Leidenfrost effect enables next-gen soft engines

But sometimes, research projects emerge from a sudden strike of curiosity. Physicist Scott Waitukaitis (Leiden University / AMOLF) stumbled across a YouTube video of bouncing hydrogel balls on a hot plate and was so inspired that he decided to write an NWO Veni grant proposal to explore the phenomenon.

The dance of water droplets in a frying pan is a well-known phenomenon called the Leidenfrost effect. The warm undersides of the droplets vaporise so quickly that they float around on a cushion of vapor. In striking contrast, hydrogel balls exhibit sustained bouncing motion.

What's more, they scream while they bounce. Why and how do they do this? Waitukaitis and group leader Martin van Hecke (Leiden University / AMOLF) discovered that an unknown effect underlies the bouncing and screaming: the elastic Leidenfrost effect, which they describe in a publication in Nature Physics.

Hydrogel balls are elastic spheres that are mostly (98 percent) water—there is no shell. When they touch the hot plate, a small amount of water vaporises, as happens with regular water droplets.

Using high-speed videography to scrutinise the vaporisation process at the interface between the sphere and the hot surface, Waitukaitis discovered that the released vapor interacts with the squishy sphere to create rapid oscillations in pressure and deformation.

The movement that follows injects energy into the sphere, leading to a sustained bouncing. "These oscillations happen rapidly, at a frequency of a few thousand cycles per second, causing the high-pitched sounds," says Waitukaitis.

This phenomenon may seem trivial, but Waitukaitis and colleagues contend that it brings to light a useful concept. "Effectively, the sphere acts like an engine in the way that it harnesses energy from the surface," he says. "What's incredible, however, is that all of the components of an engine, such as the fuel, the piston mechanism, and the mechanical output, are embedded in a single hydrogel ball."

Given that the mechanism explicitly requires the object to be squishy, the researchers call this a "soft engine." The idea could be useful in other fields, as well. "One could take advantage of this idea in soft robotics. You could, for example, run a set of wires through the arms of a hydrogel robot and heat them where you want movement."