Make something float in the middle of a water surface and see what happens. Maybe not a lot!
If you look from the side you should see a curved water surface around your floating object. This is called a meniscus.
It gets interesting when a floating object gets close to the curved surface at the edge of the container. Watch the video below to see what I mean.
So what’s actually happening?
When the pin is in the middle of the liquid there is a force pulling the pin down – its weight – and a force pushing upwards – buoyancy. The forces balance out so there is no net force and the pin stays still.
If the pin gets close to the edge, though, it can end up interacting with the meniscus in the container. The meniscus is that bit of water that curves up to meet the walls of the glass – so the water level is slightly higher at the edges. The buoyancy force is now at a slight angle so the net force is towards the glass.
Once the pin hits the side of the container the forces cancel out again – the pin can’t be pushed up any higher than it already is so it will stay in contact with the glass. It might drift around the sides of the container slightly, depending on any water movements or whether the container is tilted a little.
What about if there are two floating objects?
One floating object gets drawn towards the edges. What about two or more floating objects? Try this out with your next bowl of cereal – you should see the objects clumping together. The same thing happens with bubbles – they don’t tend to stay on their own but form together into a foamy lump. The video below shows pieces of floating wire coming together in the same way.
Why is it called the Cheerios effect?
Let’s be honest – you probably don’t need me to tell you this. It’s called the Cheerios effect after the cereal. I think this is a great example of how something really scientific can end up in the ordinary everyday world – people having their breakfasts noticed something interesting going on and those same ordinary people tried to work out what was happening. In the age of Wikipedia and instantly being able to Google things, it can be so easy to find these things out that we forget how amazing what we are actually seeing is.
The next time you are having cereal, when you get down to those last few pieces in the bowl with the left over milk, see what happens to them. They will either get dragged towards the edge of the bowl or start to stick together.
A lot of credit should go to a paper written by Dominic Vella and L. Mahadevan in the American Journal of Physics – available at http://people.maths.ox.ac.uk/vella/Vella2005.pdf
The article goes in to a lot more of the theory than I could explain here.