Watch a toy boat float upside down in a levitating puddle

By Leah Crane

Benjamin Apffel et al., Nature.

Shaking a liquid fast enough allows it to levitate, and a toy boat can float on top of the levitating fluid – or upside down beneath it.

When a viscous fluid like silicone oil is shaken up and down around 100 times a second, resulting pressure waves can cause air bubbles in the fluid to pulsate, wobble and sink. If the bubbles are big enough, this can lead to a layer of air beneath the fluid, making a sort of strange floating pond.

When Emmanuel Fort at the Langevin Institute in Paris and his colleagues poured beads into one of these floating ponds, they found that rather than falling straight through the liquid and the air below it to the bottom of the vibrating container, some beads seemed to “float” at the bottom of the liquid.

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“We were playing with the experiment,” says Fort. “We had this liquid layer and some beads, and we were surprised to see the beads floating on the lower interface. At first, it was not meant to be applied to anything practical, we were just amazed by the system and how counter-intuitive it was.”

They found that the shaking of the container stabilises the bottom of the liquid, vibrating any droplets that might start to form back into the bulk of the puddle. This also creates a stable point for floating objects at the bottom of the liquid: the researchers floated small toy boats on both the top and the bottom.

Their container levitated about half a litre of silicone oil or glycerol, but a bigger shaker could in theory make just about any amount of liquid levitate, says Fort. “There is no size limit as long as the liquid is viscous enough, so if you wanted to swim on the bottom of a levitating liquid layer you would be swimming through something more viscous like honey, which would be entertaining to watch,” he says.

More practically, he says that this method of levitating liquid and floating objects beneath it could be used for processes that involve sorting and transporting solid objects in fluids, like some kinds of mining or waste-water treatment.

Journal reference: Nature, DOI: 10.1038/s41586-020-2643-8