为什么花生掉进啤酒里会跳舞

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

Scientists investigated this process in a study involving the alcoholic drink beer. It appeared recently in the publication Royal Society Open Science. The scientists say the research helps them understand mineral extraction or bubbling magma in the layer of Earth called the crust.

Brazilian researcher Luiz Pereira is the study's lead writer. He told the French news agency AFP that he first had the idea when passing through Argentina's capital Buenos Aires to learn Spanish.

It was a "bartender thing" in the city to take a few peanuts and drop them into beers, Pereira said.

Because the peanuts are more dense than the beer, they first sink 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. The bubbles act as flotation devices that carry the peanuts upward.

"The bubbles prefer to form on the peanuts rather than on the glass...," said Pereira, a researcher at Germany's Ludwig Maximilian University of Munich.

When the bubbles reach the surface, they burst.

The peanuts sink again before newly formed bubbles send the peanuts up again. Like a dance movement, the peanuts continue sinking and floating until the carbon dioxide runs out, or someone drinks the beer.

"Beer-gas-peanut system"

In the experiments, the team of researchers in Germany, Britain and France examined how peanuts acted in what they called the "beer-gas-peanut system."

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

But it cannot grow too much — less than 1.3 millimeters across is best, the study said.

Pereira said he hoped that "by deeply researching this simple system, which everyone can grasp, we can understand a system" that would be useful for industry or explaining natural processes.

For example, he said the flotation process was similar to the one used to separate iron from ore.

Air is injected 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," he said.

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

Like peanuts, magnetite is more dense and should sit at the bottom. But the researchers suggest, a high contact angle causes gas bubbles that carry the mineral up through the magma.

Science is never settled, especially when beer is involved.

So, Pereira said the scientists hope to create a better model of the dancing peanut action by continuing to experiment with "different peanuts and different beers."

I'm Dorothy Gundy.

Pierre Celerier wrote this story for Agence France-Presse. Hai Do adapted the story for Learning English.

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bubble - v. to form small ball of air or gas inside a liquid

magma - n. hot liquid rock below the surface of the earth

bartender - n. the person who serves drinks at eating and drinking places

prefer - v. like better than someone or something else

curve - n. smooth, round line

grasp - n. an understanding of something