National Science Foundation-funded engineers have found that under the right conditions, ordinary clear water droplets on a transparent surface can produce brilliant colors, without the addition of inks or dyes. In a paper published this week, the team reports that a surface covered in a fine mist of transparent droplets and lit with a single lamp should produce a bright color if each tiny droplet is precisely the same size. This iridescent effect is due to "structural color," by which an object generates color simply due to the way light interacts with its geometric structure. The effect may explain certain iridescent phenomena, such as the colorful condensation on a plastic dish or inside a water bottle. The researchers have developed a model that predicts the color a droplet will produce, given specific structural and optical conditions. The model could be used as a design guide to produce, for example, droplet-based litmus tests, or color-changing powders and inks in makeup products.
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