Penn Physicists Explain the Science Behind 'Coffee Rings'

In a study published in the physics journal Physical Review Letters, Arjun Yodh, director of the Laboratory for Research on the Structure of Matter, and his team built upon their previous research on the “coffee-ring effect,” a commonplace occurrence when drops of liquid with suspended particles dry, leaving a ring-shaped stain at the drop’s edges.

The original team, which also included Penn doctoral candidates Peter Yunker and Matthew Lohr and postdoctoral fellow Tim Still, collaborated with Professor D.J. Durian of the Department of Physics and Astronomy and MIT's Alexei Borodin for the new research. Exploring how those particles stack up as they reach the drop’s edge, they discovered that different particles make smoother or rougher deposition profiles depending on their shape. These resultant growth profiles offer tests of deep mathematical ideas about growing interfaces and are potentially relevant for many commercial and industrial coating applications.

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