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Particles defy gravity, float upstream

Inspired by tea leaves’ reverse route, physicists demonstrate that water’s surface tension allows unexpected movement

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11:18pm, July 2, 2013

UPSTREAM FLOW  When pouring clean water into a cup filled with tea leaves, some leaves glide upstream into the kettle thanks to differences in the water’s surface tension.

Rogue tea leaves have led physicists to the discovery of a counterintuitive phenomenon: Particles can float upstream in moving water.

“It’s interesting and very cool,” says Eva Kanso, a physicist at the University of Southern California. “I’m going to have my students do an experiment like this.”

Any kayaker, plumber or physicist would probably say that things always flow downstream. But that conventional wisdom started to unravel for Sebastian Bianchini one night in 2008 when he prepared some mate tea, a South American specialty, by pouring hot water over a cup of tea leaves. Bianchini, then an undergraduate at the University of Havana in Cuba, noticed that by the time he filled his cup, a handful of tea leaves had invaded the pristine water in the kettle.

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