Colors are generated via one of two methods; the absorption of light by small molecules (pigmentary colors) or by the scattering of light by nanostructures (structural color). There are many examples of structural color in nature including the wing feathers of the green-winged teal (<i>Anas crecca</i>) and feathers of the wild turkey (<i>Meleagris gallopavo</i>), both of which use different types of melanosomes to generate these colors. There is great interest in synthetic materials capable of generating structural colors due to their tunability and improved resistance to bleaching (chemical and photo). Previously, our lab described the development of synthetic melanin-based core-shell particles that were assembled into three dimensional photonic balls, termed supraballs, which showed bright and noniridescent colors. When these synthetic melanin particles were combined in a binary system, we observed that they appeared to localize on the surface of the supraballs, which greatly affects their colors. However, the mechanism and properties that influence this surface segregation lacks understanding. Surface segregation in binary colloidal particle mixtures offers a simple way to control surface properties and colors of colloidal assemblies without affecting their bulk composition. In our study, we synthesized different sizes of synthetic melanin particles and silica particles, combined them in a binary mixture, and assembled them into supraballs by a facile one-pot reverse emulsion process. Interestingly, we observed that melanin-based and/or smaller particles preferentially segregate at the surface of the supraballs compared to their larger counterparts, while no such surface segregation is observed in films prepared by evaporative assembly. Coarse-grained molecular dynamics (CG-MD) simulations suggest that differences in the contact angle of particles at the liquid-liquid interface during emulsion assembly drives the observed surface segregation behavior with particles having a larger contact angle (melanin) being enriched at the supraball surface regardless of the relative strength of particle-interface interactions.

Surface segregation of binary particles in photonic colloidal assemblies

2019 Fall National Meeting

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