Penn Physicists Help Show Math Behind Growth of 'Coffee Rings'

Last year, a team of University of Pennsylvania physicists showed how to undo 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. Now the team is exploring how those particles stack up as they reach the drop's edge, and they discovered that different particles make smoother or rougher deposition profiles at the drop edge 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. The new research was conducted by the members of the original team: 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. New to the collaboration were D.J. Durian of the Department of Physics and Astronomy and Alexei Borodin, professor of mathematics at the Massachusetts Institute of Technology. Their study was published in the journal Physical Review Letters. In the "coffee-ring effect," drop edges are "pinned" to a surface, meaning that when the liquid evaporates, the drop can't shrink in circumference and particles are convectively pushed to its edges.