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A raisin dropped into a glass of a carbonated liquid performs a wobbly up-and-down dance for hours 鈥� and now we know why.

In 2017, at the University of Wisconsin-Madison and his daughter Carina were playing with a carbonated beverage that they had dyed red 鈥渂ecause that looked neat鈥�. He noticed that dropping a cashew into it didn鈥檛 make the nut sink or swim, but forced it to repeatedly rise to the surface of the fluid before descending towards the bottom.

鈥淚 was baffled at first. It took me an hour or two just to even have an idea about what in the heck could be happening,鈥� he says. To find out, Spagnolie and his colleagues ran computer simulations and experiments on carbonated water.

They knew the dropped object鈥檚 dance was driven by bubbles in the fluid, so they switched out the cashews for raisins, which collect more bubbles on their rougher surfaces. They used tiny 3D-printed spheres, and mathematically modelled bubble growth and diffusion. The researchers also used a high-speed camera to record how the bubbles and dropped objects interacted with each other.

They discovered that when an object is dropped into a carbonated fluid, bubbles nucleate on its exterior and push the object upwards. Once it reaches the surface, some fizz escapes into the air and the object 鈥済ets a little haircut; it loses that top layer of bubbles鈥�, says Spagnolie. Then it starts to sink again and pick up new bubbles. He says that this process can continue for hours, starting up even if the liquid looks perfectly still before an object is dropped in.

鈥淭his is a beautiful example of a system with self-sustaining oscillations, and it鈥檚 readily accessible in the kitchen,鈥� says at Princeton University.

Spagnolie says the carbonated water in his team鈥檚 study is an example of a liquid supersaturated with gas. These liquids also exist in some industrial processes and in nature 鈥� for instance, in the form of gas-filled magma. He says that understanding how raisins and gas bubbles interact may eventually lead to a clearer picture of how such liquids de-gas when something is mixed into them.