Toy boats float upside down underneath a layer of levitated liquid

The upward force of buoyancy keeps objects afloat even in unusual conditions

toy boats floating right side up and upside down

Creating an antigravity effect in the lab allows objects like toy boats to float upside down on the bottom of a layer of levitating liquid.

B. Apffel et al/Nature 2020

Going bottom-up is no problem for a boat on the underside of a levitated liquid.

In a container, liquid can be levitated over a layer of gas by shaking the container up and down because the repeated, upward jerking motion keeps fluid from dripping into the air below. Lab experiments have revealed a curious consequence of this antigravity effect. Objects can float along the bottom of a hovering liquid as well as along the top, researchers report in the Sept. 3 Nature.

Physicist Emmanuel Fort and colleagues observed this effect by injecting a layer of gas underneath either silicone oil or glycerol and shaking the container. The researchers used these thick substances because keeping a liquid aloft requires vigorous shaking — with larger pools requiring stronger vibrations — and a runny fluid like water would slosh around too much to form a stable, levitated layer.

“Technically, you could have a liquid pool of any size stay up, just by shaking it enough,” says Fort, of the École Supérieure de Physique et de Chimie Industrielles in Paris. “If you’d like to have a swimming pool upside down, it’s possible … but you’d need to have a very viscous fluid.” Here, Fort’s team stuck to levitating up to half a liter.

Physicists knew it was possible to keep a layer of liquid levitated over a cushion of air by vigorously shaking the layers up and down in a container. But new lab experiments have revealed a surprising effect of that antigravity trick. Toy boats and other objects are able to float along the bottom surface of a levitated liquid as well as its top.

Toy boats bobbed along the bottom of the hovering liquid because, like boats floating right-side up along the top, the toys were partially submerged. Any object submerged in a liquid experiences a skyward, buoyant force, whose strength depends on the amount of space the object takes up in the liquid. That physical law, discovered by the ancient Greek mathematician and inventor Archimedes, is why dense objects sink and less dense ones float. Diving rings sit at the bottom of a pool because they have a lot of mass but don’t take up much space, so the force of gravity beats buoyancy. A beach ball, on the other hand, has very little mass but takes up a lot of space, so if it were placed at the bottom of a pool, it would bob to the surface.

A partially submerged, upside-down boat experiences the same upward pull. As a result, if the right amount of the boat is submerged, the force of buoyancy is strong enough to counteract gravity pulling the boat down, and the boat floats. Bet Archimedes didn’t see that coming.    

“I was very surprised” to see the effect, says Vladislav Sorokin, an engineer at the University of Auckland in New Zealand, who coauthored a commentary on the study that appears in the same issue of Nature. Sorokin has studied another paradoxical phenomenon in vibrating liquids — the fact that bubbles sink to the bottom of the liquid rather than rising to the top. The discovery of this new phenomenon hints that perhaps other counterintuitive effects are yet to be discovered in vibrating systems, he says.

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