What mechanism explains the effect of a hot surface on hydrogel balls?

Question

Adding little hydrogel balls to a hot surface has an interesting effect as seen in this video. Hydrogels (as the name suggests) contain a lot of water but rather than melt or burst, the little balls are seen to bounce up-and-down on the hot surface making a high-frequency squeaking noise.

My thought on this is that it is related to the Leidenfrost effect. At the hot surface, the membrane is quickly heated and perhaps some small amount of vapor escapes causing the balls to bounce. However, I don't think this would have enough force to have them bounce so high. Perhaps the elasticity of the hydrogel plays a part.

Does anyone have an alternative idea?

Answer 1

Yes, the Leidenfrost effect plays a big part here. As the hydrogel balls aren't perfectly elastic (their restitution coefficient is smaller than one) they would quickly come to a halt but little jets of steam that are ejected by them when they hit the hot surface constantly provide extra upward force to compensate for inelastic losses. It's what keeps them bouncing.

The high pitch of the sound is related to the small size of the hydrogen balls.