Why Do Your Tea Leaves Move To The Middle Of The Cup?

Swirl water in a cup, and the water will move to the outer edge of the cup. Sometimes, when you swirl hard enough, it will actually start "climbing" the walls of the cup, as the water collects at the outer edge. Now take a cup of tea with a few tea leaves in it and stir it with a spoon. The tea leaves collect in the middle of the cup, proving tea leaves really are magic.

Why Do Your Tea Leaves Move To The Middle Of The Cup?

Or maybe not. Maybe there's something else going on, and Albert Einstein is just the guy to figure it out. Einstein determined that, just where the liquid meets the glass, there's a little drag on the flow of the tea. The tea right at the sides of the cup, and more importantly, all along the bottom, will be going less quickly than the rest of the liquid in the cup. This is important, as all that water pushing up against the sides of the cup also establishes a pressure gradient, with higher pressure towards the outside of the cup. The slow-moving water is displaced. When it hits the sides of the cup, it drops downwards, and when it gets to the bottom of the cup, and the relatively slow-moving water there, it's forced inwards. Once it gets to the middle it's forced upwards, and the cycle continues again in a kind of invisible roll inside the swirling tea. The only visible signs are the tea leaves themselves. They are too heavy to be lifted up by that upward flow in the middle of the cup, but they do get caught up in that roll. So they're herded towards the middle of the cup, defying centrifugal force. Thanks, Einstein!


Why Do Your Tea Leaves Move To The Middle Of The Cup?

Or maybe, thanks Ekman. You know that layer just where the sides of the cup meet the swirling tea? Some people call it the boundary layer, but some people call it the Ekman layer. Vagn Walfrid Ekman was an oceanographer who wondered why icebergs tend to move not with the prevailing wind, but at a specific angle to it. What he found was that the icebergs were a bit like tea leaves. They were acting under the influence of considerably mightier forces than a tea lovers hot little hands — specifically the coriolis forces, the currents of the ocean, and the wind — but they still had that strange layer of water that got slowed down by friction. The physics of the Ekman layer helps us construct current meters, and understand how objects drift in the ocean, and can even result in rarely-seen but dramatic swirls of water in the ocean known as Ekman spirals. And they help us understand the tea leaves.

So who deserves credit for the tea leaves? There's no question that Einstein figured out the tea leaf motion in 1926 (and just to show off, explained how that motion might change the course of rivers), but Ekman discovered the Ekman layer and its effects on the motion of tea leaves and any similar objects all the way back in 1902. Ekman discovered the overall pattern of motion. So perhaps Einstein can lay claim to the special theory of tea leaves, while Ekman gets credit for the general theory of tea leaves?

[Via The Ekman Layer and Why Tea Leaves Go To The Center of the Cup, Tea Leaf Effect, Einstein's Tea Leaves]