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August 12, 2011

The shape of particles in a drop of liquid plays a role in the "coffee ring effect."

This illustration represents a how a dried drop would appear if it contained round particles (red) or elongated particles (blue).

When a drop of coffee or tea dries, its particles (which are round) leave behind a ring-like stain called the "coffee ring effect" (upper left). But if you change the shape of the particles, the coffee stain behavior changes too. Elongated particles (blue) do not exhibit the coffee ring effect, rather they are deposited across the entire area of the drop, resulting in a uniformly dark stain (lower right).

Credit: Felice Macera, University of Pennsylvania


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