Brownian dynamics simulations with spin Brownian motion on the negative magneto-rheological effect of a rod-like hematite particle suspension

We have investigated the behaviour of a suspension of magnetic rod-like hematite particles in a simple shear flow with the addition of an applied magnetic field. A significant feature of the present hematite particle suspension is the fact that the magnetic moment of the hematite particle lies normal to the particle-axis direction. From simulations, we have attempted to clarify the dependence of the negative magneto-rheological effect on the particle aggregation and orientational distribution of particles. The present Brownian dynamics method has a significant advantage in that it takes into account the spin rotational Brownian motion about the particle axis in addition to the ordinary translational and rotational Brownian motion. The net viscosity is decomposed into three components and discussed at a deeper level and in detail: these three viscosity components arise from (1) the torque due to the magnetic particle–field interaction, (2) the torque and (3) the force due to the interaction between particles. It is found that a slight change in the orientational distribution has a significant influence on the negative magneto-rheological effect. In a relatively dense suspension, the viscosity components arising from an applied magnetic field and the interaction between particles come to change rapidly for a certain strength of the magnetic particle–particle interaction, which is due to the onset of the formation of raft-like clusters.     

Viscosity and dispersion state of magnetic suspensions

We investigate the viscosity behavior of a magnetic suspension in which magnetic particles are dispersed in a mixture of polyacrylic liquids. The size of magnetite particles is nearly 300 nm and the volume fraction of the magnetic particles is in the range of 0.003-0.03. The particle concentration dependence of the suspension viscosity yields the intrinsic viscosity [η], which varies from 25.6 at 5 s⁻¹ to 5.1 at 400 s⁻¹. The yield stress and the infinite shear viscosity of the suspension increase non-linearly as the particle concentration ? increases. We examine the effect of process conditions such as milling time and amount of dispersant on the viscosity behavior of the suspension. As milling time elapses, yield stress and low shear viscosity decrease and then reach constant values while the infinite shear viscosity remains constant. When oleic acid is added as a dispersant, the yield stress and low shear viscosity of the suspension show minimum values as the amount of oleic acid increases. These results agree with experimental results of sedimentation tests, which enable us to estimate the aggregate size of magnetic suspension. The yield stress and the low shear viscosity of the magnetic suspension are found to be useful in evaluating the dispersion state of the magnetic suspension.     

无机-高分子磁性复合粒子的制备与表征

详细地研究了乳化剂用量对种子乳液聚合反应的影响,并成功地制备出粒径约为３００的苯乙烯／丙烯酸共聚小球。另外,还报道使用化学共沉淀法使无机粒子与高分子球复合,制备出高分子球为核,无机粒子为壳层的磁性复合料子,使用ＸＲＤ、ＴＥＭ等手段对此复合料子进行了表征。同时,进一步研究了这种复合粒子悬浮液的悬浮性能以及粘度随磁场的变化情况。     

对利用动态光散射法测量颗粒粒径和液体黏度的改进

光散射技术通过测量悬浮液中布朗运动颗粒的平移扩散系数,得到颗粒流体力学直径或液体黏度.本文由单参数模型入手,建立了低颗粒浓度下,单颗粒平移扩散系数与颗粒集体平移扩散系数和颗粒浓度之间的线性依存关系并将其引入光散射法中,从而对现有的测量方法进行了改进.改进后的测量方法可实现纳米尺度球型颗粒标称直径的测量和液体黏度的绝对法测量.以聚苯乙烯颗粒+水和二氧化硅颗粒+乙醇两个分散系为参考样本,通过实验,验证了改进后方法的可行性.此外,还针对上述两个分散系,实验探讨了温度和颗粒浓度对颗粒集体平移扩散系数的影响规律,发现聚苯乙烯颗粒+水分散系中,颗粒间相互作用表现为引力;二氧化硅颗粒+乙醇分散系中,颗粒间相互作用表现为斥力.讨论了颗粒集体平移扩散系数随颗粒浓度变化规律与第二渗透维里系数的关系.     

