Tea leaves defy gravity, move upstream

Mate tea in a calabash gourd
Photo by Jorge Alfonso Hernandez
from Wiki

Water flows downstream, right? So, how can particles, such as tea leaves or chalk particles, floating on downstream-flowing water move upstream? This is a puzzle tackled by researchers recently. In work summarized in Science News here, Sebastian Bianchini describes how, one night when he was an undergraduate in 2008, he was making tea and noticed that by the time he had filled his cup containing tea leaves, some of the tea leaves had marched up into the pristine water in the kettle. In a sad comment on the difficulty of getting unusual work published, he and a physicist at the University of Havanna in Cuba, were unable to get their work published, even though it included some experiments.
    Last year, the duo met Troy Shinbrot, a physicist at Rutgers. Troy replicated the experiment, setting one tank of water 1 cm higher than another. The water flowed down an inclined channel, and into a waterfall 1 cm high.They added chalk and mate tea** to the bottom tank and observed the particles moving up to contaminate the upper tank. The flow is complicated in 3-D: the particles climb up the backside of the waterfall in a series of vortices, to the outside of the channel, and then can be transported back downwards through the center of the channel and over the front of the waterfall.

The experimental setup for the tea leaf experiment
Figure 1 in the referenced paper

     They hypothesized that the tea leaves in the lower tank disturbed the surface tension (bonds of hydrogens that create an elastic network at the surface) and that the particles moved up toward the pure water where the surface tension was higher, effectively climbing along the top of the water surface in defiance of gravity.
     The effect had been recognized earlier by physicists, but the magnitude had not been realized. In a simple back-of-the envelop analysis, the authors show that the surface tension difference between the lower (tea-contaminated) and upper (pure water) reservoirs is about 0.01 N m-1, and that the acceleration (for typical chalk particles) would be about 20 times gravity.
     To test the hypothesis that this is a surface tension effect, the authors dropped a liquid surfactant into the upper and lower reservoirs. When it was dropped into the upper reservoir, the contamination was abruptly eliminated, whereas when it was added to the lower reservoir, the contamination was initiated (if it had not already begun) or accelerated (if it was in progress.) In supplementary material, there are detailed 2-D simulations. There are many more experiments and quantitative analyses in the paper.
     Does this experiment have practical implications? The authors don't know yet, but are suspicious that particles might migrate upstream in a slow-moving river, or be able to sneak into the tips of laboratory pipettes, contaminating samples.

**Mate tea is a traditional South American tea made by infusing the leaves of yerba mate with hot water. The traditional way of brewing it is shown in the picture above.

Reference:

S. Bianchini et al. Upstream contamination by floating particles. Proceedings of the Royal Society A. July 3, 2013. doi: 10.1098/rspa.2013.0067.