‘Dancing’ raisins – a simple kitchen experiment shows how objects can draw energy from their environment and come to life

Scientific discoveries do not always require a high-tech laboratory or a large budget. Many people have a first-class laboratory in their own home: their kitchen.

The kitchen offers plenty of opportunities to view and discover what physicists call soft matter and complex liquids. Everyday phenomena, like Cheerios clustering in milk or rings left behind when coffee drops evaporate, have led to discoveries at the intersection of physics and chemistry and other tasty collaborations between food scientists and physicists.

Two students, Sam Christianson and Carsen Grote, and I published a new study in Nature Communications in May 2024 that addresses another kitchen observation. We investigated how objects can float in carbonated liquids, a phenomenon amusingly called dancing raisins.

The study examined how objects such as raisins can move rhythmically up and down in carbonated liquids for several minutes, even up to an hour.

An accompanying Twitter thread about our research went viral, receiving more than half a million views in just two days. Why did this particular experiment capture the imagination of so many?

Bubbling physics

Sparkling water and other carbonated drinks fizz with bubbles because they contain more gas than the liquid can hold; they are ‘supersaturated’ with gas. When you open a bottle of champagne or soda, the liquid pressure drops and CO₂ molecules begin to escape into the surrounding air.

Bubbles do not usually form spontaneously in a liquid. A liquid is made up of molecules that like to stick together, so molecules at the liquid boundary are a bit unhappy. This results in surface tension, a force that tries to reduce the surface area. Because bubbles increase the surface area, surface tension and fluid pressure normally push any forming bubbles right back out of existence.

But rough spots on the surface of a container, such as the etchings in some champagne glasses, can protect new bubbles from the crushing effects of surface tension, giving them a chance to form and grow.

Bubbles also form in the microscopic, tubular cloth fibers left behind after wiping a glass with a towel. The bubbles grow steadily on these tubes and once they are large enough, they break free and float to the top, transporting gas out of the container.

But as many champagne lovers who put fruit in their glasses know, surface etchings and small fabric fibers aren’t the only places where bubbles can form. Adding a small object such as a raisin or peanut to a fizzy drink also allows bubble growth. These submerged objects act as attractive new surfaces on which opportunistic molecules such as CO₂ can accumulate and form bubbles.

And once enough bubbles have grown on the object, a levitation action can be performed. Together, the bubbles can lift the object to the liquid surface. Once at the surface, the bubbles burst, causing the object to fall back down. The process then begins again, in a periodic vertical dance movement.

Dancing raisins

Raisins are particularly good dancers. It only takes a few seconds for enough bubbles to form on the wrinkled surface of a raisin before it begins to rise to the top. On smoother surfaces it is more difficult to form bubbles. When a raisin is dropped into newly opened sparkling water, it can dance a vigorous tango for twenty minutes, and then a slower waltz for another hour or so.

We found that rotation, or turning, was crucial for moving large objects to dance. Bubbles that stick to the bottom of an object can hold it up even after the top bubbles have burst. But if the object starts to spin even slightly, the bubbles underneath cause the body to spin even faster, causing even more bubbles to pop on the surface. And the sooner those bubbles are removed, the sooner the object can dance vertically again.

Small objects like raisins don’t rotate as much as larger objects, but instead do the twist and wiggle back and forth quickly.

Modeling the vibrant flamenco

In the article we developed a mathematical model to predict how many trips to the surface we would expect from an object like a raisin. In one experiment, we placed a 3D-printed sphere that acted as a model raisin in a freshly opened glass of sparkling water. The sphere traveled from the bottom of the container to the top more than 750 times in one hour.

The model included the rate of bubble growth as well as the shape, size and surface roughness of the object. How quickly the liquid loses carbonation was also taken into account based on the geometry of the container, and especially the flow created by all that fizzing activity.

Small objects covered with bubbles in carbonated water move up to the surface and back down.

Raisins with bubbles ‘dance’ to the surface and descend once their lifting agents are released. Saverio Spagnol

The mathematical model helped us determine which forces most influence the object’s dancing. For example, the fluid resistance on the object turned out to be relatively unimportant, but the ratio between the surface area of ​​the object and its volume was crucial.

Looking to the future, the model also offers a way to determine some difficult-to-measure quantities using easier-to-measure quantities. For example, by observing an object’s dance frequency, we can learn a lot about its surface at a microscopic level without having to see those details directly.

Different dances in different theaters

However, these results are not only interesting for carbonated drink enthusiasts. Supersaturated fluids also occur in nature – magma is an example of this.

As magma in a volcano moves closer to the Earth’s surface, the pressure drops rapidly, and dissolved gases from the volcano rush toward the exit, much like the CO₂ in carbonated water. These escaping gases can form into large high-pressure bubbles and come out with such force that a volcanic eruption occurs.

The particles in magma may not dance in the same way as raisins in soda water, but small objects in the magma can influence how these explosive events play out.

Other eruptions have also occurred in recent decades: thousands of scientific studies devoted to active matter in liquids. These studies look at things like swimming microorganisms and the inside of our fluid-filled cells.

Most of these active systems do not exist in water, but in more complicated biological fluids that contain the energy needed to produce activity. Microorganisms absorb nutrients from the fluid around them in order to continue swimming. Molecular motors transport charge along a superhighway in our cells by drawing nearby energy in the form of ATP from the environment.

Studying these systems can help scientists learn more about how the cells and bacteria in the human body function, and how life on this planet evolved to its current state.

Meanwhile, a fluid itself can behave strangely due to its diverse molecular composition and the bodies moving within it. Many new studies have focused on the behavior of microorganisms in fluids such as mucus, which behaves both as a viscous fluid and as an elastic gel. Scientists still have a lot to learn about these very complex systems.

Although raisins in soda water seem quite simple compared to microorganisms swimming through biological fluids, they provide an accessible way to study generic traits in those more challenging environments. In both cases, bodies draw energy from their complex fluid environment while also influencing it, and fascinating behavior emerges.

New insights into the physical world, from geophysics to biology, will emerge from bench-scale experiments – and perhaps straight from the kitchen.

This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Saverio Eric Spagnolie, University of Wisconsin-Madison

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Support for this research was provided by the Office of the Vice Chancellor for Research and Graduate Education with funding from the Wisconsin Alumni Research Foundation, and by donations to the AMEP (Applied Math, Engineering, and Physics) program at the University of Wisconsin- Madison.

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