Surface-dictated sorting in colloidal mixtures

In colloidal mixtures, different particle types coexist in the same suspension environment yet exhibit intrinsically distinct surface chemistries. Here we present surface-dictated sorting rules that enable selective assembly among mixed colloids. Using seven types of particles of equal size (∼2 μm) representing common families of negatively charged stable colloids, including inorganic spheres bearing native surface hydroxyls groups (M-OH, e.g. SiO₂, TiO₂) or grafted carboxyl groups, organic polymer particles synthesized by emulsion polymerization (sulfonated PS and PMMA, carboxylated PS), and an organosilane-derived colloid with silanol surface (TPM), we identify three general strategies: (1) pH and solvent dielectric tuning, suppresses the dissociation of weakly acidic surface groups, driving particles with near-neutral M-OH or -COOH sites to clusterize; (2) Small molecules with multiple hydrogen-bonding sites (i.e., urea), reorganize interfacial hydrogen-bond networks, selectively destabilizing the surfaces of hydroxyl-rich inorganic particles; (3) Depletion-driven assembly with Pluronic surfactants. Upon selective adsorption onto hydrophobic particles (e.g., polystyrene), Pluronic copolymers form a stable surface brush layer, increasing the effective radius of the particles. This expanded depletion zone creates greater overlap as particles approach, resulting in enhanced depletion attraction. When combined with disparities in intrinsic surface potential, this brush-mediated interaction yields distinct overall potentials between hydrophobic and hydrophilic particles, thereby driving the preferential assembly of the hydrophobic ones. Applying this strategy to SiO₂/Pt Janus particles further induces neutral charged Pt-facet clustering under depletion, yielding oriented clusters and chain-like assemblies.