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.
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.
Whatever floats your boat
Toy boats at the top and bottom surfaces of a levitated liquid (illustrated) are partially submerged in liquid, and so experience the upward force of buoyancy. That force counteracts the downward pull of gravity, allowing the toys to float on both sides of the liquid’s surface.
Buoyancy explains how a top-down boat floats on levitated liquid
Source: B. Apffel et al/Nature 2020
“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.