Experimental and theoretical evidence for molecular forces driving surface segregation in photonic colloidal assemblies

M Xiao and ZY Hu and TE Gartner and XZ Yang and WY Li and A Jayaraman and NC Gianneschi and MD Shawkey and A Dhinojwala, SCIENCE ADVANCES, 5, eaax1254 (2019).

DOI: 10.1126/sciadv.aax1254

Surface segregation in binary colloidal mixtures offers a simple way to control both surface and bulk properties without affecting their bulk composition. Here, we combine experiments and coarse-grained molecular dynamics (CG-MD) simulations to delineate the effects of particle chemistry and size on surface segregation in photonic colloidal assemblies from binary mixtures of melanin and silica particles of size ratio (Dlarge/Dsmall) ranging from 1.0 to similar to 2.2. We find that melanin and/or smaller particles segregate at the surface of micrometer- sized colloidal assemblies (supraballs) prepared by an emulsion process. Conversely, no such surface segregation occurs in films prepared by evaporative assembly. CG-MD simulations explain the experimental observations by showing that particles with the larger contact angle (melanin) are enriched at the supraball surface regardless of the relative strength of particle-interface interactions, a result with implications for the broad understanding and design of colloidal particle assemblies.

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