'Dancing' raisins – a simple kitchen experiment shows how objects can extract energy from their environment and come to life

Scientific discovery doesn't always require a high-tech laboratory or a huge budget. Many people have a first-class laboratory right in their own homes – in their kitchen.

The kitchen offers lots of opportunities to observe and explore what physicists call soft matter and complex fluids. Everyday phenomena, such as the coagulation of Cheerios in milk or the rings left when coffee drops evaporate, have led to discoveries at the intersection of physics and chemistry and other delicious collaborations between food scientists and physicists.

Two students, Sam Christianson and Carson Grote, and I published a new study in Nature Communications in May 2024 that dives into another kitchen observation. We studied how objects can fly in carbonated liquids, a phenomenon known as dancing raisins.

The study explored how objects such as raisins can rise and fall rhythmically in carbonated liquids for several minutes, even up to an hour.

A Twitter thread about our research went viral, receiving over half a million views in just two days. Why did this particular experiment catch the attention of so many people?

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 soft drink, the pressure of the liquid drops and CO₂ molecules begin to escape into the surrounding air.

Normally bubbles do not form on their own in any liquid. A liquid is made up of molecules that like to stick together, so the molecules at the liquid boundary are slightly misaligned. This results in surface tension, a force that seeks to reduce surface area. Since bubbles add surface area, surface tension and fluid pressure generally force any forming bubbles out of existence.

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

Bubbles also form inside the microscopic, tube-like cloth fibers left after wiping the glass with a towel. The bubbles on these tubes continuously grow and, once they become large enough, break apart and float to the top, carrying the gas out of the container.

But as many Champagne lovers pour fruit into their glasses know, surface scratches and tiny fibers in the fabric aren't the only places bubbles can form. Adding a small object like raisins or peanuts to a sparkling drink also helps increase the bubbles. These submerged objects act to attract opportunistic molecules like CO₂ to new surfaces to accumulate and form bubbles.

And once enough bubbles have grown on the object, the levitation operation can be performed. Together, the bubbles can lift the object to the surface of the liquid. Once on the surface, the bubbles burst and cause the object to fall back down. This process begins again, in a periodic vertical dance motion.

dancing raisins

Raisins are particularly good dancers. It only takes a few seconds for enough bubbles to form on the wrinkled surface of the raisins before they begin to rise – it takes longer for bubbles to form on smooth surfaces. When dropped into freshly opened sparkling water, a raisin may dance a vigorous tango for 20 minutes, and then a slow waltz for an hour or two.

Anyone with a few kitchen supplies can make their own dancing raisins.

We found that rotation or spin was extremely important in getting large objects to dance. Bubbles that stick to the bottom of an object can keep it afloat even after the upper bubbles burst. But if the object begins to rotate even slightly, the bubbles below rotate the body even faster, resulting in even more bubbles bursting to the surface. And the sooner those bubbles are removed, the sooner the object can return to its vertical dance.

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

Bubbly Flamenco Modeling

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

The model took into account the rate of bubble growth as well as the shape, size and surface roughness of the object. It also took into account how quickly the liquid loses carbonation depending on the geometry of the container and especially the flow created by all that bubbling activity.

Small objects covered in bubbles in carbonated water rise upward toward the surface and sink back down.
The bubble-coated raisins 'dance' on the surface and fall once their raising agents are released.
saverio spagnoli

The mathematical model helped us determine which forces most affect the object's dance. For example, the drag of the fluid on the object turned out to be relatively insignificant, but the ratio of the object's surface area to its volume was important.

Looking to the future, the model also provides a way to determine some difficult quantities using more easily measured quantities. For example, by looking at the dance frequency of an object, we can learn a lot about its surface at a microscopic level without seeing those details directly.

Different dances in different theaters

However, these results are not only of interest to carbonated beverage lovers. Supersaturated fluids also exist in nature – magma is an example.

As the magma in a volcano gets closer to the Earth's surface, it becomes increasingly pressurized, and dissolved gases from inside the volcano rush to the outside, just like CO₂ in carbonated water. These escaping gases can form into large, high-pressure bubbles and erupt with such force that a volcanic eruption can occur.

Particulate matter in magma may not dance around the same way raisins do in soda water, but small objects in magma can influence these explosive events.

The past decades have also seen a different kind of explosion – thousands of scientific studies devoted to active substances in liquids. These studies look at things like floating microorganisms and the inside of our fluid-filled cells.

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

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

Meanwhile, a fluid itself can behave strangely due to its diverse molecular structure and the bodies moving inside it. Several new studies have addressed the behavior of microorganisms in fluids such as mucus, for example, which behaves like both a viscous fluid and an elastic gel. Scientists still have a lot to learn about these highly complex systems.

While raisins in soda water seem fairly simple compared to microorganisms swimming through biological fluids, they provide an accessible way to study common characteristics in those more challenging settings. In both cases, bodies extract energy from their complex fluid environment and also influence it, and fascinating behaviors arise.

From geophysics to biology, new insights about the physical world will continue to emerge from tabletop-scale experiments – and perhaps even straight from the kitchen.

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