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Brownian dynamics simulations were employed to investigate the rheological properties and structure of suspensions of rigid spherocylinders, as a model for rodlike colloids. The spherocylinders interacted only through a soft repulsive force that mimicked a hard spherocylinder interaction. The translational and rotational diffusivities of hard spherocylinder suspensions were reproduced. Liquid crystalline phases, including isotropic, nematic, and smectic and solid phases were identified using orientational and hexatic order parameters and pair distribution functions. Typical flow curves observed experimentally for rodlike colloidal suspensions were reproduced in the simulations, with two shear thinning regions that bracketed a viscosity plateau at intermediate Peclet numbers. The transient rheology and structure of suspensions that were nematic at rest exhibited a variety of behaviors that depended on the Peclet number and concentration. Systemwide domains that aligned in and kayaked about the vorticity direction, domains that rotated coherently locally, and layered domains were observed. Oscillations in the order parameter, viscosity, and the first and second normal stress differences were associated with changes in the structure.

The effect of temperature on the apparent rheological properties of concentrated synthetic fiber suspensions was investigated experimentally. Aqueous suspensions of viscose rayon, acrylic, and nylon 6,6 fibers of various fiber concentrations, sizes, and shapes were used. At a fixed shear rate, the apparent viscosity of all the suspensions decreased reversibly with increasing temperature. The steady-state flow behavior is well described by the Bingham fluid model where the yield stress is a decreasing function of temperature and follows an Arrhenius dependence with an activation energy in the range of 2–80 kJ/mol, which is the same order of magnitude as that reported for 20 wt. % fibrous biomass suspensions below 55 °C. The fiber suspensions exhibited a negative plastic viscosity at low temperatures, and as the temperature was increased, the plastic viscosity became less negative. This temperature-dependent rheological behavior is qualitatively similar to that observed for concentrated fibrous biomass suspensions. The fiber suspensions formed heterogeneous networks where the state of aggregation depended on the experimental conditions and thus affected the macroscopic rheology.

Experimental data for the pressure-driven flow of concentrated lignocellulosic biomass (corn stover) in a circular pipe are presented. A positive curvature was observed in the pressure profile at steady state, both when the biomass was flowing, and for several minutes after the flow had stopped. After the flow into the pipe was stopped, biomass continued to be expelled for at least five minutes, suggesting that the material is compressible. Occasionally, the pressure and outlet flow rate exhibited rapid, transient fluctuations. The fluctuations would cease when dryer-than-average heterogeneities exited the pipe. A mathematical model is developed to treat the biomass as a compressible Bingham fluid with a density-dependent yield stress. This model quantitatively reproduces steady-state pressure profiles for both flowing and nonflowing states, and captures the transition between the two states after the inlet flow rate is set to zero. Our model cannot predict the rapid pressure fluctuations that appear to be associated with heterogeneities in composition.Experimental data for the pressure-driven flow of concentrated lignocellulosic biomass (corn stover) in a circular pipe are presented. A positive curvature was observed in the pressure profile at steady state, both when the biomass was flowing, and for several minutes after the flow had stopped. After the flow into the pipe was stopped, biomass continued to be expelled for at least five minutes, suggesting that the material is compressible. Occasionally, the pressure and outlet flow rate exhibited rapid, transient fluctuations. The fluctuations would cease when dryer-than-average heterogeneities exited the pipe. A mathematical model is developed to treat the biomass as a compressible Bingham fluid with a density-dependent yield stress. This model quantitatively reproduces steady-state pressure profiles for both flowing and nonflowing states, and captures the transition between the two states after the inlet flow rate is set to zero. Our model cannot predict the rapid pressure fluctuations that appear to b...

The mechanisms governing the ability of water-soluble polymers (WSP) to alter the rheological properties of lignocellulosic biomass to achieve processing advantages are investigated. Lignocellulosic fiber surface chemistry is found to be an important factor in the efficacy of WSPs as rheological modifiers, and a strong correlation between the amount of adsorbed polymer at fiber surfaces and the yield stress of the fiber suspension indicate that adsorption of polymer is important for rheological modification. Three fiber suspensions of varying physical chemistry were produced from Aspen wood chips, and a number of additional fiber suspensions with chemically functionalized surfaces were generated from cellulose pulp. Polymer adsorption and suspension rheology are found to correlate in every case examined, but the amount of adsorbed polymer alone cannot be used to predict the efficacy of a WSP as a rheological modifier, suggesting that there are contributions from additional variables such as adsorbed-polymer conformation.

