And why they start spinning on their own...
I am sure you have seen at least one video of the rare, eerie, and sometimes giant discs of ice, known as ice circles, that are often spotted spinning in rivers. These discs can range in size from 1 to 200 meters (3.3 to 656 feet), and for over 100 years, almost everything about them has bewildered physicists and environmental scientists.
The most common explanation for this natural phenomenon is that as the discs float along in a river they are spun around by eddies, which are little spinning currents that form when water flows over rocks or into an enclosed space. And even though this is probably part of what’s happening, it can’t be the whole story.
According to ScienceAlert, if eddies were the only thing at play, then smaller discs would spin faster than bigger discs. However, this doesn’t happen; discs of all different sizes spin at the same speed. Furthermore, in still water — where there are no eddies — the discs still rotate.
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This prompted a research team, led by Stéphane Dorbolo from the University of Liége in Belgium, to test the phenomenon. They created homemade ice disks — formed by freezing water in Petri dishes with little nickel balls at the center — and placed them in a tank of still water with a magnet held above it. The magnet was used to keep the disc in place as it spun, since the nickel was just magnetic enough to want to stay right below it.
The researchers found that the discs still rotated as they melted, even though they were not able to move around. How could these discs rotate if they were not being spun by eddies in the water?
Interestingly, when the researchers conducted a second experiment without nickel balls to keep the discs in place, the team discovered that the discs spun faster in warmer water. It was this clue that led the researchers to track what the water itself was doing when a disc was held still. What they found was that, right below its center, water was flowing down after it melted off the disc instead of dispersing gradually.
Why does this happen? It comes down to the strange properties of water. Most substances get denser as they get cold and even though water does follow this trend to a point, the density decreases just below freezing, which is why ice floats. Water is actually at its most dense just a couple degrees above its freezing point.
What this means is that, as water is melts off the discs, it sinks straight down because it is denser than the warmer water around it. The warmer the surrounding water is, the bigger the difference is in densities, and the faster the newly melted water sinks.
Now logically, water can’t really go straight down. If there is any sideways movement at all, which you definitely have in a river and even apparently still bodies of water, the water will start to spin as it sinks. And this is exactly what the researchers saw: The water underneath the ice circles was spinning as it sunk, and this spinning was pulling the discs around. Mystery solved!
Their results were published in the journal Physics Review E.
This goes to show that not all new physics discoveries have to be from the Large Hadron Collider or Hubble Space Telescope — there is still a lot to learn about the natural and tangible things here on Earth.
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