Explanation for why peanuts dance in beer may be used to study volcanoes

The quirky dance observed in a mug of beer holds the secrets to a volcanic phenomenon.
Loukia Papadopoulos
Beer and peanuts.jpg
Beer and peanuts.


Scientists have explained why peanuts dance when dropped in a glass of beer and now hope this revelation can lead to a better understanding of volcanoes.

This is according to a report by Phys.org published on Wednesday.

Brazilian researcher Luiz Pereira, the study's lead author, revealed the idea came to him in a trip to Argentina's capital Buenos Aires he took to learn Spanish.

Bartenders in the cosmopolitan city would take a few peanuts and drop them into beers, Pereira said.

The peanuts would first sink down to the bottom of the glass but would then quickly become a "nucleation site” and rise and dance at the top of the glass. 

This phenomenon happened because hundreds of tiny bubbles of carbon dioxide would form on the peanuts’ surface, acting as buoys to drag them upwards.

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

Once the peanuts reach the surface, the bubbles burst leading the food to dive down again before being propelled up once more by freshly formed bubbles.

This creates an entertaining dance of peanuts rising to the surface and heading back again down south.

Pereira decided to lead a series of experiments that examined how roasted, shelled peanuts behaved in a lager-style beer and found two conclusions.

The first was 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.

The second was that despite having the space, the angle cannot grow too much, limiting itself to a radius of under 1.3 millimeters.

What does this mean for volcanoes?

Pereira told Phys.org that "by deeply researching this simple system, which everyone can grasp, we can understand a system" that could explain natural phenomena such as volcanoes.

In the past, volcanologists have found that the mineral magnetite rises to higher layers in the crystallized magma of the Earth's crust than would be expected.

In this case the magnetite is like the peanuts (denser). Logic would dictate that it has to stay at the bottom but due to a high contact angle the mineral rises through the magma with help from gas bubbles.

Pereira added that with the help of his researchers he will continue to "play with the characteristics of different peanuts and different beers" to draw more conclusions about volcanoes and other unexplained phenomena.

 The study is published in the journal Royal Society Open Science.

Study abstract:

In Argentina, some people add peanuts to their beer. Once immersed, the peanuts initially sink part way down into the beer before bubbles nucleate and grow on the peanut surfaces and remain attached. The peanuts move up and down within the beer glass in many repeating cycles. In this work, we propose a physical description of this dancing peanuts spectacle. We break down the problem into component physical phenomena, providing empirical constraint of each: (i) heterogeneous bubble nucleation occurs on peanut surfaces and this is energetically preferential to nucleation on the beer glass surfaces; (ii) peanuts enshrouded in attached bubbles are positively buoyant in beer above a critical attached gas volume; (iii) at the beer top surface, bubbles detach and pop, facilitated by peanut rotations and rearrangements; (iv) peanuts containing fewer bubbles are then negatively buoyant in beer and sink; and (v) the process repeats so long as the beer remains sufficiently supersaturated in the gas phase for continued nucleation. We used laboratory experiments and calculations to support this description, including constraint of the densities and wetting properties of the beer–gas–peanut system. We draw analogies between this peanut dance cyclicity and industrial and natural processes of wide interest, ultimately concluding that this bar-side phenomenon can be a vehicle for understanding more complex, applied systems of general interest and utility.