Researchers from Pennsylvania State University (Penn State) and the Massachusetts Institute of Technology (MIT) have shown how a clear surface covered in transparent droplets and lit with a single white-light lamp can produce bright, iridescent colors if each droplet is precisely the same size. While studying transparent droplet emulsions made from a mixture of oils of different density and water-based surfactants, the researchers observed that the drops appeared blue. Initially they thought that the effect was due to Mie scattering (the same effect that causes rainbows). However, the hemispherical droplets were not perfect spheres. The concave surface of a hemisphere allows total internal reflection (TIR), an optical effect that is not possible in perfect spheres. A petri dish containing transparent droplet emulsions made from a mixture of oils of different density and water-based surfactants photographed from different angles. When illuminated with white light, the oil droplets reflect different colors depending on the viewing direction. New research explains how this “structural color” is formed based on the size and curvature of the droplets, along with the droplet’s total internal reflection. Courtesy of Zarzar laboratory, Penn State. When light entered a single droplet, it was reflected by TIR along its concave interface. The researchers further found that once light made its way into a droplet, it could take different paths, bouncing two, three, or more times before exiting at another angle. The way the light rays “added up” as they exited the droplet determined whether the droplet produced color. For example, two rays of white light, containing all visible wavelengths and entering and exiting a droplet at the same angle, could take different paths within a droplet. If one ray bounced three times, it would lag behind a ray that bounced twice before exiting. If this lag resulted in the wavelengths of both rays being in phase, the color corresponding to that wavelength would be visible. This interference effect appeared much stronger in small than in large droplets, the researchers observed. Structural color from clear water droplets. Microscale water droplets condensed onto a clear plastic sheet reflect different bright colors based on their size. Courtesy of Zarzar laboratory, Penn State. The color that the droplets produced also depended on structural conditions, such as the size and curvature of the droplets, along with the droplet’s refractive indices. The researchers incorporated these parameters into a mathematical model to predict the colors that droplets would produce under certain structural and optical conditions. They tested the model’s predictions against actual droplets they produced in the lab. First, they created droplet emulsions whose sizes could be controlled using a microfluidic device. They produced a “carpet” of same-size droplets that they illuminated with a single, fixed white light. They then recorded the droplets with a camera that circled around the petri dish, and observed that the droplets exhibited brilliant colors that shifted as the camera circled around. This demonstrated how the angle at which light was seen to enter the droplet affected the droplet’s color. The team also produced droplets of various sizes on a single film and observed that from a single viewing direction, the color would become redder as the droplet size increased, and then would loop back to blue and cycle through again. This made sense according to the model, as larger droplets would give light more room to bounce, creating longer paths and larger phase lags. To demonstrate the importance of curvature in a droplet’s color, the team produced water condensation on a transparent film that was treated with a hydrophobic solution, with the droplets forming the shape of an elephant. The hydrophobic parts created more concave droplets, whereas the rest of the film created more shallow droplets. Light could more easily bounce around in the concave droplets, compared to the shallow droplets. An image of a penguin reflected from oil in water droplets; images of each type of droplet reflecting the blue and green light are shown. The penguin is made by using a light-responsive surfactant and photopatterning the droplet shape. Courtesy of Zarzar laboratory, Penn State. In addition to liquid droplets, the researchers 3D-printed tiny, solid caps and domes from various transparent, polymer-based materials, and observed a similar colorful effect in these solid particles that could be predicted by the team’s model. By tuning size, illumination angle, and curvature, engineers can produce brilliant colors, in patterns they can predict, in otherwise transparent droplets. Courtesy of Felice Frankel/MIT. This newly explained form of structural color could aid in the development of brilliantly colored, dye-free cosmetics; color-changing paints; or adaptive camouflage. It could also be applied to lighting displays. The team believes that its model could be used to design droplets and particles for an array of color-changing applications. “There’s a complex parameter space you can play with,” said MIT professor Mathias Kolle. “You can tailor a droplet’s size, morphology, and observation conditions to create the color you want.” “The typical way you get color is with dyes or pigments, which have molecules that selectively absorb and scatter specific wavelengths of light,” said Penn State professor Lauren Zarzar. “Structural color is different. It’s a product of light interacting with a material in a way that causes light interference.” The research was published in Nature (https://doi.org/10.1038/s41586-019-0946-4). Engineers at MIT and Penn State University 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. Courtesy of MIT.