A viral video of gel beads bouncing and squealing in a hot frying pan has led to the discovery that hydrogels slapping against a hot surface can generate substantial kinetic energy. This could one day give a power boost to bots made of such materials.
Scott Waitukaitis at the University of Leiden in the Netherlands was inspired by a YouTube video showing balls of water-saturated polyacrylimide gel shrieking when they hit the skillet’s hot surface, and leaping up and down as if the pan were a trampoline.
To find out why this was happening, Waitukaitis and his colleagues soaked hydrogel beads, often used in flower arranging, until they were about 99 per cent water by weight and dropped them onto pans heated to 215°C from heights ranging from a few hundred micrometres to 20 centimetres.
If the balls were dropped from less than 4 centimetres, they seemed to defy gravity by bouncing higher and higher with every leap until they reached that height, where they bounced stably for several minutes. Dropped from a greater altitude, they lost height with each bounce until they reached that sweet spot of 4 centimetres.
Snapshot of steam
To find out what was happening, the team focused a high-speed camera on the interface between the ball and the pan. “We saw a gap under the ball that opened and closed 2000-3000 times per second, like the piston of an engine,” says Waitukatis.
Every time a ball hit the pan, in the fraction of a second it was in contact, steam built up under the elastic surface until the gel was forced to curve up and release the vapor. This is similar to the Leidenfrost effect, in which steam cushions form under water droplets on a hot pan, causing them to skate around like hovercrafts.
Once the steam was released from under the gel, the material’s elasticity forced the bottom to rebound back onto the hot plate, and the process repeated, about 10-15 times for each bounce. These “micro-slaps” occur because water in the gel vaporises on contact and presses back up on the ball, generating kinetic energy that sends the balls leaping to surprising heights over and over again.
The frequency of the slapping matched the frequency of the shriek, suggesting the ball is acting like a speaker, vibrating the air to make sound. The energy pumped into the ball eventually helped propel it back up into the air. Four centimetres was the height at which the energy provided by the hot plate during each bounce balanced the energy lost on impact, at least for a few minutes. At different temperatures and with different formulations for the gel, that sweet spot would be different.
Hydrogel beads are used for more than flower arrangements. Among other things, they can be found in disposable nappies, contact lenses and soft-bodied robots.
In these robots, the hydrogels allow claws to open and close or for the whole thing to move when, say, water is pumped through tubes that run through them or an electric current is applied.
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Hydrogel robots move slowly, but this discovery could mean we can develop ones with high power densities, says Xuanhe Zhao of MIT’s Soft Active Materials Laboratory, which has built a hydrogel bot that can capture and release live fish in a tank.
A heat source could be used to make a hydrogel robot travel, maybe with the gel flapping against a surface, says Michael Dickey at North Carolina State University. But it might screech while it moved.
Journal reference: Nature Physics, DOI: 10.1038/nphys4194