Structurally determined Brownian dynamics in ordered colloidal suspensions : self-diffusion in fluid, supercooled, and crystalline phases

Abstract

Electrostatically interacting colloidal suspensions at medium to very low concentrations were prepared in differently ordered phases using the method of continuous deionization. Equilibrium phase states include fluid, mono- and polycrystalline material as well as coexistence between fluid and monocrystal. A nonequilibrium supercooled fluid state is reproducibly reached by shear melting. In these phases the long time self-diffusion coefficient DL was measured by forced Rayleigh scattering, while the potential of interaction was systematically varied by changing salt concentration c and volume fraction Φ. In the equilibrium fluid DL decreases by roughly an order of magnitude as the interaction increases. This trend extends continuously into the supercooled state. In all cases crystallization is observed for DL/DO < 0.10 (1). In the polycrystalline phases self-diffusion coefficients are still 1 to 2 orders of magnitude lower than in the supercooled state. Here self-diffusion increases with increasing interaction. For the monocrystalline case upper limits of DL are given. These data on the solid phases are discussed in terms of grain boundary and vacancy diffusion.