Viscosity of liquid metal suspensions — experimental approaches and open issues

Knowing the viscosity of metal melts with suspended particles is necessary to interpret experimental results and to simulate fluid flow in such materials. At present, reliable viscosity data is only available for pure metals and alloys. In order to study the viscosity behavior of metal melts with suspended solid particles in detail, samples with defined particle amounts are needed. Various methods of incorporating particles into the metallic melts were evaluated, and viscosity was experimentally determined using an oscillating cup technique. It was shown that solid particles in suspension change the melts’ viscosities dramatically, far beyond the effects expected from normal colloidal rheology.     

Rheological properties of a vesicle suspension

The rheological properties of a vesicle suspension have been investigated in the limit of strong flows destroying the stationary form of vesicles. The dependence of the effective viscosity of the suspension on the velocity gradient and the properties of vesicles has been obtained for the case of the plane flow. In particular, it has been shown that the effective viscosity of the suspension can strongly depend on its initial state. The effect of thermal fluctuations on the rheological properties of the suspension has been analyzed.     

Polarization and transmission of microwaves in the magnetic suspension in the applied magnetic field

The production method of magnetic suspension consisting of ferromagnetic particles dispersed in cedarwood oil is presented at the beginning of this article. Next, the set-up for microwaves generation using a klystron is described. The main part of this paper concerning microwave transmission and polarization during its passage in samples of the produced magnetic suspension placed in a magnetic field is based on the following parameters: induction of this field, filling factor of magnetic suspension by ferromagnetic particles, dimensions of particles, viscosity of liquid carrier, and ratio of the magnetic field changes. Conducted investigations show that microwaves are damped and polarized in these magnetic suspensions. Obtained results are discussed and observed effects are explained by ordering of ferromagnetic particles in magnetic suspension by applied magnetic field.     

Flow rate increased by electrorotation in a capillary

In this paper, we show that the viscosity of a suspension can be drastically reduced if its particles are rotating faster than the surrounding liquid. We propose to induce particle rotation by applying a DC electric field to a suspension of insulating particles dispersed in a slightly conducting liquid (Quincke rotation). We have studied the flow of such a suspension in a rectangular channel and measure the increase of the flow rate induced by the application of an electric field along the velocity gradient direction (shortest dimension of the channel).     

Effect of the bulk viscosity on the sound propagation in nonequilibrium suspensions of microparticles in gas

The sound propagation in a mixture of gas with uniformly dispersed solid particles, whose temperature is maintained above that of the gas by an external source, is considered. The dispersion properties of this kind of suspensions are studied, and expressions for the second viscosity and the sound velocity in such suspensions are derived. It is shown that, in a nonequilibrium suspension, the second viscosity may be negative. The ranges of the suspension parameters, for which the propagation of low-frequency sound is impossible, are determined.     

Effect of viscosity on the magnetic permeability of Sendust-filled polymer composites

A flake-shaped Sendust/acryl suspension is tape cast and the effect of the viscosity of the suspension on the magnetic permeability of the resulting composite is investigated. The real part of the permeability of the composite is inversely proportional to the viscosity of the suspension, indicating that the lower the viscosity of the suspension, the higher the permeability of the composite. The viscosity of the suspension is controlled by adding a small amount of surfactant; an anionic surfactant is most effective for lowering the viscosity at a given concentration range. It is thought that using a suspension with a relatively low viscosity improves the permeability of the resulting composite due to the easy alignment of the flake-shaped filler with the plain direction of the sheet.     

Hydromagnetic transverse instability of two highly viscous fluid-particle flows with finite ion Larmor radius corrections

