Packing polydisperse colloids into crystals: when charge-dispersity matters
Monte-Carlo simulations and small-angle x-ray scattering experiments were used to determine the phase diagram of aqueous dispersions of titratable nano-colloids with a moderate size polydispersity over a broad range of monovalent salt concentrations, 0.5 mM $\leq c_s \leq$ 50 mM and volume fractions, $\phi$. Under slow and progressive increase in $\phi$, the dispersions freeze into a face-centered-cubic (fcc) solid followed unexpectedly by the formation of a body centered cubic (bcc) phase before to melt in a glass forming liquid. The simulations are found to predict very well these observations. They suggest that the stabilization of the bcc solid at the expense of the fcc phase at high $\…
Hiding in plain view: Colloidal self-assembly from polydisperse populations.
We report small-angle x-ray scattering (SAXS) experiments on aqueous dispersions of colloidal silica with a broad monomodal size distribution (polydispersity 18%, size 8 nm). Over a range of volume fractions the silica particles segregate to build first one, then two distinct sets of colloidal crystals. These dispersions thus demonstrate fractional crystallization and multiple-phase (bcc, Laves AB$_2$, liquid) coexistence. Their remarkable ability to build complex crystal structures from a polydisperse population originates from the intermediate-range nature of interparticle forces, and suggests routes for designing self-assembling colloidal crystals from the bottom-up.