Development of surface composition and ordering during reverse-emulsion assembly of binary colloidal particle mixtures

Nanoparticle assembly at a fluid-fluid interface is a proven route to precisely engineer materials with controlled optical properties. In this talk, we will present our recent work using coarse-grained Langevin dynamics simulation that mimics reverse-emulsion directed assembly of binary nanoparticle mixtures into supraballs. We model a binary mixture of silica and synthetic melanin particles in implicit solvent within a shrinking spherical confinement to replicate the shrinking reverse-emulsion droplet. The simulation protocol captures the physics of this process by reproducing the experimental observation of melanin and/or smaller nanoparticles enriching the water-octanol interface. For all mixtures, we observe enrichment of the melanin particles at the supraball surface compared to the supraball interior. This enrichment decreases with increasing melanin/silica size ratio. We observe appreciable crystalline ordering only for systems with similarly sized particles. Particle size dispersity, finite assembly timescale, and curvature of the supraball surface all serve to suppress particle ordering. These findings serve as design rules for tailoring the supraballs for structural color applications and improve our fundamental understanding of particle assembly near curved interfaces.