United States. Researchers at the University of Pennsylvania, led by Arjun Yodh, discovered how to take advantage of the effect known in industry as the "ring coffee" to discover new techniques or formulations in coatings.
This effect can be recognized the moment a drop of coffee is spilled on a surface and the curious way in which the color is concentrated at the edges when the coffee is dried is observed. It is for that reason that the researchers determined that the shape of the particles in the liquid is an important factor in creating that pattern.
"We've found that if you change the shape of the particles in the solution, the effect of the 'coffee ring' goes away, and you can end up with a uniform coating," said Peter Yunker, a graduate student in Yodh's lab.
"If the particles are lengthened, the air-water interface is deformed, causing the particles to attract strongly to each other. This effect can be observed in a cup of cereals. If there are only a few left, they are grouped in the center of the cup, due to the tension of the milk surface," Yunker explained.
"This work gives us a new idea on how to make a uniform layer relatively simply. If you change the shape of particles, you can change the way a particle is deposited. In some cases, even just a small amount of ellipsoids can change the way particles are deposited when they dry," Yodh said.
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