Jumping and bouncing of hydrogel drops on a heated plate

The contact between a liquid drop and a solid surface is ubiquitous. Significant efforts have been invested in understanding how liquid drops move along, bounce from, or roll off different types of surfaces. On super-hot surfaces, drops of liquid hover over a layer of their over vapor, due to rapid evaporation – this is known as the Leidenfrost effect. Interestingly, when the drop possesses some elasticity by exchanging it for a hydrogel, the drop no longer hovers, but it instead bounces. Using high speed video microscopy, we demonstrate that hydrogel balls, initially at rest, jump upon rapidly heating the surface. Jumping is governed by both the surface as well as the hydrogel properties. After jumping, the hydrogel can continue to bounce for extended periods of time. Our results illustrate how the interplay between solid and liquid characteristics of hydrogels can result in intriguing dynamics.