Certain vibrant colors found in nature, such 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 that researchers have tried to produce using structural color typically carry a blue tint. These properties limit the application of structural color in coatings and paints, where pure, matte colors are the goal. Now, scientists have found a way to make purer shades of red by patterning a polymer with air pockets —and the resulting material can be tuned to produce a full spectrum of matte structural colors (ACS Appl. Mater. Interfaces 2016, DOI: 10.1021/acsami.6b03217).
Arranging nanoparticles in orderly crystals produces iridescent structural colors, but making matte colors requires less orderly, amorphous arrangements. Matte blues are easy to produce this way, but reds are tricky. The space between particles can be adjusted so that a material reflects red light, but since nanoparticles themselves tend to scatter blue light, any red structural color holds hints of blue.
In 2014, a theoretical study predicted that the best way to make matte reds would be with particles that had a refractive index similar to that of air (Phys. Rev. E, DOI: 10.1103/PhysRevE.90.062302). To test this idea, Shin-Hyun Kim of Korea Advanced Institute of Science & Technology and his colleagues dispersed tiny silica particles in polyethylene glycol dimethacrylate. The particles packed closely into an amorphous array, but the team controlled the particle concentration so that no two silica particles touched one another. They cross-linked the acrylate molecules, creating a solid resin, which they treated with strong acid to dissolve the silica, leaving the polymer intact. The resulting film—a polymer layer with pockets of air approximately 200 nm in diameter—produced a matte red color. By using 168-nm- or 185-nm-diameter silica particles instead, the team made blue and green films. In future work, the group will try producing films that incorporate multiple colors into a single sheet.
Xiaodong Yang of Missouri University of Science & Technology notes that this method and the polymer used are low cost and can likely be manufactured easily at large scales.
Vinothan N. Manoharan of Harvard University, an author of the 2014 theoretical study and a previous collaborator of Kim’s, says that these are “some of the best results yet” for matte red and green structural colors.
Bird feathers that produce bright blues in nature also rely on air pockets of varied shapes, sizes, and patterns. “It will be interesting to see how changing the structure of pores in addition to their size and shape will influence color,” Manoharan adds.