Beer Dancing Peanuts Explained

Physics 16, 121

The buildup and bursting of carbon dioxide bubbles can cause a peanut to float and sink repeatedly in a glass of beer. The process can help understand phenomena in terrestrial magmas.

M. Schmit/Ludwig Maximilian University

Bubbles build up on the surface of a peanut when it’s dropped into a glass of carbonated beer. The bubbles increase the buoyancy of the peanut, allowing it to float even if its density is greater than that of the liquid.

In Argentina, it is common for a bartender to entertain a patron with a “dancing” peanut. Placing a beer in front of a customer, the bartender drops a peanut into the fizzy liquid. The peanut sinks because the legume is denser than the liquid that surrounds it. But then the peanut starts to rise, floating to the surface of the beer. The peanut then sinks and rises again, continuing in this cycle until the customer gets bored and drinks the beer. Researchers now have a physical description of this fun bar trick, which has similarities to processes occurring in terrestrial magmas [1].

For their experiments, Luiz Pereira of the Ludwig Maximilian University of Munich and colleagues dropped whole, shelled and roasted peanuts into a 1-liter tank of lager. They then recorded the peanuts’ movements using a camera.

The videos show that as soon as a peanut entered the liquid, gas bubbles began to stick to its surface. These bubbles continued to build up after the peanut had sunk until there were enough of them to float the peanut to the surface of the beer. As the peanut reached the surface, it began to rotate around its long axis, causing the bubbles to burst and dissipate their buoyant air. Once a large enough fraction of the bubbles burst, the peanut lost its buoyancy, sinking again. In a liter of beer, this process took only seconds and was repeated for an average of 150 minutes until the peanut sinks forever.

The researchers modeled the process, finding that it is energetically favorable for the bubbles to nuclearize on the surface of the peanut rather than on the glass or in the liquid. This preference results from the relatively high contact angle that the peanut surface provides for bubble nucleation.

Michael Manga, a planetary scientist at the University of California, Berkeley, says the motion of dancing peanuts in beer could help understand gas-induced processes in other systems, such as in the Earth’s crust, that are difficult and dangerous to probe experimentally. . The surfaces of liquid magma regions of the Earth’s crust are often decorated with magnetite, the crystallized mineral form of magma. As a solid mineral, magnetite is denser than liquid magma, so why does it float? The primary explanation for magnetite’s buoyancy is that it comes from gas bubbles in the magma that stick to the mineral’s surface, causing it to rise.

Once it has risen, the magnetite crystals do not sink, as is the case with peanuts. But Manga – and Pereira – think the beer and peanuts could allow researchers to take a safe and close look at how the bubble nucleation and outgassing processes in magma might work. The process of nucleating bubbles on an object’s surface is something scientists don’t fully understand, says Manga. “[Bubble nucleation] it’s a phase transition where you go from one state to another, and all phase transitions involve non-trivial physics,” he says.

Pereira says he and his colleagues are now testing how different types of beer and different styles of peanuts affect how well a peanut dances. For example, Pereira claims that in a highly carbonated beer, such as a craft beer, the peanuts stay on the surface longer than they do in a less carbonated beer, such as the lager they used for their experiments. This is because the bubbles continue to accumulate on the peanut when it is on the surface and do so almost as fast as they disappear. Meanwhile, broken peanuts appear to float forever, as they spin less easily and thus may retain their bubble buoyancy aids.

Allison Gasparini

Allison Gasparini is a freelance science writer based in Santa Cruz, California.

References

  1. L. Pereira et al.The physics of dancing peanuts in beer Soc.R. Open ski. 10 (2023).

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