Ever noticed that if you spill coffee on to a table and let it dry, the colour will be concentrated at the edges of the stain?
The intriguing phenomenon has been put under the microscope, and scientists say their findings may encourage a revolution in printing, paints and product coatings.
The "coffee-ring effect", they report, derives from two factors: the shape of the particles - molecules of coffee, ink, dye, and so on - in the liquid and the way these particles respond to surface tension.
Round particles tend to gather at the perimeter of the drop, which explains why they remain in a ring once it has dried, according to the research. But particles that are elongated or ellipsoid distribute themselves in looser clumps, which makes it easier to smooth them across the entire surface.
"This work gives us a new idea about how to make a uniform coating, relatively simply," Arjun Yodh of the University of Pennsylvania said.
"If you change the particle shape, you can change the way a particle is deposited. "You can also make mixtures. In some cases, even just a small amount of ellipsoids can change the way the particles deposit when they dry."
The investigation was published in Nature, the British science journal.