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UPenn's 'Coffee Ring Effect' Research Could Pave Way for Better Coatings, Inks & Paints Formulation

Published on 2011-08-19. Author : SpecialChem

PHILADELPHIA - 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. The discovery provides 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.

"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.

University of Chicago physicists Sidney Nagel, Thomas Witten and their colleagues wrote an influential paper about this process in 1997, which focused mainly on suspended spherical particles, but it was not until the Yodh team's recent experiments that the surprising role played by suspended particle shape was discovered.

About University of Pennsylvania

With 165 research centers and institutes, research is a substantial and esteemed enterprise at Penn. As of fiscal year 2011, the research community includes over 4,000 faculty and over 1,100 postdoctoral fellows, over 5,400 academic support staff and graduate student trainees, and a research budget of $814 million. The scale and interdisciplinary character of our research activities make Penn a nationally-ranked research university.

Source: University of Pennsylvania Research


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