‘Dancing’ raisins – a simple kitchen experiment shows how things can harness energy from their surroundings and come to life

Scientific discovery does not always require a high-tech laboratory or a big budget. Many people have a first class laboratory in their own homes – in their kitchen.

The kitchen offers plenty of opportunities to see and explore what physicists call soft matter and complex fluids. Everyday phenomena, like the cluster of Cheerios in milk or the rings left when a drop of coffee evaporates, are the result of 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 dives into another kitchen observation. We studied how things can levitate in carbonated liquids, a phenomenon whimsically called dancing raisins.

The study examined how things like raisins can move rhythmically up and down in a carbonated liquid for several minutes, even up to hours.

An accompanying Twitter thread about our research went viral, garnering over half a million views in just two days. Why did this particular experiment capture so much imagination?

Bubbling physics

Sparkling water and other carbonated drinks bubble because they contain more gas than the liquid can support – they are “supersaturated” with gas. When you open a bottle of champagne or soft drink, the fluid pressure drops and the CO₂ molecules begin to escape to the surrounding air.

Bubbles do not usually form spontaneously in liquids. A fluid is made up of molecules that like to stick together, so molecules at the boundary of the fluid are somewhat disorganized. This results in surface tension, a force that tends to reduce the surface area. Since bubbles add to the surface area, surface tension and fluid pressure force any formed bubbles back out of existence.

But rough patches on the surface of the container, like the etchings in some champagne glasses, can protect new bubbles from the crushing effects of surface tension, allowing them to form and grow.

Bubbles also form inside the microscopic, tubular cloth fibers that are left behind after wiping glass with a towel. The bubbles grow steadily on these tubes and, when they are large enough, detach and float upwards, carrying gas out of the container.

But as many champagne enthusiasts who put fruit in their glasses know, surface etching and tiny cloth fibers aren’t the only places where bubbles can form. Adding a little something like a raisin or peanut to a fizzy drink can also grow bubbles. These submerged objects act as new surfaces that attract opportunistic molecules such as CO₂ to accumulate and form bubbles.

With enough bubbles expanding on the object, an act of levitation may be performed. Together, the bubbles can lift the object up to the surface of the liquid. Once at the surface, pop the bubbles, drop the thing back down. The process then begins again, in a periodic vertical dance motion.

Dance raisins

Raisins are especially good dancers. It only takes a few seconds for enough bubbles to form on a wrinkled raisin surface before it starts to rise – bubbles have a harder time forming on smoother surfaces. When dropped into freshly opened sparkling water, a raisin can dance a lively tango for 20 minutes, then a slower waltz for another hour or so.

We found that rotation, or spinning, was key to getting large objects to dance. Bubbles that stick to the bottom of an object can keep it aloft even after the top bubbles pop. But if the object starts to spin even a little, the bubbles below the body cause it to spin even faster, causing more bubbles to come to the surface. And the sooner those bubbles are removed, the sooner the object can go back to its vertical dance.

Small objects like raisins do not rotate as much as larger objects, but instead do the spinning, spinning rapidly back and forth.

Modeling the bubbly flamenco

In the paper, we developed a mathematical model to predict how many trips to the surface we would expect for an object like a raisin. In one experiment, we placed a 3D-printed sphere that served as a resin model in a straight-open glass of sparkling water. The sphere traveled from the bottom of the container to the top over 750 times in one hour.

The bubble growth rate as well as the shape, size and surface roughness of the object were incorporated into the model. He also took into account how quickly the liquid loses carbonation based on the geometry of the container, and especially the flow created by all that bubbly action.

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

Bubble-coated raisins ‘dance’ to the surface and fall off when their agents are lifted up. Game Saverio Spagnolie

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

Looking to the future, the model also provides a way to measure some hard-to-measure quantities using easier-to-measure ones. For example, by observing the dancing frequency of an object, we can learn a lot about its surface at the microscopic level without seeing that detail directly.

Different dances in different theatres

These results are not only interesting for carbonated drink lovers, however. There are also supersaturated fluids in nature – magma is one example.

As magma in a volcano rises closer to the Earth’s surface, it quickly expels, and dissolved gases from inside the volcano make a break for the exit, just like the CO₂ in carbonated water. These escaping gases can form large high-pressure bubbles and develop to such an extent that a volcanic eruption occurs.

The particulate matter in magma may not dance the way raisins do in soda water, but small things in the magma can affect how these explosive events play out.

Over the past decades there has also been an eruption of another kind – thousands of scientific studies focused on active matter in fluids. These studies look at things like swimming microorganisms and the inside of our fluid-filled cells.

Most of these active systems are not in water but instead in more complex biological fluids that contain the energy necessary to produce activity. Microorganisms absorb nutrients from the fluid around them to keep swimming. Molecular motors transport cargo along a highway 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 different molecular compositions and bodies moving around within it. Many new studies have addressed the behavior of microorganisms in fluids such as mucus, for example, which behaves like a viscous fluid and an elastic gel. Scientists still have a lot to learn about these very complex systems.

Although raisins in soda water seem relatively simple compared to microorganisms swimming through biological fluids, they provide an accessible way to study generic features in those more challenging settings. In both cases, bodies extract energy from their complex fluid environment while also disrupting it, and interesting behaviors occur.

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

This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. Written by: Saverio Eric Spagnolie, University of Wisconsin-Madison

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

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