Daniel J. Klingenberg

Electrorheology : mechanisms and models

Abstract Electrorheological (ER) suspensions, typically composed of nonconducting or weakly conducting particles dispersed in an insulating liquid, undergo dramatic, reversible changes when exposed to an external electric field. Apparent suspension viscosities can increase several orders of magnitude for electric field strengths of the order of 1 kV mm −1 , with simultaneous ordering of the microstructure into particulate columns. While this electronic control of momentum transport and structure has many applications, development is severely inhibited by a lack of suitable materials and an incomplete understanding of the underlying mechanisms. This review focuses on the current understanding of the microscopic phenomena believed to control ER and the models used to describe macroscopic behavior. Particular emphasis is placed upon comparing model predictions with experimental observations, relating macroscopic behavior to microscopic mechanisms, and demonstrating the utility of mechanistic models for furthering our understanding of electrorheology.

Magnetorheological fluids: a review

Magnetorheological (MR) materials are a kind of smart materials whose mechanical properties can be altered in a controlled fashion by an external magnetic field. They traditionally include fluids, elastomers and foams. In this review paper we revisit the most outstanding advances on the rheological performance of MR fluids. Special emphasis is paid to the understanding of their yielding, flow and viscoelastic behaviour under shearing flows.

Dynamic simulation of electrorheological suspensions

A simulation method is developed to investigate structure formation in electrorheological suspensions. The suspension is treated as polarizable, spherical particles in a nonconducting medium, with the spheres subject to electric polarization forces due to an applied electric field and to hydrodynamic resistance due to their motion through the continuous phase. The fibrous structures obtained from these simulations are independent of electric field strength and continuous phase viscosity in agreement with experimental observation. We have also found that the details of the simulated structures are sensitive to the treatment of the short‐range forces preventing particle overlap. When this force is represented by a form that accurately approximates a hard‐sphere interaction, the simulated structures agree well with those obtained experimentally, both with respect to their appearance and the time scale for structure formation.

The small shear rate response of electrorheological suspensions. I. Simulation in the point–dipole limit

The electrorheological (ER) response is defined as the rapid and reversible change in the rheological properties of nonaqueous suspensions due to the application of large electric fields [∼O(1 kV/mm)]. Orders of magnitude increases in suspension viscosities at small shear rates are commonly observed, and are believed to be due to the field‐induced formation of fibrous structures. A molecular dynamics‐like simulation technique is developed to investigate the ER response at small shear rates. The suspensions are modeled as monodisperse suspensions of hard, dielectric spheres contained in a Newtonian fluid between parallel plate electrodes, and subjected to electrostatic and hydrodynamic forces. The results predict a dynamic yield stress with a concentration dependence that agrees well with experimental results. The magnitudes of the simulated stresses are smaller than the experimental values, a result of approximations in the model. This issue is addressed in the second paper of this series.

The small shear rate response of electrorheological suspensions. II. Extension beyond the point–dipole limit

Electrorheological (ER) suspensions display dramatic changes in their rheological properties when subjected to large electric fields. In an accompanying paper, a molecular dynamics‐like simulation technique was developed to investigate sheared ER suspensions. The results of that study predicted a dynamic yield stress that saturates at large concentrations of the disperse phase. Experimental results are presented here which confirm the predicted saturation of the yield stress. The experimentally determined yield stresses are larger than the values predicted by the simulations in the point–dipole limit. This discrepancy is accounted for by reconciling approximations made in the simulation model. Incorporation of multipole and multibody contributions to the electrostatic interaction between spheres through a perturbation analysis suggests that accurate predictions of yield stresses can be made. However, we find that the ER response is sensitive not only to the material dielectric properties, but also the det...

Simulations of fiber flocculation: Effects of fiber properties and interfiber friction

Non-Brownian fibers commonly flocculate in flowing suspensions at relatively low concentrations (<1% by weight). We have developed a particle-level simulation technique modeling fibers as chains of rods connected by hinges to probe fiber flocculation. The model incorporates fiber flexibility, irregular fiber equilibrium shapes, and frictional fiber interactions. Model fibers reproduce known orbits of isolated rigid and flexible fibers in shear flow. Simulation predictions of first normal stress differences in homogeneously dispersed, dilute flexible fiber suspensions agree with experimental data. Fiber features such as flexibility and irregular equilibrium shapes strongly impact single fiber and suspension behavior. Fibers aggregate in simulations with interfiber friction, in the absence of attractive forces between fibers. Strong flocculation is observed in suspensions of stiff fibers with irregular equilibrium shapes. Flocs contain many fibers with three or more contact points, and derive cohesiveness f...

Rheology of sheared flexible fiber suspensions via fiber-level simulations

We employ a particle-level simulation technique to investigate the rheology of non-Brownian, flexible fiber suspensions in simple shear flow. The model incorporates a variety of realistic features including fiber flexibility, fiber deformation, and frictional contacts. The viscosity of fiber suspensions is strongly influenced by the fiber equilibrium shape, interfiber friction, and fiber stiffness. The viscosity of the suspension increases as the fiber curvature, the coefficient of friction, or the fiber stiffness is increased. The yield stress of fiber suspensions scales with the volume fraction in a manner similar to that observed experimentally. Fiber suspensions that flocculate exhibit a shear thinning regime that extends to shear rates lower than those observed for homogeneous suspensions.

Material Parameters for Electrostriction.

Electrostriction is often described by a phenomenological tensor relating a material’s deformation to an applied electric field. However, this tensor is not a material parameter; for deformable, weakly compressible materials (e.g., elastomers), the field‐induced deformations depend strongly upon boundary conditions. A different approach that relates the deformation to material properties as well as boundary conditions is required. In this article, we describe a linear theory which introduces five material parameters governing electrostriction: the relative dielectric constant, e0, two derivatives of the dielectric constant tensor, a1 and a2, Young’s modulus, Ey and Poisson’s ratio, ν. Knowledge of these parameters and appropriate boundary conditions allow one to predict field‐induced deformations for arbitrary configurations. We demonstrate an experimental procedure for measuring deformations and permittivity changes, from which the parameters a1 and a2 may be extracted (e0, ν, and Ey can be measured by a...

Dynamic simulation of flexible fibers composed of linked rigid bodies

A particle-level simulation method is employed to study the dynamics of flowing suspensions of rigid and flexible fibers. Fibers are modeled as chains of prolate spheroids connected through ball and socket joints. By varying the resistance in the joints, both flexible and rigid fibers can be modeled. Repulsive interactions between fibers are included, but hydrodynamic interactions and particle inertia are neglected in this implementation. The motion of a fiber is determined by solving the translational and rotational equations of motion for each spheroid. Simulations of isolated fibers in shear flow demonstrate that the method can reproduce known dynamical behavior of both rigid and flexible fibers. The transient behavior in the suspension relative viscosity under simple shear flow was also investigated. An oscillatory response similar to the experimental observations of Ivanov et al. was obtained for rigid fibers. Fiber flexibility reduced the period of oscillation, but had little effect on steady-state ...

Electro- and magneto-rheology

Electro- and magneto-rheological (ER and MR) fluids share many characteristics and potential applications in stress transfer and damping devices. Recent advances include improved understanding of the underlying physical mechanisms, particularly with regard to nonlinear conduction in ER fluids and magnetic saturation in MR fluids, and the development of more effective fluids. Although both technologies have advanced in recent years, commercial applications have been realized only for MR fluids.