The Cheerios Effect


[MUSIC] The breakfast table’s probably the last
place you’d expect to find cool physics, but there is some awesome science happening right
here, and you’ve probably seen lots of times without even realizing it. Ever notice how cereal tends to stick together
in the middle of the bowl? Or it clumps to the edges. That makes it easy to eat, but why does it
happen? We see this same clumpage with other objects
too: paper clips, thumb tacks, even bubbles in a beverage will snap together. Maybe you’ve noticed this, but scientists
didn’t fully understand what was going on until 2005, when a pair of mathematicians
decided to hit the lab, it hthe kitchen, and hit the books. What they found is cool. I’m super cereal. Check this out. Breakfast cereal is less dense than water
(and milk is mostly water). It’s buoyant, it weighs less than the milk
it displaces. That force of buoyancy pushes up on each ring,
until it matches the downward force of gravity. This interaction holds the Cheerios at the
surface of the liquid, like little toasty rafts drifting together on top of a sea of
cereal milk (mmmm cereal milk). It’s a really complicated way of saying cereal
floats. But look closely at where the cereal meets
the liquid. It’s curved up. The same thing happens at the edge of the
container, thanks to the meniscus effect. Water molecules are sticky – they’re attracted
to each other, but they’re even more attracted to the edges of your bowl or glass, or to
the edge of the cereal itself. That “adhesion” forms a U-shape wherever
the liquid meets an edge. A buoyant object will always be pushed up
the liquid to the highest point on a meniscus. That’s what makes them stick to the edge,
and what causes the cheerios to become cheeri-amigos. Any two nearby Os are pushed to a high point
between them, and clumps are pushed towards the overall highest point in the bowl, around
the edge. Let’s try something denser. I don’t recommend eating paperclips, but
toss them in water and they sink. Place them carefully though, and you can get
them to float. They’re too dense to be buoyant, they float
because of surface tension. Water molecules like to stick to each other
so much, they can behave like a membrane that’s strong enough to hold up tiny things. Let’s try it with these thumbtacks. Like the paper clips, you can see they’re
pushing that membrane dow, just not hard enough to break through. If I place another one nearby, watch what
happens. They’re attracted to each other, just like
the Cheerios. But the water around each one is curving down. Instead of climbing up the water like cereal
did, they fall into each other’s sinkhole. We can mess this scenario up just by adding
soap. The chemical properties of soap lower the
surface tension of water, so anything relying on surface tension to stay afloat will sink. But buoyant objects don’t rely on surface
tension, so they continue surfing the meniscus. The first time I did this, I wondered if the
tacks were being pulled together by static attraction on the plastic coating or something. So I put in just the plastic bit to see. But instead of being pulled toward the tacks,
something strange happened… they repelled each other. The same thing happens with Cheerios and a
paper clip. That’s because light, floaty objects run
away from the low points caused by the heavy objects. A buoyant object will always repel something
held up by surface tension’s stretchy membrane. Just to be clear, you should never put thumbtacks
in your cereal. But this is what would happen if you did. All of this made me wonder: What could happen
if we could reverse the direction of water’s meniscus? I coated this glass with a hydrophobic coating
that does just that. When I put thumbtacks on top of the water
in here, they floated to the edge instead of the center. And that buoyant object did the opposite,
it floated to the middle. So that’s cool and all, but does the physics
of cereal clumping actually matter in the real world? It does if you’re a tiny insect. Take water striders,. These pond skaters are nature’s Cheerios. They float so well that even a load 15 times
their body weight won’t make them sink. They can even jump on water. Tiny hairs on their legs trap air bubbles
and increase their buoyancy. They’re basically wearing swim floaties
on their feet. Other aquatic insects like water treaders
exploit surface tension, just like thumbtacks and paper clips. But they get in trouble when it’s time to
get out. Gravity is pushing them into the depressions
under their feet, but they’ve come up with a clever way to climb the meniscus. A running start doesn’t work. But by arching their bodies and lifting their
front and back ends, the bugs curve the water up, and are pulled to the edge just like the
Cheerios were. They’re carried uphill by a physics-powered
water escalator. That’s pretty cool. If you can find science like this at breakfast,
imagine what else you might see the rest of day. Try this for yourself, and see what other
floating objects you can get to attract or repel. Leave a comment and let me know what you find. And if you see any cool physics in everyday
life I should check out in a future video, let me know. Stay curious.

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