Structure and dynamics of polymer colloid suspensions from dynamic light scattering and Brownian dynamics simulation

Short- and long-time diffusion and dynamic scaling in suspensions of charged colloidal particles

We report on a comprehensive theory-simulation-experimental study of collective and self-diffusion in concentrated suspensions of charge-stabilized colloidal spheres. In theory and simulation, the spheres are assumed to interact directly by a hard-core plus screened Coulomb effective pair potential. The intermediate scattering function, f_c(q, t), is calculated by elaborate accelerated Stokesian dynamics (ASD) simulations for Brownian systems where many-particle hydrodynamic interactions (HIs) are fully accounted for, using a novel extrapolation scheme to a macroscopically large system size valid for all correlation times. The study spans the correlation time range from the colloidal short-time to the long-time regime. Additionally, Brownian Dynamics (BD) simulation and mode-coupling theory (MCT) results of f_c(q, t) are generated where HIs are neglected. Using these results, the influence of HIs on collective and self-diffusion and the accuracy of the MCT method are quantified. It is shown that HIs enhance collective and self-diffusion at intermediate and long times. At short times self-diffusion, and for wavenumbers outside the structure factor peak region also collective diffusion, are slowed down by HIs. MCT significantly overestimates the slowing influence of dynamic particle caging. The dynamic scattering functions obtained in the ASD simulations are in overall good agreement with our dynamic light scattering (DLS) results for a concentration series of charged silica spheres in an organic solvent mixture, in the experimental time window and wavenumber range. From the simulation data for the time derivative of the width function associated with f_c(q, t), there is indication of long-time exponential decay of f_c(q, t), for wavenumbers around the location of the static structure factor principal peak. The experimental scattering functions in the probed time range are consistent with a time-wavenumber factorization scaling behavior of f_c(q, t) that was first reported by Segrè and Pusey [Phys. Rev. Lett. 77, 771 (1996)] for suspensions of hard spheres. Our BD simulation and MCT results predict a significant violation of exact factorization scaling which, however, is approximately restored according to the ASD results when HIs are accounted for, consistent with the experimental findings for f_c(q, t). Our study of collective diffusion is amended by simulation and theoretical results for the self-intermediate scattering function, f_s(q, t), and its non-Gaussian parameter α₂(t) and for the particle mean squared displacement W(t) and its time derivative. Since self-diffusion properties are not assessed in standard DLS measurements, a method to deduce W(t) approximately from f_c(q, t) is theoretically validated.

Highly charged inorganic-organic colloidal core-shell particles

Highly defined, hybrid inorganic-organic colloidal core-shell particles consisting of a silica core and a shell of fluorinated acrylate are prepared in a two-step route. The core-shell structure of the particles is investigated by means of small-angle X-ray scattering (SAXS). Because of highly acidic sulfonic acid surface groups resulting from the radical initiator sodium peroxodisulfate at the organic shell, long-range electrostatic interactions lead to the formation of liquidlike mesostructures. Increasing the effective interaction by reducing the next-neighbor distances induces a freezing of the liquidlike structures, i.e., a transition to crystalline and glassy structures. Because of the high electron density in the core and the fluorinated polymer shell, these particles are strong X-ray scatterers. In combination with the large number of effective charges and the outstanding monodispersity, these core-shell particles are a promising model system for the investigation of the glass transition by photon correlation spectroscopy employing coherent X-rays.

Long-time dynamics of concentrated charge-stabilized colloids

Dynamic light scattering was used to study the dynamic structure factor, S(q,t), of suspensions of charged colloidal silica spheres over the full colloidal time range. We show that a dynamic scaling relation for S(q,t) found by Segrè and Pusey [Phys. Rev. Lett. 77, 771 (1996)10.1103/PhysRevLett.77.771] for hard spheres, relating long-time and short-time dynamics, and collective and self-diffusion, also applies to charged colloids up to the freezing concentration. The universality of this scaling is analyzed theoretically. Our experimental data confirm dynamic freezing criteria proposed for the long-time self- and cage-diffusion coefficients, along with a theoretical prediction for the self-diffusion coefficient.

Experimental study of nonequilibrium fluctuations during free diffusion in nanocolloids using microscopic techniques

We report quantitative experimental results regarding concentration fluctuations based on a small-angle light-scattering setup. A shadowgraph technique was used to record concentration fluctuations in a free-diffusion cell filled with colloids. Our experimental setup includes an objective attached to the CCD camera to increase the field of view. We performed two separate experiments, one with 20 nm gold and the other with 200 nm silica colloids, and extracted both the structure factors and the correlation time during the early stages of concentration fluctuations. The temporal evolution of fluctuations was also qualitatively investigated using recursive plots and spatial-temporal sections of fluctuating images. We found that the correlation time versus wavenumber for gold nanocolloids is concave shaped, whereas, for silica colloids, it is convex shaped. The difference in correlation time behavior is not only due to the size of the particle, but also to possible plasmonic interactions in gold colloids.

Field induced anisotropy of charged magnetic colloids: A rescaled mean spherical approximation study

The liquidlike structure of colloidal suspensions with both electrostatic and magnetic interactions is investigated by means of small angle x-ray scattering (SAXS) dependent on an external magnetic field. For weak magnetic interactions, without external field, the magnetic dipoles are randomly oriented. Under this condition, isotropic structures are observed. In an external field, however, the magnetic momenta arrange parallel to the external field and induce anisotropic liquidlike structures. For weak magnetic interactions, the structure factor can be described within the framework of the rescaled mean spherical approximation. Due to the high experimental accuracy of synchrotron SAXS, from the anisotropic distortion of liquidlike structures, interparticle forces smaller than 10(-15) N can easily be detected.

Self-consistent theory of collective Brownian dynamics: theory versus simulation

A recently developed theory of collective diffusion in colloidal suspensions is tested regarding the quantitative accuracy of its description of the dynamics of monodisperse model colloidal systems without hydrodynamic interactions. The idea is to exhibit the isolated effects of the direct interactions, which constitute the main microscopic relaxation mechanism, in the absence of other effects, such as hydrodynamic interactions. Here we compare the numerical solution of the fully self-consistent theory with the results of Brownian dynamics simulation of the van Hove function G(r,t) and/or the intermediate scattering function F(k,t) of four simple model systems. Two of them are representative of short-ranged soft-core repulsive interactions [(sigma/r)(mu), with mu>>1], in two and in three dimensions. The other two involve long-ranged repulsive forces in two (dipolar, r(-3) potential) and in three (screened Coulomb, or repulsive Yukawa interactions) dimensions. We find that the theory, without any sort of adjustable parameters or rescaling prescriptions, provides an excellent approximate description of the collective dynamics of these model systems, particularly in the short- and intermediate-time regimes. We also compare our results with those of the single exponential approximation and with the competing mode-mode coupling theory.