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Here’s a curious phenomenon.

The south American drink ‘mate’ is a tea-like beverage in which finely cut, dried leaves from the yerba mate herb are infused in hot water. The dried leaves, just a few square mm in size, tend to float on the surface of the water

While pouring the hot water onto the leaves, Ernesto Althsuler and buddies at the University of Havana in Cuba, noticed a puzzling phenomenon. They found that, sometimes, the leaves would somehow travel upstream and end up contaminating the upstream container of pure water.

Being diligent physicists, they decided to investigate. They found that the leaves (and also chalk powder) were able to navigate upstream if the waterfall was less than about a centimetre in height. “For distances of the order of 1 cm or less, some of the floating particles eventually start to “climb up the stream”,” they say.

How can this be? Altshuler and co say the geometry of the system is important. Their set up consists of a relatively long horizontal channel, along which the water flows, followed by a short drop into the infusion (see diagram above).

In the past, physicists have observed vortices in horizontal flows which can set up a counterflow. When this happens, there is a main flow through the middle of the channel and a counterflow along the edges, which can carry small particles in the opposite direction.

What’s new, however, is the idea that the vortices can survive the drop and continue to generate a counterflow even after the water has plunged over the lip of the channel.

Altshuler and co think that this counterflow lifts particles out of the lower container and pumps them back into the upper container. But only if the drop is not too big–less than a centimetre or so.

That’s an interesting effect that you could probably recreate in your kitchen.

But it’s more than a mere curiosity. Altshjuler and co say their discovery “may have potential implications in many chemical, medical, pharmaceutical and industrial processes.”

So if you’re mixing processes are plagued by problems of contamination, perhaps the “mate” effect is to blame.

Ref: arxiv.org/abs/1105.2585: Upstream Contamination In Water Pouring

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