We designed, constructed, and tested a new device that can measure the rheological properties of lignocellulosic biomass slurries with high solids concentrations (>25%) containing large particles (>10 mm), and that can operate at high temperatures (>230 °C), high pressures (>2.8 MPa), and low pH (

Dielectric suspensions consisting of sterically coated, nano-scale barium titanium particles in the organic ester, isoamyl isovalerate, were developed to enhance the field forces in electrostatic machinery. The relative permittivity and conductivity of the suspensions were measured experimentally, as was the electrostatic force developed upon application of significant electric fields. At a particle concentration of 10% by volume, the relative permittivity of the suspension was approximately 50% greater than that of the base solvent. However, this improvement came at the expense of detriments to the conductivity and rheology of the suspension. These challenges must be addressed before dielectric suspensions can be used in such challenging applications.

We investigate model magnetorheological (MR) fluids (inverse ferrofluids) under both steady and dynamic oscillatory shear. Analytical theories, particle-level simulations, and magnetorheometry are used in an attempt to obtain universal master curves. Steady shear flow data can be collapsed when plotted as a function of a dimensionless Mason number. The critical Mason number associated with the transition from magnetostatic to hydrodynamic control of the suspension structure is demonstrated to linearly increase with particle concentration which is in good agreement with theories and our simulations. Experimental linear viscoelastic moduli are in good agreement with micromechanical and macroscopic models in the dilute regime. However, upon increasing particle concentration, theoretical predictions underestimate experimental data while particle-level simulations are in good agreement. The accordance with particle-level simulations suggests that the mean (average) magnetization approximation gives a good pred...

Additives that alter the rheology of lignocellulosic biomass suspensions were tested under conditions of variable pH, temperature, and solid concentration. The effects of certain ions, biomass type, and time after the addition of rheological modifier were also examined. Torque and vane rheometry were used to measure the yield stress of samples. It was found that the effectiveness of rheological modifiers depends on pH over a range of 1.5 to 6, biomass type, concentration of certain ions, and time after addition. The time-dependent properties of rheologically modified biomass are sensitive to the type of rheological modifier, and also to mixtures of these additives, which can result in unexpected behavior. We show that time-dependent rheology is not correlated with time-dependent changes of the water-soluble polymer (WSP) in the aqueous environment, such as slow polymer hydration, suggesting that time-dependent changes in the polymer-fiber interaction may play a more significant role.

An electro-acoustic method for measuring volume charge distributions in dielectric films (50 µm–0.2 mm thick) is described. A high voltage (>1 kV) sinusoidal signal (frequency ∼ 2 kHz) is applied across the dielectric sample. The charges inside the dielectric material will mechanically respond to the input electric signal and excite acoustic waves. The acoustic waves can be detected and measured using a piezoelectric sensor. By analyzing the received acoustic signal, we are able to compare the amount of charge in various samples.

An inexpensive, rapid method for measuring the rheological properties of yield stress fluids is described and tested. The method uses an auger that does not rotate during measurements, and avoids material and instrument-related difficulties, for example, wall slip and the presence of large particles, associated with yield stress fluids. The method can be used for many types of yield stress fluids, including concentrated lignocellulosic biomass. Sample preparation prior to measurement is minimal, reducing, or eliminating disruption of the sample. We show that measurements using this technique compare well with measurements obtained with a vane rheometer. A variation of the described method is proposed that would make it easier to measure time-dependent rheological properties. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1523–1528, 2014

After a brief summary of previous work on multicomponent diffusion theory, we review how the thermodynamics of irreversible processes leads us to an expression for the generalized driving force for diffusion. We then give the generalized Fick’s law expression containing the diffusivities D α γ ; then we show how this may be turned “wrong-side out” using Merk’s method to give the generalized Maxwell–Stefan equations containing the diffusivities Ð 12 ≡ x 1 x 2 / C 12 . Finally, we show how the latter are related to the diffusivities D 12 that are usually reported in the literature. All results are applicable to gases or liquids. This review is restricted to presenting the basic theory in a consistent notation, and not to the applications of the theory.

We use fiber-level simulations to investigate the combined effects of carbon nanotube (CNT) aspect ratio and shear rate on the microstructure and electrical properties of CNT/polymer composites. In our previous studies, we studied the effects of aspect ratio at a constant shear rate as well as the effects of shear rate for a constant aspect ratio. In this study electrical properties and microstructure changes (e.g. agglomeration/deagglomeration, network strength, nanotube orientation) of CNT/polymer composites are investigated for varying aspect ratios at different shear rates. When shear rate is increased, we observe a decrease in the electrical conductivity and an increase in the anisotropy factor due to the deagglomeration and flow induced orientation. Increasing aspect ratio shifts the conductivity vs. shear rate curve to larger values and anisotropy vs. shear rate curve to lower values showing that there is a tendency for tube agglomeration when high aspect ratio nanotubes are used. On the other hand when low aspect ratio nanotubes are used, conductive networks can be more easily destroyed by the shear forces because networks formed by low aspect ratio nanotubes have lower strength than those formed by high aspect ratio nanotubes. Our results show that electrical conductivity is anisotropic with a larger component in the flow direction. The critical shear rate defined as the shear rate where the conductive network is destroyed and all components of the composite conductivity decrease to the matrix conductivity, shifts to higher values when the aspect ratio is increased. Reduced alignment and increased entanglement are the reasons of this decrease.

