HOW COME?: Loose tea leaves gather in a cup's center

Kathy Wollard
October 13, 2008

If you make a cup of tea using loose tea, and then stir it, why do the leaves gather in the center at the bottom of the mug? asks a reader.Albert Einstein is perhaps best known for the Theory of Relativity, which explained the puzzle of space, time, and gravity. But in 1926, he also discovered the Tea Leaf Paradox, explaining the puzzling behavior of leaves in a tea cup.If you've ever ridden a swiftly moving merry-go-round, or gone around a curve too fast in a car, you're familiar with centrifugal force. Centrifugal force throws objects outward from the center; it's why a washer's spin cycle leaves clothes plastered to the wall of the washer basket.

So when we stir loose tea in a cup of hot water, the leaves "should" end up in an outward-flung ring around the bottom.Instead, they group obediently in the center. How come?According to Einstein, the leaves' motion reveals the circulation of water in the cup. Stirring makes the water spin around a central axis, and spiral out from the center. But the water down below is slowed by friction with the cup's bottom; its spin is weakened.

The rotation difference creates a circulation system in the cup: Water at the top, strongly spun outward, travels down the wall and across the cup's bottom, and then flows back up the central axis. It's this current that ferries tea leaves to the center bottom of the cup.But Cleveland State University physicist Jearl Walker says that there is one thing Einstein failed to notice in his cup of tea. Just after the spoon is removed, before the leaves settle, they form a central ring. Tea leaves outside the ring are dragged inward by the current.

Tea leaves inside the ring are pulled out to join it. As the spinning water slows, the ring contracts in size. Finally, the leaves come to rest in a central heap.Some 80 years after Einstein wrote his paper, the Tea Leaf Paradox has inspired one scientist to invent a new way of "reading" blood cells. At the University of Notre Dame in South Bend, Ind., scientist Leslie Yeo was using an electric field to spray drops of liquid from a needle. When one drop landed in the wrong place, he found that small bits of solid material were spinning around in the drop. The liquid was being stirred by a "wind" of charged particles created near the electrified needle. So Yeo decided to do some experiments in a tiny cylinder. He discovered that the liquid's swirling particles always settled in the center on the cylinder's bottom. But why?Puzzled, Yeo found the answer in Einstein's paper on the tea paradox.

Yeo, now based in Australia, realized that the effect could be used to spin blood cells out of whole blood. Using a credit-card sized kit, he says, doctors could do blood tests in the office, with no waiting for outside lab results. Reading the tea leaves, Yeo expects that it might take up to 10 years to develop a working device.