balls of water-saturated polyacrylimide gel shrieking when they hit a pan's heated surface, and bouncing up and down.
Water droplets float in a hot pan because of the so-called Leidenfrost effect. Physicists from Leiden University and AMOLF have discovered a variation: the so-called Elastic Leidenfrost effect. It explains why the hydrogel balls jump around in a frying pan making high pitched sounds.
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, nothing is known about the effect with materials whose stiffness lies between these extremes. Here the physicists introduce a new phenomenon that occurs with vaporizable soft solids—the elastic Leidenfrost effect. By dropping hydrogel spheres onto hot surfaces they found that, rather than hovering, they energetically bounce several times their diameter for minutes at a time. With high-speed video during a single impact, they uncover high-frequency microscopic gap dynamics at the sphere/substrate interface. The team show how these otherwise-hidden agitations constitute work cycles that harvest mechanical energy from the vapour and sustain the bouncing. These findings suggest a new strategy 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.
Source and top image: Leiden Institute of Physics, Nature Physics