Facts So Romantic

Reading the Tea Leaves: How Particles Can Travel Upstream

It’s been said that the true harbinger of scientific discovery is not “Eureka!” but “Huh… that’s funny….” That certainly proved to be the case for Sebastian Bianchi: a simple cup of tea led him to some intriguing, counter-intuitive insights into the surface tension of water.

Back when he was an undergraduate physics major at the University of Havana in Cuba in 2008, Bianchi noticed something unusual while brewing a fresh cup of mate—a popular (and potent) South American tea. Preparing mate involves pouring hot water over mate leaves packed into a cup and letting them steep. Generally, things tend to flow downstream, yet he found a few mate leaves inexplicably ended up back in the kettle. Somehow, they had traveled upstream.

Bianchi found a mentor in fellow Havana physicist Ernesto Altshuler, who helped him conduct a few experiments, although they never published their conclusions. But Altshuler continued to be intrigued by the question of what might be going on, and last year joined forces with Rutgers University physicist Troy Shinbrot to replicate those earlier experiments.

The set-up involved two tanks of water, set side by side, with one positioned higher than the other. The tanks were connected by a channel, through which water could flow from the upper to the lower tank. Then they sprinkled mate tea leaves and chalk particles onto the surface of the water in the bottom tank. Sure enough, leaves and particles soon found their way into the water in the upper tank:

The tea leaves aren’t really defying gravity; they’re simply responding to complex fluid dynamics. In a paper published July 3 in the Proceedings of the Royal Society A, the physicists concluded that this behavior resulted from a combination of factors: namely, differences in the water’s surface tension and vortex currents.

The primary culprit is the Marangoni Effect, discovered in 1855 by James Thomson, a physicist with quite the family pedigree—his brother was Lord Kelvin.  This was another “that’s funny” moment: Thomson noticed that drops and rivulets formed on the side of his wine glass at dinner and wondered why this might occur—particularly for wines with high alcoholic content. Wine is mostly alcohol and water, and alcohol has a lower surface tension than water. So water will tend to flow away from any pockets with higher alcohol concentration, because of those slight differences in surface tension.

Then there is capillary action, in which surface tension pulls a liquid up a narrow channel (or the wall of a container, like a wine glass) until its mass becomes heavy enough so that it falls back down. Plants rely on capillary action to transport water and nutrients from stems to leaves. It can also be seen in the so-called “wick effect”: in a kerosene lamp, the kerosene travels up the wick to ensure a slow, steady burn.

In the case of wine (or other adult beverages with high alcohol content, like bourbon, as seen below), the liquid sloshes against the side of the glass and starts to climb up, thanks to capillary action, forming a film. The alcohol and water both evaporate in the process—except they evaporate at different rates because alcohol has a higher vapor pressure. This reduces the alcohol concentration in that region, increasing the surface tension, so even more wine is drawn upward, forming drops that eventually become heavy enough to collapse and run back down the glass in rivulets, sometimes known as tears, legs, curtains, or church windows.

Something similar happens with the tea leaves and chalk particles, helped along by “riding” the vortex currents that inevitably form (the Marangoni effect is technically a kind of convection). We’ve all seen vortices when we stir milk into our coffee, blow smoke rings, or watch water in the bathtub form whirlpools as it swirls down the drain.

These aren’t new phenomena, by any means, but it’s surprising to find they can combine to exert sufficient force to propel tea leaves upstream. The physicists’ conclusions may also explain why contaminating particles sometimes sneak into pipettes holding laboratory samples, or why pollutants may occasionally move upstream in a slowly flowing river.

Bianchi’s “that’s funny” moment isn’t the first time a physics experiment has been inspired by a cup of tea either. A few years ago, Daniel Ives, then a graduate student in mechanical engineering in Colorado, came up with a flow visualization experiment [pdf] to explore how hibiscus tea leaves infuse hot water. He placed a clear glass of hot water against a white paper background and sprinkled a few hibiscus leaves onto the surface, photographing various stages of the diffusion process.

Ives found that hibiscus flowers absorb water and swell, and that absorbed moisture pulls water-soluble particles out of the flowers so that they leach into the water, gradually diffusing outward, and downward, until the tea-infused water reaches equilibrium. The tea leaves, being denser than the water, exhibit an upward buoyant force equivalent to the mass of the water they displace, per the Archimedes principle—the original, and possibly apocryphal, “Eureka!” moment. But gravity is also pulling each particle in the tea leaves downward. It’s a small effect, but just enough to cause the water already saturated with the leached particles to slowly sink in the glass.

Perhaps one day, Ives and his fellow physicists will get around to testing George Orwell’s 1946 assertion that in order to achieve a perfectly infused cup of tea, the water must be boiling, among other exacting conditions. In the meantime, it’s nice to know we can learn something interesting about nature from a simple cup of tea.


Jennifer Ouellette is a science writer and the author of The Calculus Diaries and the forthcoming Me, Myself and Why: Searching for the Science of Self. Follow her on Twitter @JenLucPiquant.


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