DRIP coffee on a table, let it dry and the stain will be shaped like a ring. Now there鈥檚 a way to stop this pesky 鈥渃offee ring effect鈥�, which causes paints and inks to coat surfaces unevenly.
As a splotch of coffee loses water to evaporation, its outer rim refuses to shrink inwards, held in place by the microscopic roughness of the tabletop. Instead, water flows from the drip鈥檚 centre to the rim and evaporates there, leaving behind the dark particles that give coffee its colour. The same thing occurs in particle-laden inks and paints.
In a bid to beat the problem, Peter Yunker of the University of Pennsylvania in Philadelphia and colleagues watched drops of alcohol dry. When laden with polystyrene spheres, the particles concentrated into a ring. But when the particles were torpedo-shaped, they coated the surface uniformly after drying. This was also true when a small fraction of elongated particles made up a solution of mainly spherical ones (Nature, ). The torpedoes don鈥檛 slide along as easily as the spheres, it seems.
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Adding a few jamming torpedoes to ink or paint could be enough to block the flow of particles, leading to an even coat.
