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Whatever floats your boat? Scientists defy gravity with levitating liquid [theguardian.com]:

Scientists have turned the world upside down with a curious quirk of physics that allowed them to float toy boats the wrong way up beneath a levitating body of liquid.

In a striking demonstration of the mind-bending effect, the boats seem to defy the laws of gravity as they bob about on the water above them with their sails pointing down.

The bizarre phenomenon makes for a nifty trick, but researchers say the finding may have practical implications, from mineral processing to separating waste and pollutants from water and other liquids.

“We were playing around,” said Emmanuel Fort, a researcher on the team that discovered the effect at the Higher School of Industrial Physics [theguardian.com] and Chemistry in Paris. “We had no idea it would work.”

The scientists made their finding while studying the curious impact vibrations can have on the behaviour of liquids. Researchers already knew that, given the right kind of vibrations, bubbles can plunge downwards in liquids, while heavy particles that would normally settle out float to the surface instead.

Another strange effect of vibrations allows a layer of liquid to float in air, provided it is in a closed container. The explanation lies in the ability of vibrations to stabilise what are otherwise unstable systems.

If a container holding a runny liquid is swiftly flipped over, the liquid will fall to the bottom. The liquid does not fall in one go, however. Instead, droplets first form on the underside of the liquid, which instigate the rest of the liquid’s collapse.

But set the container on a vibrating plate and the liquid can behave very differently. Vertical shaking at the right frequency prevents the droplets forming. Without them, the liquid remains aloft: instead of falling, it rests on the cushion of air.

This much was already known. What Fort and his colleagues showed was that objects could experience an “antigravity” effect and float in the underside of the levitated liquid. Writing in Nature [nature.com], they demonstrate this with small toy boats floating in levitating layers of glycerol and silicon oil.

A number of forces keep the boats in place. The first comes from air pressure, which is raised by the weight of the liquid above, and pushes the boat up into the water. But the liquid pushes back on the boat itself, a force that decreases higher up the layer. Gravity also pulls the boat down. It all makes for a delicate balance that could fall apart at any moment, were it not for the vibrations.

“If you move the boat down in the air it will fall, and if you move it up, it will go up to the interface. The whole trick is not only to make the situation possible in the first place, but to stabilise the equilibrium,” Fort said.

The researchers believe the work opens up new avenues for study, and suspect the effect might be useful for industrial processes, such as removing plastic particles from liquids. But for now, the scientists are simply enjoying the weird effect.

“The fun 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 floats upside down. It’s counterintuitive. It gets people talking about science fiction and fantasy and that is very nice.”

Original Submission