Additives were tested for their ability to modify the rheology of lignocellulosic biomass. Additive types included water-soluble polymers (WSPs), surfactants, and fine particles. WSPs were the most effective rheological modifiers, reducing yield stresses of concentrated biomass by 60–80% for additive concentrations of 1–2 wt. % (based on mass of dry biomass solids). Yield stress and plastic viscosity of rheologically modified biomass depended on WSP molecular weight and degree of substitution. The apparent shear stress-shear rate data are represented with the Bingham model. In the absence of WSP, the biomass exhibited a positive yield stress and a negative plastic viscosity, which suggests a nonmonotonic dependence of shear stress on shear rate. When WSP was added, the yield stress decreased and the plastic viscosity increased, becoming positive for sufficiently large WSP concentrations.

Economic processing of lignocellulosic biomass requires handling the biomass at high solids concentration. This creates challenges because concentrated biomass behaves as a Bingham-like material with large yield stresses. Here we employ torque rheometry to measure the rheological properties of concentrated lignocellulosic biomass (corn stover). Yield stresses obtained using torque rheometry agree with those obtained using other rheometric methods, but torque rheometry can be used at much larger solids concentration (weight fractions of insoluble solids greater than 0.2). Yield stresses decrease with severity of hydrolysis, decrease when water-soluble polymers are added (for nonhydrolyzed biomass), and increase with particle length. Experimental results are qualitatively consistent with those obtained from particle-level simulations.

We employ a fiber-level simulation technique to simulate carbon nanotube (CNT)/polymer composites in simple shear flow. This model incorporates CNT flexibility, irregular CNT equilibrium shapes and CNT interactions. Electrical conductivity of the composites is determined using a resistor network algorithm. Tunneling resistance of the insulating matrix film between nanotubes is also considered. We show that the rate of imposed shear flow influences the composite conductivity by facilitating the formation or destruction of the conductive aggregates. In addition, the conductivity evolution during shearing for different concentrations is investigated. At low concentration, percolating clusters form and break simultaneously which causes large conductivity fluctuations during the simulations. When sufficiently large concentrations are reached, percolating clusters persist during shearing and the conductivity fluctuations decrease. In agreement with previous research we determine that increasing the shear rate causes alignment of the nanotubes in the flow direction. We show that upon shearing at constant shear rate, the system attains a state with substantially constant electrical conductivity, nanotube orientation and agglomerate size that is a function of the applied shear rate. The state reached for a given shear rate is independent of the initial state of orientation and aggregation.

Economical biorefining of lignocellulosic biomass (LCB) requires processing high-solids particulate streams. We have developed new techniques and testing protocols to measure the rheological properties of high- solids LCB using a modified torque rheometer (TR). The flow field in the TR is similar to that of a twin-screw extruder and for modeling purposes can be adequately represented as a dual-Couette viscometer. Our experiments show that LCB exhibits Bingham plastic behavior with very large yield stresses. We observe that in the initial stages of mixing, torque values are extremely large and erratic. During this period, considerable particle-size reduction takes place with correspondingly large energy consumption. We show that the addition of a rheological modifier (e.g., carboxy - methyl cellulose) reduces biomass apparent viscosity and mixing energy requirements. We take advantage of this effect to further investigate the viability of continuous processing by extrusion. Application: This research was conducted to measure the rheological properties of high-solids biomass for bio - refining applications. Determining the rheological properties of high-solids biomass is challenging. In this paper, we attempt to elucidate these challenges, as well as devise experimental techniques and quantitative measures for their determination.

