Mark Megahan
If you want to impress a date, here’s one of those “go ahead and try it at home” science experiments involving nothing more dangerous than beer and a few peanuts. You can show your new friend how to make them dance. While you’re having fun playing with your food, you can also show off your knowledge of geophysics.
Magically dancing peanuts
The peanuts dancing in beer trick is nothing new in South America but most North Americans never heard of it. There’s never anything good on TV in Argentina so, long ago, bartenders learned a way of entertaining visiting gringos. Well, they know quite a few ways of doing that but this one happens to be legal. It also involves only two things and both are commonly at hand in any neighborhood bar.
It turns out that geophysicists go spastic over the science behind how it works. Some Argentinian bartender solved a scientific mystery of how magnetite forms on the surface of magma. It has interesting applications in other ways too, including nuclear waste disposal.
Dr. Luiz Pereira of the Ludwig Maximilian University of Munich, Germany and his colleagues did an in-depth analysis of exactly what happens when you drop peanuts in beer to find out why they dance, with all sorts of fancy graphs and charts of the data they collected. In the classic experiment, your typical Argentinian bartender would casually place a beer in front of a new “patron.” Then, just as casually, drop a peanut in. It instantly sinks. That’s because it happens to be “denser than the liquid around it.”
After a short while, “the peanut starts to rise, floating to the beer’s surface.” It doesn’t stay there long before it starts dancing. The trick won’t work with cashews, almonds or any other kind of nut. You can drop things in your beer all night and they won’t float back up and start dancing. The magically special thing about a typical shelled, roasted peanut is it’s shape.
In the lab, for scientific accuracy, Pereira and his team measured and weighed everything they used. They “dropped whole, shelled, roasted peanuts into a 1-liter tank of lager-style beer.” That’s about a quart here in America. Then, they “recorded the peanuts’ motions using a camera.”
Videos clearly show “that as soon as a peanut entered the liquid, gas bubbles started attaching to its surface.” That’s just the beginning. “These bubbles continued to accumulate after the peanut had sunk until there were enough to float the peanut to the surface of the beer.”
What makes it dance
Once the peanut floats to the surface is when things get really interesting. It will begin “to rotate around its long axis, causing the bubbles to burst and their buoyant air to dissipate. Once a large enough fraction of the bubbles had burst, the peanut lost its buoyancy, sinking again.” In a quart of Munich house lager, “this process took as little as a few seconds and repeated for an average of 150 minutes until the peanut sinks for good.”
Part of the reason is because bubbles of carbonation really like to stick to peanuts. “This favorability comes from the relatively high contact angle the surface of the peanut provides for bubble nucleation.” It’s exactly round enough in it’s curved surface, which also happens to be longer than wide. The length is what makes it dance. A broken peanut won’t do that.
What they found out is useful outside both the lab and the bar. According to Michael Manga, a planetary scientist at the University of California, Berkeley, “the motion of the beer-dancing peanuts could aid in understanding gas-induced processes in other systems, such as in Earth’s crust, that are difficult and dangerous to probe experimentally.” You are a lot more familiar with magnetite than you realize. It’s in everything from fridge magnets to recording tape to the strip on credit cards. It has a tendency to be found floating on the top of cooled magma. It’s not supposed to do that. Like a peanut, it’s denser than the surrounding liquid and should be found at the bottom. For generations, aerodynamic experts were baffled by the bumble bee, once considered too heavy to fly. The secret there is extra lift from the figure-eight pattern a bee beats his wings in. Just like the extra lift of bubbles in beer when attaching to a peanut, magnetite picks up gas bubbles and it floats to the top.
Now that they discovered the basic principles behind the effect, “Pereira says that he and his colleagues are now testing how different beer types and different peanut styles impact how a peanut dances.” The type of beer you use matters. “For example, Pereira says that in a highly carbonated beer, such as a craft beer, the peanuts stay bobbing at the surface for longer than they do in a less carbonated beer, such as the lager they used for their experiments.” Also, they note, “broken peanuts seemingly float forever, as they rotate less easily and so can maintain their bubble-flotation aids.“
