Self-diffusion in dilute colloidal suspensions with attractive potential interactions

Short-time diffusion of charge-stabilized colloidal particles: generic features

Analytical theory and Stokesian dynamics simulations are used in conjunction with dynamic light scattering to investigate the role of hydrodynamic interactions in short-time diffusion in suspensions of charge-stabilized colloidal particles. The particles are modeled as solvent-impermeable charged spheres, repelling each otherviaa screened Coulomb potential. Numerical results for self-diffusion and sedimentation coefficients, as well as hydrodynamic and short-time diffusion functions, are compared with experimental data for a wide range of volume fractions. The theoretical predictions for the generic behavior of short-time properties obtained from this model are shown to be in full accord with experimental data. In addition, the effects of microion kinetics, nonzero particle porosity and residual attractive forces on the form of the hydrodynamic function are estimated. This serves to rule out possible causes for the strikingly small hydrodynamic function values determined in certain synchrotron radiation experiments.

Collective diffusion in charge-stabilized suspensions: Concentration and salt effects

The authors present a joint experimental-theoretical study of collective diffusion properties in aqueous suspensions of charge-stabilized fluorinated latex spheres. Small-angle x-ray scattering and x-ray photon correlation spectroscopy have been used to explore the concentration and ionic-strength dependence of the static and short-time dynamic properties including the hydrodynamic function H(q), the wave-number-dependent collective diffusion coefficient D(q), and the intermediate scattering function over the entire accessible range. They show that all experimental data can be quantitatively described and explained by means of a recently developed accelerated Stokesian dynamics simulation method, in combination with a modified hydrodynamic many-body theory. In particular, the behavior of H(q) for de-ionized and dense suspensions can be attributed to the influence of many-body hydrodynamics, without any need for postulating hydrodynamic screening to be present, as it was done in earlier work. Upper and lower boundaries are provided for the peak height of the hydrodynamic function and for the short-time self-diffusion coefficient over the entire range of added salt concentrations.

Latex diffusion at high volume fractions studied by fluorescence microscopy

The behavior of fluorescent latex probes (radii 0.05, 0.1, and 0.5 mum) in latex host particle suspensions was investigated by fluorescence microscopy with image analysis. The volume fraction of the host latex was varied between 0 and 0.50. A careful statistical analysis was performed to examine the accuracy of the fluorescence microscopy method, from which the direct observation of the Brownian motion gives the diffusion coefficient. The method was found to meet all statistical requirements. From rheological measurements, the maximum volume fraction and the intrinsic viscosity can be obtained. The Krieger-Dougherty equation can be used for the prediction of sample viscosities. The predicted viscosities were used to obtain the theoretical diffusion coefficients with the Stoke-Einstein equation. When comparing the theoretical diffusion coefficients with the experimental ones, it turned out that all models tested yielded acceptable predictions of the diffusion coefficients.