A team of University of Pennsylvania physicists has shown how to disrupt the “coffee ring effect” — the ring-shaped stain of particles left over after coffee drops evaporate — by changing the particles' shape, a discovery that might provide new tools for engineers to deposit uniform coatings.
The research was conducted by professor Arjun Yodh, director of the Laboratory for Research on the Structure of Matter; doctoral candidates Peter Yunker and Matthew Lohr; and postdoctoral fellow Tim Still, all of the department of Physics and Astronomy in Penn’s School of Arts and Sciences.
Their research will be published in the journal Nature on August 18.
“The coffee ring effect is very common in everyday experience,” Yunker said. “To avoid it, scientists have gone to great lengths designing paints and inks that produce an even coating upon evaporation. We found that the effect can be eliminated simply by changing the shape of the particle.”
The edges of a water drop sitting on a table or a piece of paper, for example, are often “pinned” to the surface. This means that when the water evaporates, the drop can’t shrink in circumference but instead flattens out. That flattening motion pushes water and anything suspended in it, such as coffee particles, to its edges. By the time the drop fully evaporates, most of the particles have reached the edge and are deposited on the surface, making a dark ring.
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