|Mate tea in a calabash gourd|
Photo by Jorge Alfonso Hernandez
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
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.
S. Bianchini et al. Upstream contamination by floating particles. Proceedings of the Royal Society A. July 3, 2013. doi: 10.1098/rspa.2013.0067.