Structural Color Production in Melanin-based Disordered Colloidal Nanoparticle Assemblies in Spherical Confinement

Melanin is a ubiquitous natural pigment that exhibits broadband absorption and high refractive index. Despite its widespread use in structural color production, how the absorbing material melanin affects the generated color is unknown. Using a combined molecular dynamics and finite-difference time-domain computational approach, we investigate structural color generation in one-component melanin nanoparticle-based supraballs as well as binary mixtures of melanin and silica (non-absorbing) nanoparticle-based supraballs. We validate our approach by demonstrating that experimentally produced one-component melanin and one-component silica supraballs produce reflectance profiles similar to the computational analogues. We isolate the influence of melanin's absorption, packing fraction, and nanoparticle size dispersity on color generation in one-component melanin supraballs in our optical simulations. We also extend our computational approach to study how nanoparticle stratification and degree of interparticle mixing/demixing of melanin and silica nanoparticles affect optical properties. These results provide design principles for synthesizing melanin-based systems to control color, saturation, and lightness factor for applications as pigments for cosmetics, paints, and food coloring.