One of my favorite breakfast cereals growing up was Cheerios. When General Mills brought out Honey Nut Cheerios, I thought breakfast couldn’t get any better than that. And on top of their nutritional value and crunchy sweetness, Cheerios are supposed to help lower cholesterol. What more could you want from breakfast?
Well, have you ever heard of the Cheerio Effect? It is an interesting physics phenomenon you can engage with every morning if you like.
We all know Cheerios are naturally buoyant. They are light and crunchy and filled with air pockets so they naturally float on the surface. If you place a single Cheerio in the middle of a bowl of milk, it will happily rest there until it becomes too waterlogged and sinks to the bottom.
But what happens if you place two Cheerio rings into a bowl of milk? If you haven’t ever tried this, feel free to go ahead. I’ll wait.
Two rings will attract each other. They somehow get pulled together. Indeed, add a few more rings and they will form a raft with a hexagonal structure. Too many rings in a breakfast bowl will result in some migrating to the edge of the liquid. But in a large flat dish, you can create interesting floating rafts of Cheerios. And when you are done, you can even eat your experiment! Not something I can say about every physics lab.
So what is happening? Is this something only Cheerio rings do? No. Paper clips, thumb tacks, plastic disks, and even dimes can be made to float on the surface of water with a little care. And they aggregate into floating rafts in much the same way Cheerios do. (Go ahead and try it. I’ll wait.)
Many of these objects, such as paper clips, are actually denser than water. They should sink but they don’t because of surface tension.
Water is a very unusual liquid. It is made up molecules containing two hydrogen atoms and one oxygen (hence, its chemical formula of H-2-O) arranged in a v-shape. In addition with the hydrogen atoms at the top of the ‘V’, there are two lone pairs of electrons dangling from the oxygen at the bottom of the molecule giving an overall tetrahedral shape.
Molecules of water join with one another through the interaction a hydrogen atom on one water molecule being attracted to a lone pair of electrons on another water molecule. This interaction is called a ‘hydrogen bond’ and it is one of the most truly marvelous interactions in all of nature. It isn’t as strong as the H-O bond inside a water molecule but the O-H --- O interaction is strong enough to keep water in a liquid state until 100 degrees Celsius.
It also keeps water molecules on the surface of a bowl of water from escaping easily. Each surface water molecule is only hydrogen bonded on one side – the side pointing into the bulk of the water – and as a consequence, engages in a tight mesh with the other water molecules around it. The energy holding these molecules together on the surface is called surface tension and it means water molecules form a skin-like layer preventing other objects from sinking through. It is surface tension which keeps a paper clip afloat. The molecules of water don’t want to let go of the other water molecules around them to allow the paper clip to intrude.
This is part of the reason for the Cheerio effect. By clustering the rings on the surface of the milk (which is mostly water), you cause less disruption to the surface and lower the energy of the system. The water molecules essentially kick the Cheerios out of the way. But it is not the only reason for the Cheerio Effect. Gravity also comes into play.
Each Cheerio ring generates a dimple on the surface – a little well in which it floats. Approaching Cheerios fall to the bottom of each other’s well due to gravity. And a recent study has shown each ring actually tips producing a deeper well and enhancing the effect. This was observed by using Cheerio-sized plastic disks with magnets on them and then trying to separate the disks. By slowly applying a magnetic field, physicists were able to measure the forces involved demonstrating there is more than just surface tension involved.
While this is something to contemplate over a bowl of breakfast cereal, is there a point to studying these interaction? Yes. Scientists and engineers are building nano-scale objects and robots for a wide variety of purposes and the forces involved in surface tension have a profound effect on their behaviour. At the other extreme, Cheerios floating in milk have been used as a model by astrophysicists to understand the formation of galaxies.
All from a bowl of breakfast cereal.