Strongly coupled fluids and solids

In one avenue of our ongoing work, we study systems where soft elastic solids interact with fluids (liquids or gases). Our interest here started with the traditional Leidenfrost effect, which occurs when a liquid droplet is placed above a surface whose temperature is far above its boiling point. Counterintuitively, the droplet does not boil, but rather floats above the surface. This occurs because the droplet vaporizes rapidly enough on its underside to hover on its vapor.

We have discovered a new type of Leidenfrost that occurs with vaporizable soft solids, e.g. hydrogels (see article in Nature Physics). Rather than hovering, these materials spontaneously start bouncing on a hot surface. This effect is dramatic—a 1 cm hydrogel sphere easily bounces to 5 cm heights for several minutes. The mechanism comprises an unanticipated new way to extract mechanical energy, with the sphere acting as a soft engine. This has potential in fields like active matter and soft robotics, where robustly actuating and activating soft materials is notoriously challenging. Ongoing work includes improving gel synthesis to change the mechanical properties, using shape to control the dynamics, and further understanding the transition to the ‘regular’ Leidenfrost effect that occurs in the velocity-temperature plane.  To find out more about what we’ve already done, check out the papers in (Nature Physics, Physical Review Letters, Soft Matter), or read this nice news article by Ben Guarino in The Washington Post.

Publications

Microwave induced mechanical activation of hydrogel dimers
Hamza Khattak, Scott Waitukaitis, and Aaron Slepkov
Soft Matter 15, 5804-5809 (2019). 

From Bouncing to Floating:  The Leidenfrost Effect with Hydrogel Spheres 
Scott Waitukaitis and Martin van Hecke
Physical Review Letters 121, 048001 (2018).

Coupling the Leidenfrost effect and elastic deformations to power sustained bouncing
Scott Waitukaitis, Antal Zuiderwijk, Anton Souslov, Corentin Coulais and Martin van Hecke
Nature Physics 13, 1095-1099 (2017).