Lattice-Boltzmann simulation of the sedimentation of charged disks

Changes in Electrical Conductance of Polymer Composites Melts Due to Carbon Nanofiller Particles Migration

In this work, the results of investigation of the effect of polymer composite melts electrical conductance increase with time are presented. The conductance time dependencies were obtained for composites based on polypropylene filled with carbon nanoparticles of different types. The dependencies were analyzed to demonstrate the possibility of correlation of the conductance kinetics with different composite parameters, such as the filler geometry. Additional studies were carried out, such as electron microscopy study, conductance measurements after consecutive surface layer removal, and composite melt conductance measurements using a three-electrode scheme. The results showed that the increased electrical conductance of the composite materials can be attributed to the formation of an enriched with the filler particles surface layer, which happens during the stay of the composite in a melt state. Analysis of the experimental data, along with the results of numerical modeling, allowed to suggest a possible filler distribution transformation scheme. The physical premises behind the investigated effect are discussed.

Primary charge effects on prolate spheroids with moderate aspect ratios

In this article, we investigate the behavior of charged spheroidal colloids with moderate aspect ratios in linear flow fields. We use direct numerical simulation with body-fitted grids for the solution of the Stokes-Poisson-Nernst-Planck system to include all non-linear effects. Therefore, we propose an efficient semi-implicit time discretization based on a splitting of the Stokes equation. We will study the effects of the electric double layer on the forces, torques and on the motion of spheroidal particles. For low Reynolds numbers, we find approximating linear expressions between the ambient fluid flow and the force and torque on the particle. The description of this linear behavior is based on the resistance functions, whose dependencies on the Debye length and the zeta potential are investigated. It is recovered that the resistance functions obey a quadratic dependence on the zeta potential in the small zeta potential regime. For low values of the zeta potential, approximate formulas for the resistance functions are given. The approximation properties are carefully studied by comparing the approximate results with direct numerical simulations. For the case of a shear flow, the approximate formulas can be used to avoid time-consuming direct numerical simulations.

Determination of the zeta potential for highly charged colloidal suspensions

We compute the electrostatic potential at the surface, or zeta potential ζ, of a charged particle embedded in a colloidal suspension using a hybrid mesoscopic model. We show that, for weakly perturbing electric fields, the value of ζ obtained at steady state during electrophoresis is statistically indistinguishable from ζ in thermodynamic equilibrium. We quantify the effect of counter-ion concentration on ζ. We also evaluate the relevance of the lattice resolution for the calculation of ζ and discuss how to identify the effective electrostatic radius.

Orientational microdynamics and magnetic-field-induced ordering of clay platelets detected by 2H NMR spectroscopy

The orientation of montmorillonite clays induced by a static magnetic field is quantified by using (2)H NMR spectroscopy. Indeed, the residual quadrupolar splitting of the (2)H resonance line measured for heavy water is a direct consequence of the specific orientation of the clay platelets in the static magnetic field. In the dilute regime, this residual splitting increases linearly with clay concentration, which confirms that the clay/clay electrostatic repulsions remain negligible by comparison with the diamagnetic coupling of these anisotropic platelets. At higher concentration, the electrostatic repulsion between clay particles markedly enhances the detected splitting. Such enhancement is well predicted by numerical simulations. By varying the size of the clay platelets and the strength of the static magnetic field, it is possible to evaluate the order of magnitude of the diamagnetic susceptibility of these anisotropic colloids.

Hindrance function for sedimentation and creaming of colloidal disks

We report the first measurement of the hindrance function for sedimentation or creaming of disk-shaped colloids via the analytical centrifugation. Disks align with the external flow right above a volume fraction of a few percent, and this effect is extremely sensitive to the aspect ratio of disks. Due to this alignment effect, disk sedimentation or creaming demonstrates distinct trends in dilute and semidilute regions.

Simulating (electro)hydrodynamic effects in colloidal dispersions: smoothed profile method

Previously, we have proposed a direct simulation scheme for colloidal dispersions in a Newtonian solvent (Phys. Rev. E 71, 036707 (2005)). An improved formulation called the "Smoothed Profile (SP) method" is presented here in which simultaneous time-marching is used for the host fluid and colloids. The SP method is a direct numerical simulation of particulate flows and provides a coupling scheme between the continuum fluid dynamics and rigid-body dynamics through utilization of a smoothed profile for the colloidal particles. Moreover, the improved formulation includes an extension to incorporate multicomponent fluids, allowing systems such as charged colloids in electrolyte solutions to be studied. The dynamics of the colloidal dispersions are solved with the same computational cost as required for solving non-particulate flows. Numerical results which assess the hydrodynamic interactions of colloidal dispersions are presented to validate the SP method. The SP method is not restricted to particular constitutive models of the host fluids and can hence be applied to colloidal dispersions in complex fluids.

