Why do tea leaves gather in the centre of the cup?

July 22, 2008 6:54 AM

Why do stirred tea leaves tend to move towards the centre of the cup?

When I pour a cup of tea, then stir it, then wait, the tea leaves gather in the centre of the cup as they slow down from their circular motion.
Why do they disobey the rule I learned, obviously wrongly, about circular motion creating an outwards force from a central point? What is the name of the force pushing the tea leaves inwards, and how does it work?

posted by Fiasco da Gama to science & nature (12 comments total)

i believe the word you are looking for is vortex.
posted by phil at 7:01 AM on July 22

Why do they disobey the rule I learned, obviously wrongly, about circular motion creating an outwards force from a central point?

Centrifugal force is a phantom. The only force pulling outward is the force that is opposite the centripetal force that holds a spinning object to a circular path.

I believe what you are asking about is cyclonic separation.
posted by mkb at 7:01 AM on July 22

wouldn't cyclonic separation put them on the outside?

i suspect it's a density thing - similar to why a helium filled balloon moves "the wrong way" when you go round a corner in a car (this really happens! we tried it a couple of months ago while taking a balloon to my grandma's 100th birthday party....)
posted by not sure this is a good idea at 7:04 AM on July 22

Einstein explains. More here.
posted by steef at 7:09 AM on July 22 [3 favorites]

Friction.
posted by gjc at 7:14 AM on July 22

The tea is lighter than the water. The heavier water moves to the outside and the lighter tea particles stay on the inside.
posted by caddis at 7:35 AM on July 22

wow. that's neat. fwiw when the einstein article uses "stationary" near the end of the fourth paragraph it means "steady" not "motionless". maybe that's obvious, but i thought it was a clumsy translation. (and i was completely wrong about it being like the helium balloon).
posted by not sure this is a good idea at 7:38 AM on July 22

How To Fossilize Your Hamster covers this plus another few dozen physics at home experiments. Recommended.
posted by Paid In Full at 8:28 AM on July 22

Stuff avoids fast water and air, and gathers in the slow water and air. That's why rivers meander--because the outside of the river bend gets cut away by the faster water, and the dirt gets deposited on the inside curve. It's why the intake for the air in your car is at the base of the windshield, where the pressure is high, and there is therefore less dust in the air. It is also why, during that blizzard in Montreal, I should have parked in the middle of the parking lot, where the wind was fierce, instead of behind my friend's apartment, where I couldn't find my car under the eight foot pile of snow in the morning.
posted by weapons-grade pandemonium at 8:43 AM on July 22

It's also why airplanes fly.
posted by weapons-grade pandemonium at 8:51 AM on July 22

What steef said. Congratulations — You have asked mefi a question which Einstein answered!
posted by exphysicist345 at 5:05 PM on July 22

If the tea were lighter then it'd surely be floating on the surface, not sitting on the bottom of the teacup. And the effect is quite obvious with materials that are obviously heavier than water too. Go with steef.
posted by edd at 1:51 AM on July 24