“Coffee Ring Effect” Counteracted by Bacterial Surfactants

Ever notice how a dried coffee stain has a thicker outer rim, while the middle of the stain remains almost unsoiled? This “coffee ring effect” also occurs in other materials. Researchers from the Departments of Chemical Engineering and Chemistry at KU Leuven in Belgium have now discovered how to counteract coffee rings with “surfactants,” i.e., soap. The key to the discovery was not a kitchen towel, but a bacterium that counteracts the coffee ring effect at the microscopic level. The findings were published on April 23, 2013 in an open-access article in Nature Communications. When a coffee ring dries, its edges become noticeably darker and thicker. This occurs because the coffee particles move toward the edge of the stain while the water in the liquid evaporates. At a microscopic level, this coffee ring effect can also be seen in liquids with particles of other materials such as plastic and wood. In various industrial applications – applying an even coat of paint or varnish, for example – the coffee ring effect can be particularly troublesome and scientists have long been seeking ways to counteract it. Dr. Raf De Dier and Dr. Wouter Sempels (Departments of Chemical Engineering and Chemistry) have now described a solution based on examples found in nature. Drs. De Dier and Sempels carried out experiments and calculations on nanomaterials, as well as on a particularly promising bacterium, Pseudomonas aeruginosa. Pseudomonas aeruginosa is a dangerous bacterium that can cause infections in open wounds. “A Pseudomonas aeruginosa bacteria colony wants to find as large a breeding ground as possible.
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