Moment of Science: The Cheerios Effect

“I won’t eat any cereal that doesn’t turn the milk purple.” -Bill Watterson, ‘Calvin & Hobbes’
Ever notice the last few bits of your morning cereal clumping together? (Or evening cereal, we don't judge). Here's a quick primer on the "Cheerios Effect". 🥣
Published: May. 23, 2023 at 10:41 AM EDT
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Have you ever noticed how your last little bits of cereal tend to stick together, or to the sides of the bowl? This week, we’re taking a closer look at -- and this is the published name for it -- the “Cheerios effect”.

* First off, the whole reason your cereal even floats in milk is due to buoyancy. We visited this during our cruise episodes, with a balancing act between gravity and upthrust. The cereal is less dense than the milk, hence it floats... but that doesn’t explain the clumping thing. That’s where surface tension comes into play. The liquid molecules are more attracted to each other than they are the adjacent air molecules. We have that thin membrane of sorts at the surface, allowing a light enough object to sit on top of that surface, causing it to bend around to accommodate. A floating object will always want to float to the highest point in that medium. That explains why your last spoonful of Cheerios sort of gravitates to the edge of the bowl.

* When two of your cereal bits get close enough to each other, they essentially want to travel up the sides of those curves that each object has created themselves, causing them to clump together. Even coins can float as long as they stick together -- otherwise, you’ve got a wishing well and not much else. Even aside from having an object there, the liquid will still bend at the surface, forming what’s called a “meniscus”. Water or milk will appear to climb slightly up the sides of a container, but some liquids like mercury actually prove the opposite due to density and a variety of other factors. Note that for measurements, you should always read the level at the center of that curve, and not what appears on the sides.

* You can also go the other way with this. If you pour liquid into a container like a bowl, and barely overfill... it turns out the water bulges ever so slightly over the lip, but still contains itself. Now your highest point is in the middle of the bowl, and your buoyant objects float right to the center of the action -- a simple demonstration of an everyday concept.

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