Scientific discovery doesn’t always require a high-tech laboratory or a hefty budget. Many people have a first-rate laboratory in their own homes – in the kitchen.
The kitchen offers many opportunities to see and explore what physicists call soft matter and complex fluids. Everyday phenomena, such as the accumulation of Cheerios in milk or rings left when coffee drips evaporate, have led to discoveries at the intersection of physics and chemistry and other tasteful 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, which dives into another kitchen observation. We study how objects can levitate in carbonated fluids, a phenomenon that is whimsically known as dancing raisins.
The study explored how objects like raisins can move rhythmically up and down in carbonated fluids for several minutes, even an hour.
A follow-up Twitter thread about our research went viral, racking up over half a million views in just two days. Why did this particular experiment capture so many people’s imaginations?
Bubbly Physics
Sparkling water and other carbonated drinks bubble because they contain more gas than the fluid can handle – they are “supersaturated” with gas. When you open a bottle of champagne or a refrigerant, the fluid pressure drops and CO₂ molecules begin to escape into the surrounding air.
Bubbles generally do not form spontaneously in a fluid. A fluid is made up of molecules that like to stick together, so the molecules on the boundary of the fluid are a little unhappy. This results in superficial tension, a force that seeks to reduce surface area. Because bubbles add surface area, surface tension and fluid pressure typically compress any forming bubbles, squeezing them out of existence.
But rough patches on the surface of a container, like the engravings on some champagne glasses, can protect new bubbles from the crushing effects of surface tension, offering them the opportunity to form and grow.
Bubbles also form within the microscopic tube-shaped fibers left behind after wiping a glass with a towel. Bubbles constantly grow in these tubes and when they are large enough, release them and float upwards, carrying the gas out of the container.
But as many champagne enthusiasts who pour fruit into their glasses know, surface engravings and tiny fabric fibers aren’t the only places where bubbles can form. Adding a small object like a raisin or a peanut a sparkling drink also allows bubbles to grow. These immersed objects act as attractive new surfaces for opportunistic molecules like CO₂ to accumulate and form bubbles.
And once enough bubbles have grown on the object, an act of levitation can be performed. Together, the bubbles can lift the object to the surface of the liquid. Once at the surface, the bubbles burst, knocking the object down again. 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 starts to rise – bubbles have a harder time forming on smoother surfaces. When placed in freshly opened sparkling water, a raisin can dance a vigorous tango for 20 minutes and then a slower waltz for an hour or so.
We found that rotation, or rotation, was extremely important for coaxing large objects to dance. Bubbles that stick to the bottom of an object can keep it aloft even after the upper bubbles have burst. But if the object starts to spin just a little, the bubbles underneath cause the body to spin even faster, which results in even more bubbles bursting onto the surface. And the sooner these bubbles are removed, the sooner the object can return to its vertical dance.
Small objects, such as raisins, do not spin as much as larger objects, but instead spin, rocking rapidly back and forth.
Modeling bubbly flamenco
In the paper, we developed a mathematical model to predict how many trips to the surface we would expect an object like a raisin to make. In one experiment, we placed a 3D-printed sphere that acted as a raisin model 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 embedded model the growth rate of the bubble, as well as the shape, size and surface roughness of the object. It also took into account how quickly the fluid loses carbonation based on the geometry of the container and, mainly, the flow created by all that bubbling activity.
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 relationship between the object’s surface area and its volume was critical.
Looking ahead, the model also provides a way to determine some hard-to-measure quantities using more easily measurable ones. For example, just by observing the dancing frequency of an object, we can learn a lot about its surface at the microscopic level without having to see those details directly.
Different dances in different theaters
However, these results are not only interesting for lovers of carbonated drinks. Supersaturated fluids also exist in nature – magma is an example.
As the magma in a volcano approaches the Earth’s surface, it quickly depressurizes and dissolved gases from inside the volcano rush to the outlet, just like the CO₂ in carbonated water. These escaping gases can form large, high-pressure bubbles and emerge with such force that a a volcanic eruption follows.
Particulate matter in magma may not dance the same way as raisins in sparkling water, but small objects in magma can affect how these explosive events unfold.
The last few decades have also seen an eruption of a different kind – thousands of scientific studies devoted to active matter in fluids. These studies look at things like swimming microorganisms and the inside our liquid-filled cells.
Most of these active systems do not exist in water, but rather in more complicated biological fluids that contain the energy necessary to produce activity. The microorganisms absorb nutrients from the fluid around them to continue swimming. Molecular motors transport cargo along a superhighway in our cells, pulling energy nearby in the form of ATP of the environment.
Studying these systems can help scientists learn more about how cells and bacteria in the human body work and how life on this planet evolved to its current state.
Meanwhile, the fluid itself can behave strangely due to the diverse molecular composition and bodies moving within it. Many new studies addressed the behavior of microorganisms in fluids such as mucus, for example, which behaves both as a viscous fluid and as an elastic gel. Scientists still have much to learn about these highly complex systems.
Although the raisins in sparkling water seem quite simple when compared to the microorganisms that swim in biological fluids, they offer an accessible way to study generic features in these more challenging environments. In both cases, the bodies extract energy from their complex fluid environment at the same time as they affect it, resulting in fascinating behaviors.
New insights into the physical world, from geophysics to biology, will continue to emerge from experiences at the table scale – and perhaps even in the kitchen.
This article was republished from The conversation, an independent, nonprofit news organization that brings you trusted facts and 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 Studies with funding from the Wisconsin Alumni Research Foundation and by grants to the AMEP (Applied Math Engineering and Physics) program at the University of Wisconsin-Madison.