Influence of hydrodynamic interactions on lane formation in oppositely charged driven colloids

The influence of hydrodynamic interactions on lane formation of oppositely charged driven colloidal suspensions is investigated using Brownian dynamics computer simulations performed on the Rotne-Prager level of the mobility tensor. Two cases are considered, namely sedimentation and electrophoresis. In the latter case the Oseen contribution to the mobility tensor is screened due to the opposite motion of counterions. The simulation results are compared to that resulting from simple Brownian dynamics where hydrodynamic interactions are neglected. For sedimentation, we find that hydrodynamic interactions strongly disfavor laning. In the steady state of lanes, a macroscopic phase separation of lanes is observed. This is in marked contrast to the simple Brownian case where a finite size of lanes was obtained in the steady state. For strong Coulomb interactions between the colloidal particles a lateral square lattice of oppositely driven lanes is stable similar to the simple Brownian dynamics. In an electric field, on the other hand, the behavior is found in qualitative and quantitative accordance with the case of neglected hydrodynamics.

Interplay between hydrodynamic and Brownian fluctuations in sedimenting colloidal suspensions

We apply a hybrid molecular dynamics and mesoscopic simulation technique to study the steady-state sedimentation of hard sphere particles for Peclet number (Pe) ranging from 0.08 to 12. Hydrodynamic backflow causes a reduction of the average sedimentation velocity relative to the Stokes velocity. We find that this effect is independent of Pe number. Velocity fluctuations show the expected effects of thermal fluctuations at short correlation times. At longer times, nonequilibrium hydrodynamic fluctuations are visible, and their character appears to be independent of the thermal fluctuations. The hydrodynamic fluctuations dominate the diffusive behavior even for modest Pe number, while conversely the short-time fluctuations are dominated by thermal effects for surprisingly large Pe numbers. Inspired by recent experiments, we also study finite sedimentation in a horizontal planar slit. In our simulations distinct lateral patterns emerge, in agreement with observations in the experiments.

Translational and rotational dynamics of colloidal rods by direct visualization with confocal microscopy

We report an experimental method to characterize the dynamics of colloidal rods by measuring their rotation and translation in three dimensions with confocal microscopy. The method relies on solvent viscosification to retard dynamics to time scales that are compatible with 3D confocal optical microscopy. Because the method yields a full three-dimensional characterization of rod displacement and orientation, it is applicable to situations in which complex, anisotropic dynamics emerge. Examples include behavior in liquid crystal phases with both orientational and positional order, suspensions subjected to applied fields such as shear flow or sedimentation, and the emerging area of anisotropic particle dynamics. We demonstrate the performance of the method by quantifying the Brownian motion of fluorescent poly(methyl methacrylate) rods (aspect ratio, L/D=3.1 and 7.0) grafted with poly(dimethylsiloxane) stabilizer. The rods are dispersed at dilute concentration in a solvent mixture of viscosity 2.0 Pa s. Rod translational and rotational diffusivities are extracted from the measured translational mean square displacement of the centroid positions and of the rod unit vector u(t), respectively. Rod orientational dynamics are characterized relative to both their azimuthal and polar angles. Probability distributions for the translation and rotation in the frame of rod are computed from the measurements. Experimental values obtained agree well with theory available for the dynamics of isolated rods.

A macroion electrokinetics algorithm

A numerical algorithm is presented for the standard model of macroion electrokinetics and certain generalizations of it. The macroion consists of a cylindrical section with identical, hemispheroidal endcaps, each piece having arbitrary length. The system of one macroion and adjoining salt solution is subjected to an arbitrary sequence of pulsed electrical fields and pulsed translational and rotational velocities. Numerical solutions are obtained for the time dependent electrostatic and mobile ion concentration fields and the solvent velocity. From these fields the dielectric response, force, and torque are calculated. Generalizations of the standard model include the diffusive motion of macroion surface charges, partial slip of solvent motion at the macroion surface, and a simple model for the reactive exchange of surface charge with solution ions. The primary illustrative application is to recent measurements of electric birefringence versus applied field frequency for poly-(tetrafluorothylene) colloidal particles, but a few results are presented for the dielectric response of DNA fragments and of spherical colloidal particles. The source code and additional details are provided as supplementary documentation.