The strange phenomenon is a clever trick, but the researchers say the discovery may have practical implications, from processing minerals to separating waste and pollutants from water and other liquids.
“We were playing,” said Emmanuel Fort, a researcher from the team that discovered the effect at the Paris School of Industrial Physics and Chemistry. “We had no idea it would work.”
Scientists made their discovery by studying the curious impact that vibrations can have on the behavior of liquids. Researchers already knew that with the right kind of vibrations, bubbles can dive downward into liquids, while heavy particles that settle normally float to the surface.
Another strange effect of vibrations allows a layer of liquid to float in the air, provided it is in a closed container. The explanation lies in the ability of vibrations to stabilize otherwise unstable systems.
If a container with flowing liquid is quickly inverted, the liquid will sink to the bottom. However, the liquid does not fall suddenly. Instead, droplets form on the underside of the liquid first, causing the rest of the liquid to collapse.
But put the container on a vibrating plate and the liquid can behave very differently. Vertical shaking at the correct frequency prevents the formation of droplets. Without them, the liquid stays in the air: instead of falling, it rests on the air cushion.
This was already known. What Fort and his colleagues showed was that objects could feel an “anti-gravity” effect and float under the levitating liquid. Writing in Nature, they demonstrate this with little toy boats floating in levitating layers of glycerol and silicone oil.
A number of forces hold the boats in place. The first comes from the pressure of the air, which is increased by the weight of the liquid above, and pushes the boat through the water. But the liquid pushes back the boat itself, a force that decreases higher up in the layer. Gravity also pulls the boat down. All of this makes for a delicate balance that could crumble at any time, minus the vibrations.
“If you move the ship through the air, it will fall, and if you move it up, it will go up to the interface. The trick is not just to make the situation possible in the first place, but to stabilize the balance, ”Fort said.
The researchers believe the work opens up new avenues for study and suspect the effect could be useful for industrial processes, such as removing plastic particles from liquids. But for now, scientists are just enjoying the strange effect.
“The funny thing is that it triggers reactions from people who aren’t scientific,” Fort said. “People say it’s like the scene in Pirates of the Caribbean when the boat is floating upside down. It’s counterintuitive. It gets people talking about sci-fi and fantasy and that’s great.