A linear analysis of the combined effect of viscosity, finite ion Larmor radius and suspended particles on Kelvin-Helmholtz instability of two superposed incompressible fluids in the presence of a uniform magnetic field is carried out. The magnetic field is assumed to be transverse to the direction of streaming. A general dispersion relation for such a configuration has been obtained using appropriate boundary conditions. The stability analysis is discussed analytically, and the obtained results are numerically confirmed. Some special cases are recovered and corrected. The limiting cases of absence of suspended particles (or fluid velocities) and finite Larmor radius, absence of suspended particles are discussed in detail. In both cases, all other physical parameters are found to have stabilizing as well as destabilizing effects on the considered system. In the former case, the kinematic viscosity is found to has a stabilizing effect, while in the later case, the finite Larmor radius is found to has a stabilizing influence for a vortex sheet. It is shown also that both finite Larmor radius and kinematic viscosity stabilizations for interchange perturbations are similar to the stabilization effect due to a magnetic field for non-interchange perturbations. Received 13 January 2003 Published online 24 April 2003 RID="a" ID="a"Also at: Department of Mathematics, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt. e-mail: m.elsayed@uaeu.ac.ae     

Apparent viscosity and particle pressure of a concentrated suspension of non-Brownian hard spheres near the jamming transition

We consider the steady shear flow of a homogeneous and dense assembly of hard spheres suspended in a Newtonian viscous fluid. In a first part, a mean-field approach based on geometric arguments is used to determine the viscous dissipation in a dense isotropic suspension of smooth hard spheres and the hydrodynamic contribution to the suspension viscosity. In a second part, we consider the coexistence of transient solid clusters coupled to regions with free flowing particles near the jamming transition. The fraction of particles in transient clusters is derived through the Landau-Ginzburg concepts for first-order phase transition with an order parameter corresponding to the proportion of “solid” contacts. A state equation for the fraction of particle-accessible volume is introduced to derive the average normal stresses and a constitutive law that relates the total shear stress to the shear rate. The analytical expression of the average normal stresses well accounts for numerical or experimental evaluation of the particle pressure and non-equilibrium osmotic pressure in a dense sheared suspension. Both the friction level between particles and the suspension dilatancy are shown to determine the singularity of the apparent shear viscosity and the flow stability near the jamming transition. The model further predicts a Newtonian behavior for a concentrated suspension of neutrally buoyant particles and no shear thinning behavior in relation with the shear liquefaction of transient solid clusters.     

Effect of cavitation bubble on the dispersion of magnetorheological polishing fluid under ultrasonic preparation

In the ultrasonic dispersion process, the ultrasonic cavitation effect can seriously affect the dispersion efficiency of magnetorheological polishing fluid (MRPF), but the mechanism remains unclear now. Through considering the continuity equation and Vand viscosity equation of the suspension, a revised cavitation bubble dynamic model in the MRPF was developed and calculated. The effects of presence or absence of solid particles, the volume fraction of solid particles, and viscosity on the cavitation bubble motion characteristics in the MRPF were discussed. Settlement experiments of the MRPF under ultrasonic and mechanical dispersion were observed. Analysis of particle dispersion is made by trinocular biomicroscope and image processing of the microscopic morphology of the MRPF. The results show that the high volume fraction of carbonyl iron particle (CIP) will significantly weaken the cavitation effect, and the low volume fraction of green silicon carbide (GSC) has a negligible effect on the cavitation effect in the MRPF. When the liquid viscosity is greater than or equal to 0.1 Pa·s, it is inconvenient to produce micro-jets in the MRPF. The sedimentation rate of the MRPF prepared by ultrasonic dispersion is lower than mechanical dispersion when the volume fraction of CIP is between 1% and 25%. The dispersion ratio under ultrasonic dispersion is lower than that under mechanical dispersion. The experimental results fit the simulation well. It offers a theoretical basis for exploring the ultrasonic cavitation effect in the industrial application of the MRPF.     

Nonequilibrium Brownian dynamics simulations of shear thinning in concentrated colloidal suspensions

The effect of interparticle forces on shear thinning in concentrated aqueous and nonaqueous colloidal suspensions was studied using nonequilibrium Brownian dynamics. Hydrodynamic interactions among particles were neglected. Systems of 108 particles were studied at volume fractions of 0.2 and 0.4. For the nonaqueous systems, shear thinning could be correlated with the gradual breakup of small flocs present because of the weak, attractive secondary minimum in the interparticle potential. At the highest shear rate for=0.4, the particles were organized into a hexagonally packed array of strings. For the strongly repulsive aqueous systems, the viscosity appeared to be a discontinuous function of the shear rate. For=0.4, this discontinuity coincided with a transition from a disordered state to a lamellar structure for the suspension.     

