Coffee stain helps smarter inks

Ever noticed that if you spill coffee onto 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 believe 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 in the liquid and the way these particles respond to surface tension. "Particles" mean the molecules of coffee, ink or dye or whatever that are in suspension in the liquid. 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," said Arjun Yodh of the University of Pennsylvania in a press release. "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 is published on Thursday in Nature, the British science journal. The university has posted an illustrative video on YouTube (http://www.youtube.com/watch?v=ZaCGoSTMHyc).