Scientists at Keio University in Yokohama, Japan, have developed a colloidal “ink” that becomes iridescent upon drying as a result of diffraction. Such materials that produce colors structurally have a variety of potential applications, including as colorimetric chemical and physical sensors and as alternatives to the variable pigments used as anticounterfeiting measures on documents and banknotes.The colloidal “ink” becomes iridescent upon drying as a result of diffraction. It works equally well on glass, plastic and gold. Courtesy of Haruma Kawaguchi, Keio University.The new ink is an aqueous dispersion of poly(N-isopropylacrylamide) microspheres produced by precipitation polymerization, with the size of the particles determined by the reaction parameters. As the swollen particles dry on a surface to which they have been applied, capillary forces cause them to settle into a regular pattern. When all of the water has evaporated, the flattened spheres remaining form a periodic structure that diffracts light.In tests with spheres that had hydrodynamic diameters of 964 and 848 nm, the researchers found that the suspension of larger particles produced more brilliantly iridescent films. They also found that the ink formed the films when it was sprayed or written onto a surface, and that the films formed equally well on glass, plastic and gold.