Viscosity of Colloidal Suspensions

Simple expressions are given for the Newtonian viscosity $\eta_N(\phi)$ as well as the viscoelastic behavior of the viscosity $\eta(\phi,\omega)$ of neutral monodisperse hard sphere colloidal suspensions as a function of volume fraction $\phi$ and frequency $\omega$ over the entire fluid range, i.e., for volume fractions $0 < \phi < 0.55$. These expressions are based on an approximate theory which considers the viscosity as composed as the sum of two relevant physical processes: $\eta (\phi,\omega) = \eta_{\infty}(\phi) + \eta_{cd}(\phi,\omega)$, where $\eta_{\infty}(\phi) = \eta_0 \chi(\phi)$ is the infinite frequency (or very short time) viscosity, with $\eta_0$ the solvent viscosity, $\chi(\phi)$ the equilibrium hard sphere radial distribution function at contact, and $\eta_{cd}(\phi,\omega)$ the contribution due to the diffusion of the colloidal particles out of cages formed by their neighbors, on the P\'{e}clet time scale $\tau_P$, the dominant physical process in concentrated colloidal suspensions. The Newtonian viscosity $\eta_N(\phi) = \eta(\phi,\omega = 0)$ agrees very well with the extensive experiments of Van der Werff et al and others. Also, the asymptotic behavior for large $\omega$ is of the form $\eta_{\infty}(\phi) + A(\phi)(\omega \tau_P)^{-1/2}$, in agreement with these experiments, but the theoretical coefficient $A(\phi)$ differs by a constant factor $2/\chi(\phi)$ from the exact coefficient, computed from the Green-Kubo formula for $\eta(\phi,\omega)$. This still enables us to predict for practical purposes the visco-elastic behavior of monodisperse spherical colloidal suspensions for all volume fractions by a simple time rescaling.Comment: 51 page

Electrorotation of colloidal suspensions

When a strong electric field is applied to a colloidal suspension, it may cause an aggregation of the suspended particles in response to the field. In the case of a rotating field, the electrorotation (ER) spectrum can be modified further due to the local field effects arising from the many-particle system. To capture the local field effect, we invoke the Maxwell-Garnett approximation for the dielectric response. The hydrodynamic interactions between the suspended particles can also modify the spin friction, which is a key to determine the angular velocity of ER. By invoking the spectral representation approach, we derive the analytic expressions for the characteristic frequency at which the maximum angular velocity of ER occurs. From the numerical caculation, we find that there exist two sub-dispersions in the ER spectrum. However, the two characteristic frequencies are so close that the two peaks actually overlap and become a single broad peak. We report a detailed investigation of the dependence of the characteristic frequency and the dispersion strength of ER on various material parameters.Comment: RevTeX, 4 eps figures; clarifying discussion added in accord with referees' reports; accepted by Physics Letters

Local influence of boundary conditions on a confined supercooled colloidal liquid

We study confined colloidal suspensions as a model system which approximates the behavior of confined small molecule glass-formers. Dense colloidal suspensions become glassier when confined between parallel glass plates. We use confocal microscopy to study the motion of confined colloidal particles. In particular, we examine the influence particles stuck to the glass plates have on nearby free particles. Confinement appears to be the primary influence slowing free particle motion, and proximity to stuck particles causes a secondary reduction in the mobility of free particles. Overall, particle mobility is fairly constant across the width of the sample chamber, but a strong asymmetry in boundary conditions results in a slight gradient of particle mobility.Comment: For conference proceedings, "Dynamics in Confinement", Grenoble, March 201

Glassy dynamics and dynamical heterogeneity in colloids

Concentrated colloidal suspensions are a well-tested model system which has a glass transition. Colloids are suspensions of small solid particles in a liquid, and exhibit glassy behavior when the particle concentration is high; the particles are roughly analogous to individual molecules in a traditional glass. Because the particle size can be large (100 nm - 1000 nm), these samples can be studied with a variety of optical techniques including microscopy and dynamic light scattering. Here we review the phenomena associated with the colloidal glass transition, and in particular discuss observations of spatial and temporally heterogeneous dynamics within colloidal samples near the glass transition. Although this Chapter focuses primarily on results from hard-sphere-like colloidal particles, we also discuss other colloidal systems with attractive or soft repulsive interactions.Comment: Chapter of "Dynamical heterogeneities in glasses, colloids, and granular media", Eds.: L. Berthier, G. Biroli, J-P Bouchaud, L. Cipelletti and W. van Saarloos (Oxford University Press, to appear), more info at http://w3.lcvn.univ-montp2.fr/~lucacip/DH_book.ht

Phase separation in mixtures of colloids and long ideal polymer coils

Colloidal suspensions with free polymer coils which are larger than the colloidal particles are considered. The polymer-colloid interaction is modeled by an extension of the Asakura-Oosawa model. Phase separation occurs into dilute and dense fluid phases of colloidal particles when polymer is added. The critical density of this transition tends to zero as the size of the polymer coils diverges.Comment: 5 pages, 3 figure

Schematic Models for Active Nonlinear Microrheology

We analyze the nonlinear active microrheology of dense colloidal suspensions using a schematic model of mode-coupling theory. The model describes the strongly nonlinear behavior of the microscopic friction coefficient as a function of applied external force in terms of a delocalization transition. To probe this regime, we have performed Brownian dynamics simulations of a system of quasi-hard spheres. We also analyze experimental data on hard-sphere-like colloidal suspensions [Habdas et al., Europhys. Lett., 2004, 67, 477]. The behavior at very large forces is addressed specifically

Charge reversal of colloidal particles

A theory is presented for the effective charge of colloidal particles in suspensions containing multivalent counterions. It is shown that if colloids are sufficiently strongly charged, the number of condensed multivalent counterion can exceed the bare colloidal charge leading to charge reversal. Charge renormalization in suspensions with multivalent counterions depends on a subtle interplay between the solvation energies of the multivalent counterions in the bulk and near the colloidal surface. We find that the effective charge is {\it not} a monotonically decreasing function of the multivalent salt concentration. Furthermore, contrary to the previous theories, it is found that except at very low concentrations, monovalent salt hinders the charge reversal. This conclusion is in agreement with the recent experiments and simulations