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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

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.

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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.

He described his strange observation to a physicist at the school, Ernesto Altshuler, and they ran some experiments. Although they thought they understood what was going on, Altshuler says they received skepticism from other physicists and so never published their findings.

Then last year Altshuler met Troy Shinbrot, a physicist at Rutgers University in Piscataway, N.J., who agreed to replicate the experiment. Shinbrot set up two tanks side-by-side and elevated one of them, with water flowing down through a channel to bridge the 1-centimeter height gap. Sure enough, within seconds of adding chalk and mate tea to the bottom tank, particles began climbing up the channel to contaminate the upper tank.

Shinbrot’s experiments led him to the conclusion that Altshuler’s team had also reached: The particles overcome gravity and the current thanks to a property of water called surface tension. The linkup of hydrogen atoms among water molecules tends to create an elastic, trampoline-like surface. But small particles like tea leaves disturb that network, causing the hydrogen bonds to pull apart and thrust the particles toward purer water where the surface tension is higher. Physicists have long known that particles could get a push via this process, Shinbrot says, but they never considered the force strong enough to propel particles upstream.

The researchers describe the surface tension theory and experiments July 3 in Proceedings of the Royal Society A.

Shinbrot and Altshuler concede that they don’t know whether their discovery has implications outside of tea preparation. But Shinbrot says it’s possible that pollutants can also migrate upstream in a slow-moving river. He also demonstrated that particles can sneak into the tips of pipettes, potentially contaminating laboratory samples if the pipettes are reused.

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