Why exactly do cheerios stick to each other in milk? Doomed love between Os? Tiny magnets? Micro-bacterial pirates boarding a merchant cheerio? Only physics has the answer.
Everyone has started at least one day listlessly toying with some bland, wheat-based cereal while wishing for hash browns. It's understandable that such a dispiriting breakfast wouldn't inspire close observation from its consumer, but most people have observed two cheerios, or bran flakes, oat bits, drifting lethargically through the bowl until they get close together.
After they get close enough, they suddenly push together, and for the rest of the breakfast they stick together like they were in an old Western and one had saved the other from a cattle stampede.
What is it that makes these two cereal pieces such good friends?
Some people have described it as ‘sugaring.' Soaking in milk causes the sugars and starches that coat the cereal to become sticky.
One physicist has proposed a less chemical and more mechanical argument.
Molecules in liquids cling to each other. Under certain circumstances, they also cling to other substances. When the molecules are more attracted to the other substance, they ‘climb' the substance slightly, like the edge of water seems to climb the sides of a test tube.
When the molecules in the liquid are more attracted to each other than to the other substance, the surface of the liquid seems to shrink away from the other substance. This results in surface tension – when liquids cling to each other so tightly that they form a kind of skin over the top of the liquid. There are many times when this effect is observable; for example when water skeeters move over the surface of water, denting it slightly, without falling in and drowning like the freaky bugs they are.
Cheerios also dent the surface of the liquid they float in. Because they are less dense than milk, there is no chance of them ‘falling in,' but when the water shrinks back from them, it creates divots. When one cheerio floats close enough to another, they float ‘downhill,' into the divots, just the way a person would float downstream on a river. Because it takes energy to float uphill, and breakfast cereals are a lazy bunch, they stay close. It's not so much that they cling together than that they can't float apart unless ants are using them as kickboards.
That would spice up a morning.