The Cheerios Effect is caused by a combination of buoyancy, surface tension, and the meniscus effect. Buoyancy is the force that makes an object float or sink in a liquid. Surface tension is the property of a liquid that makes it act like a flexible membrane. The meniscus effect is the curve on the surface of a liquid caused by unequal forces between the molecules of the liquid.
It’s almost impossible to count the number of physical phenomena taking place right in front of your eyes in every passing moment. From the second you wake up in the morning to the time you finally close your eyes to go to sleep, you’re continually witnessing a host of physical phenomena without even realizing it.
For instance, consider this: have you ever wondered why breakfast cereals tend to stick together or cling to the sides of the bowl?
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Buoyancy and Surface Tension
To state it plainly: Put a ship in an ocean and it will float; fling a nail in the same ocean and… well, say goodbye to your nail.
Whether an object floats or sinks depends on a physical property referred to as buoyancy. If the object in question is denser than the liquid surrounding it, then it will sink; however, if it’s less dense, then it will float. It’s as simple as that!
Surface tension is another important concept related to liquids. It makes liquids act like ‘flexible membranes’. Surface tension is caused due to weak forces between the molecules of the liquid. The water molecules at the surface of a liquid experience ‘pulling’ forces from different directions; the air molecules above pull them upwards, while the neighboring water molecules pull them in every other direction! Depending on which of these forces is stronger, the surface of the liquid caves in that direction, giving rise to a ‘meniscus’.
The Meniscus Effect
As mentioned above, the forces between the water molecules and air molecules slightly alter the surface of the liquid in a container. The resulting curve on the surface due to these unequal forces is called a meniscus. A meniscus can come in two types: upward (convex) and downward (concave). The type that is formed depends on the properties of the liquid in question.
Water, the most common liquid in our daily lives, forms a concave (or downward) meniscus, whereas mercury forms a convex meniscus.
How does this relate to the cereal in my bowl?
Patience, please… I’m coming to that part.
Every phenomena that we discussed above, when combined together, cause the cereal to stick together in your bowl. The object floating on the surface of the liquid (let’s say water) can either be hydrophilic (affinity towards water) or hydrophobic (repulsion to water).
If the object is hydrophilic, then water molecules gather around the sides of the floating object, creating a small dent or depression on the surface of the water. In such a scenario, small bits of the object will clump together or appear to ‘fall into each other’, due to the large depression that they end up creating beneath them.
Now, consider a couple of Cheerios in a bowl of milk. As fate would have it, Cheerios are ‘milk-philic’, as the little O’s, quite elegantly, make small depressions on the surface of the milk beneath themselves. This is why they appear to cling to each other like inseparable lovers. The same phenomenon causes these Cheerios to stick to the sides of the bowl, as that force may cause the cereals to curve slightly upwards against the sides of the bowl!
Next time you sit down with a bowl of milk and you see those Cheerios forming a clump all by themselves, don’t let it freak you out. It’s just a trick that nature likes to play, and don’t expect it to change any time soon!