Coffee stains point way to making tiny lines

As a drop of coffee evaporates, capillary flows shuttle residue to the diminishing drop's edges, leaving behind dark rings. Saurabh Vyawahare, Kate M. Craig, and Axel Scherer of California Institute of Technology have found that this phenomenon can be exploited to make extremely fine lines by using surfactant-containing liquids (Nano Lett., published online Jan. 5, dx.doi.org/10.1021/nl0522678). Potential applications include making nanowires and light-manipulating photonic crystals constructed from arrays of tiny ceramic or polymeric spheres that can be carried along and organized by the capillary flows, Vyawahare says. Instead of letting the line-forming solutions evaporate in the open the way a coffee drop does, the researchers sandwich the liquids between wettable surfaces. When they use photolithography or other means to create "pinning points" on the surfaces, the evaporating liquid recedes from the points, reeling out a filament of liquid. By including fluorescent quantum dots in the solution, the researchers can visualize the process and the result, such as the line formed between two pinning points shown above.