The influence of structure on diffusion in screened Coulombic suspensions

Abstract

The static structure factor and hydrodynamics, characterized by the functions S(q) and H(q), respectively, will modify the short-time q-dependent diffusion coefficient from its free particle value, D0, as D(q)=D0[H(q)/S(q)]. We probe these dynamics with diffusing wave spectroscopy on low volume fraction suspensions of highly charged, monodisperse colloidal particles, where we can neglect the hydrodynamic contributions and probe the effect of structure alone. We vary the range of the screened Coulombic interactions via suspension ionic strength to obtain a range of interparticle structures from liquid-like to ordered crystalline arrays. From the initial decay of the autocorrelation function of the multiply scattered laser light, we obtain a q-weighted average of D(q). Modeling the interactions with the hard-core Yukawa pair potential and the structure with the rescaled mean spherical approximation, we attribute slowed dynamics in the suspension to peaks in S(q). We vary the particle size, laser wavelength, and suspension volume fraction to probe the effect of various features in S(q) on the dynamics.