Making bright colors out of thin air

Certain vibrant colors found in nature, such as the blues and purples in bird feathers and butterfly wings, are created not by pigments but by physical nanostructures that interact with light, a phenomenon known as structural color. Such colors are often shimmery, and the reds and greens typically carry a blue tint—both properties that limit using structural color to make paints and coatings, where matte colors are often the goal. Shin-Hyun Kim of Korea Advanced Institute of Science & Technology and colleagues have found a way to make a full spectrum of pure matte colors without pigments by patterning a polymer with air pockets (ACS Appl. Mater. Interfaces 2016, DOI: 10.1021/acsami.6b03217). Prior theoretical work predicted that the best way to make matte reds would be with particles that have a refractive index similar to that of air. Kim and coworkers tested this idea by dispersing silica nanoparticles in polyethylene glycol dimethacrylate, which they then cross-linked to create a solid resin. Next, they used acid to dissolve the silica, leaving a hodgepodge of air pockets approximately 200 nm in diameter, which produced a matte red film that could conceivably be used to color various surfaces. By creating material with 168-nm or 185-nm silica particles, the team made blue and green films.