When the liquid at the edge of the drop begins to dry, surface tension draws liquid from the centre dragging spherical particles with it (Source: Luiz Baltar/stock.xchng)
Shape matters 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.Their investigation is published in the journal Nature.
The 'coffee-ring effect', they report, derives from two factors: the shape of the particles in the liquid and the way these particles - whether it be molecules of coffee, ink or dye - respond to surface tension.
When the liquid at the edge of the drop begins to dry, surface tension draws liquid from the centre dragging spherical particles with it, according to the research.
To eliminate this effect, the researchers used ellipsoid particles suspended in liquid. They found these particles distribute themselves in looser clumps, which make it easier to smooth them across the entire surface.
They found that adding small amounts of ellipsoids to a suspension of spheres - as little as 0.015 per cent - was enough to suppress the formation of rings.
"This work gives us a new idea about how to make a uniform coating, relatively simply," says Arjun Yodh of the University of Pennsylvania.
"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."