Linear response theory of the viscosity of suspensions of spherical brownian particles

We study the frequency-dependent viscosity of a suspension of spherical particles on the basis of linear response theory applied to the generalized Smoluchowski equation. Hydrodynamic interactions are fully taken into account. We derive expressions for the frequency-dependent viscosity based on a cluster expansion of the linear response.     

Motion of a spherical body in a viscous suspension

The problem of drag of an arbitrary-size solid sphere with its motion in a uniform monodispersed viscous suspension is considered in the Stokes approximation. The expression for the effective suspension viscosity is derived in the first approximation over the volume concentration of the viscous suspension. The coefficient before the concentration is found in the form of an explicit analytical function, which depends on the ratio of sizes of the dispersed particles and the body. The found coefficient coincides with Einstein’s result at the limit of “point” dispersed particles, the size of which is negligibly small compared with the size of the moving sphere, but can substantially differ from it in the case of finite-size particles.     

Kinematics,equilibrium, and shape in Hamiltonian systems: the "LAB" effect

We demonstrate that a system of a finite number of particles in a container with a "typical" shape does not get mixed in the sense that in an (asymptotic) steady state distribution the particles are not equally likely to occupy (almost) any position inside the container. Our incomplete mixing result should be contrasted with the complete mixing of noninteracting or elastically colliding particles in a box, which does occur because the box is not a "typical" shape. The effect we describe does not disappear in the limit of infinitely many particles. However, it is unlikely to be observed in nature but rather in laboratory (the "LAB" effect) experiments. Ideal gas in a container, which is a product of billiards, serves as a transparent model to understand the "LAB" (look at billiards) effect.     

Lattice-Boltzmann Simulation of Particle Suspensions in Shear Flow

Inclusion of short-range particle–particle interactions for increased numerical stability in a lattice-Boltzmann code for particle-fluid suspensions, and handling of the particle phase for an effective implementation of the code for parallel computing, are discussed and formulated. In order to better understand the origin of the shear-thickening behavior observed in real suspensions, two simplified cases are considered with the code thus developed. A chain-like cluster of suspended particles is shown to increase the momentum transfer in a shear flow between channel walls, and thereby the effective viscosity of the suspension in comparison with random configurations of particles. A single suspended particle is also shown to increase the effective viscosity under shear flow of this simple suspension for particle Reynolds numbers above unity, due to inertial effects that change the flow configuration around the particle. These mechanisms are expected to carry over to large-scale particle-fluid suspensions.     

Some aspects concerning the design of an immunomagnetic separation process

We used a stochastic model and have performed a series of numerical simulations to optimise the step of microorganisms collection on magnetic carrier particles. We obtained that the time of capture depends on concentration of carrier particles in suspension, their size, the volume fraction of magnetite in the composite material, the applied magnetic field intensity and its gradient, the size and density of micro-organisms to be collected and the density and viscosity of the liquid in which the particles are suspended.     

Lattice Boltzmann simulation of the two-dimensional flow of a magnetic suspension between two parallel walls under a non-uniform magnetic field

We have developed a lattice Boltzmann method based on fluctuation hydrodynamics that is applicable to the flow problem of a particle suspension. In this method, we have introduced the viscosity-modifying method, rather than the velocity-scaling method, in which a modified viscosity is used for generating random forces in lattice Boltzmann simulations. The viscosity-modifying method is found to be applicable to the simulation of a magnetic particle suspension. We have applied this method to the two-dimensional Poiseuille flow of a magnetic suspension between two parallel walls in order to investigate the behavior of magnetic particles in a non-uniform applied magnetic field. From the results of the snapshots, the pair correlation function between the magnetic pole and the magnetic particles and the averaged local particle velocity and magnetization distributions, it was observed that the behavior of the magnetic particles changes significantly depending upon which factor dominates the phenomenon in the balance between the magnetic particle–particle interaction, the non-uniform applied magnetic field and the translational and rotational Brownian motion.