Soft lubrication in the elastic Leidenfrost effect*

When a hydrogel sphere is lowered onto a hot plate, its bottom begins to vaporize. The resulting vapor flow couples tightly to elastic deformations within the sphere. This results in two regimes of the so-called elastic Leidenfrost effect: steady-state floating or limit-cycle-like bouncing. Despite experimental evidence, a fundamental theory of these phenomena remains a challenge: How high above the hot plate does the sphere sit? What is the spatial profile of its underbelly? 

We explore an asymptotic theory of elastic Leidenfrost drops. Stiff sublimable solids support an increase in weight by sitting closer to the hot surface. We instead discover a super-squishy elastic limit, in which the underbelly of the sphere tends to a Hertzian contact profile. In this case, the larger the drop the higher it floats, due to increasing contact area. We capture this crossover in a single dimensionless parameter balancing vapor pressure against elastic stresses. We compare these theoretical predictions against simulations and experiments.  

We explore tailoring hydrogel motion using shape and connect Leidenfrost dynamics to the phenomena of soft lubrication underpinning, for example, the motion of human joints.