Scientists explain why peanuts 'dance' when dropped in beer

When peanuts are dropped into a pint of beer, they initially sink to the bottom before floating up and "dancing" in the glass.

"Beer–gas–peanut system"

Scientists have dug deep seeking to investigate this phenomenon in a new study published recently, saying it has implications for understanding mineral extraction or bubbling magma in the Earth's crust.

Because the peanuts are denser than the beer, they first sink down to the bottom of the glass.

Then each peanut becomes what is called a "nucleation site". Hundreds of tiny bubbles of carbon dioxide form on their surface, acting as buoys to drag them upwards.

The bubbles prefer to form on the peanuts rather than on the glass walls.

When the bubbles reach the surface, they burst.

The peanuts then dive down before being propelled up again by freshly formed bubbles, in a dance that continues until the carbon dioxide runs out—or someone interrupts by drinking the beer.

In a series of experiments, the team of researchers in Germany, Britain and France examined how roasted, shelled peanuts fared in a lager-style beer.

They found that the larger the "contact angle" between the curve of an individual bubble and the surface of the peanut was, the more likely it was to form and grow.

But it cannot grow too much—a radius of under 1.3 millimeters is ideal, the study said.

The floatation process was similar to the one used to separate iron from ore. Air is injected, in a controlled way, into a mixture in which a mineral—such as iron—"will rise because bubbles attach themselves more easily to it, while other (minerals) sink to the bottom". 

The same process could also explain why volcanologists find that the mineral magnetite rises to higher layers in the crystallized magma of the Earth's crust than would be expected.

Like peanuts, magnetite is denser, so should sit at the bottom. But due to a high contact angle, the researchers theorize, the mineral rises through the magma with help from gas bubbles.

 Luiz Pereira et al, The physics of dancing peanuts in beer, Royal Society Open Science (2023). DOI: 10.1098/rsos.230376