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Colour effects inspired by instant soup

September 20, 2002

The shimmering colors of beetle and butterfly wings are not usually due to pigments. Instead, these and rainbow effects are produced by interference - as in opal gemstones, or an oil slick on a wet road. Layers of colorless substances, of a thickness that lies within the wavelength of visible light, cancel out certain parts of the spectrum. The reflected wavelengths create the color impression.

Not surprisingly, industrial manufacturers have long been making use of this effect to produce coatings for optical devices such as lamps and filters. One method, not only used by the company Prinz Optics, is the sol-gel process. In this particular example, a solution of titanium chloride in ethyl alcohol is applied to a glass substrate. As the solvent gradually evaporates. Eventually sol particles in solution form a layer of gel. Subsequent heating dries them out completely, forming a transparent layer of ceramic titanium dioxide. The application of further layers finally leads to a colored interference filter that will never bleach.

This process has a drawback: The solutions used are difficult to handle, because suspended solid particles slowly form and aggregate inside the storage container. This significantly influences the optical characteristics of the filters to be produced. In order to improve the process, Prinz turned to the scientists of the Fraunhofer Institute for Silicate Research ISC in Würzburg. Dr. Peer Löbmann describes how they solved the problem: "Our method is rather similar to that of making soup using dehydrated instant stock," he says. "Fresh soup doesn't keep for very long, and its ingredients eventually spoil. In more or less dehydrated form, these problems don't occur." The researchers therefore developed a coating system based on powders, which can be stored almost infinitely. The solvent is not added until shortly before the coating process, and even this resulting "soup" stays fresh for longer than before.

Despite the lighthearted description, the new system is based on a great deal of serious expertise. Further advantages: Thicker individual layers can be produced and less of them create the desired effect. It is even easier to produce uniformly changing colors by gradually increasing the thickness of the layers as they are deposited across the substrate. "This is where our 'recipe' differs from the culinary version: Our instant soup 'tastes' better than the original product," concludes Löbmann. The improved coating systems can be seen at the Materialica trade fair in Munich from 30 September to 3 October.

Fraunhofer-Gesellschaft