In Levitating Liquids, Boats Also Float Upside Down

Plastic boats floating above and below a levitating liquid layer. Credit: Benjamin Apffel et al., Nature.

One of the oldest known laws of physics is the Archimedes principle. It describes the upward buoyant force that a body receives when submerged in a fluid. Researchers have literally turned this principle upside down. And it still works.

As reported in Nature, if the fluid is made to levitate, boats can float on either surface, with the one on the lower surface doing so against gravity. The counter-intuitive scenario allows researchers to better test the properties of levitating liquids. They established that the ability to float on the lower surface depends on the mass of the floating object. Beyond a certain limit, they end up “sinking” into the air below.

The French researchers were curious about the behavior of liquid boundaries and so sought to understand the properties of objects in levitating liquids. To make the liquid levitate, the team used an approach known as vertical shaking. By inserting bubbles of air into glycerol or silicon oil and then shaking the container vertically it is possible to make a dense liquid float on the less dense air. It is also possible to create multiple layers of levitating liquid.

This method has been well-established and researchers have marveled at the peculiar phenomena they have witnessed. The most mind-boggling discovery so far was that bubbles of air in the suspended layer can sink instead of rising. The team took it one step further and showed that there is certainly a selective falling when it comes to heavy bodies. They don't just sink down to the bottom of the container. A force of inverse floating similar to the Archimedes principle acts on the bodies, even on the lower side.

In the experiment, the team used half a liter of liquid with a maximum width of 20 centimeters (7.9 inches). In this liquid objects floated on both surfaces if they were up to 7 grams and 2.5 centimeters (about 1 inch). The inverse-floating worked only when the system was made to oscillate tens of times per second.

Sound on for this one.

The team believes that the setup of the experiment is key to explaining the antigravity pull of the liquid on the little boat and the various masses tested in the study. In a lift, depending on if it's going up or down, you would experience an effective gravity that it is higher or lower than the gravitational pull of the Earth, depending on the direction of travel. A free-falling lift might make you feel like you are floating.  

The same forces are at play here. The rapid oscillation keeps the liquid floating. This produces a buoyancy pull instead of a push, an anti-gravity effect, that keeps objects floating upside-down instead of falling.

Other than a very cool video, the possible applications of this study might go beyond physics. Systems with high-frequencies excitations are found in biology and chemistry and counter-intuitive phenomena might be found there as well.

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