Silica-based inverse ferrofluids (IFFs) are synthesized and their pre-yield and post-yield rheological properties are investigated as a function of magnetic field strength (8.8–276 kA/m), volume fraction (12.6–26.1 vol %), silica particle size (104–378 nm radius), and ferrofluid Newtonian viscosity (44–559 mPa s). The Mason number (Mn) provides a good scaling of the data in the steady simple shear flow regime. Special emphasis is made on the low and moderate Mason number region. At low Mn values, two different behaviors are observed depending on the IFF formulation and magnetic field strength applied: (i) either the viscosity monotonically increases with decreasing shear rate suggesting the existence of a yield stress (ii) or a low-shear plateau is reached. At medium Mn values, a power law behavior is found η/η∞∝MnΔ with −1

Controlling the electrical conductivity is a critical issue when processing material systems consisting of an insulating matrix filled with conductive particles. We provide experimental evidence that given shear rates result in specific conductivity levels in such different systems as high-viscosity carbon nanotube/polymer melt or low-viscosity carbon nanotube/epoxy fiber suspensions. The steady-state conductivities are independent of the initial state of the dispersion. The observed behavior is modeled phenomenologically by the competition between build-up and destruction of conductive filler network. A particle-level simulation of flowing fiber suspension also reflects the observed behavior. Our results show that properties of particulate suspensions can be controlled by steady shear. They should be considered to obtain reproducible properties in shear-based processing technologies as injection molding or resin transfer molding.

The conversion of biomass, specifically lignocellulosic biomass, into fuels and chemicals has recently gained national attention as an alternative to the use of fossil fuels. Increasing the concentration of the biomass solids during biochemical conversion has a large potential to reduce production costs. These concentrated biomass slurries have highly viscous, non-Newtonian behavior that poses several technical challenges to the conversion process. A collaborative effort to measure the rheology of a biomass slurry at four separate laboratories has been undertaken. A comprehensive set of rheological properties were measured using several different rheometers, flow geometries, and experimental methods. The tendency for settling, water evaporation, and wall slip required special care when performing the experiments. The rheological properties were measured at different concentrations up to 30% insoluble solids by mass. The slurry was found to be strongly shear-thinning, to be viscoelastic, and to have a significant concentration-dependent yield stress. The elastic modulus was found to be almost an order of magnitude larger than the loss modulus and weakly dependent on frequency. The techniques and results of this work will be useful to characterize other biomass slurries and in the design of biochemical conversion processing steps that operate at high solids concentrations.

The transient response of electrorheological suspensions in shear flow subjected to a suddenly imposed electric field is investigated experimentally. Barium titanate∕silicone oil and alumina∕mineral oil suspensions are employed. The evolution of both the rheological properties and the suspension structure are investigated. Results are compared with predictions from a two-fluid continuum model reported previously. Transient responses appear above a critical field strength, and the critical Mason number for the onset of a transient rheological response is equivalent to the critical Mason number for the onset of lamella formation, within experimental uncertainty. These results are consistent with predictions. The experimentally determined values of the critical Mason number agree with those predicted, with differences of the order of the experimental uncertainty. However, we find that the critical Mason number depends on shear rate, rather than being independent of shear rate as predicted.

We employ particle-level simulations to show that body forces, such as gravity or centrifugal forces, can significantly influence the structure and rheology of ER and MR suspensions even when the magnitude of the body force acting on a particle is small compared to the field-induced force. We also report an experimental investigation of the effects of body forces on the structure of ER suspensions. Experimental results agree qualitatively with predictions.

The electric field strength and shear rate dependence of the apparent shear viscosity of electrorheological (ER) suspensions can often be represented by a function of only the Mason number. A Mason number defined for magnetorheological (MR) suspensions by direct substitution of magnetostatic variables for electrostatic variables does not produce a similar collapse of shear viscosity data for MR suspensions. We show that a Mason number defined in terms of the suspension magnetization can be employed to produce a collapse of experimental data at various magnetic field strengths and shear rates. As for ER suspensions, this Mason number can be calculated from experimentally measured quantities.

Particle-level simulations are conducted to study magnetorheological fluids in plane Poiseuille flow. The importance of the boundary conditions for the particles at the channel walls is examined by considering two extreme cases: no friction and infinite coefficient of friction. The inclusion of friction produces Bingham fluid behavior, as commonly observed experimentally for MR suspensions. Lamellar structures, similar to those reported for electrorheological fluids in shear flow, are observed in the post-yield region for both particle boundary conditions. The formation of these lamellae is accompanied by an increase in the bulk fluid velocity. The slip boundary condition produces higher fluid velocities and thicker lamellar structures.

Particle-level simulations are employed to investigate the rheological properties of bidisperse magnetorheological fluids. These suspensions are treated as nonlinearly magnetizable, neutrally buoyant, non-Brownian spheres immersed in a nonmagnetizable Newtonian continuous phase. We examine the effects of particle size ratio, composition, and field strength on the dynamic yield stress. The dynamic yield stress of bidisperse suspensions is larger than that of monodisperse suspensions at the same particle volume fraction. The smaller particles cause the larger particles to form more chainlike aggregates than those formed in monodisperse suspensions.

An experimental investigation of the rheological response of magnetorheological suspensions subjected to step changes in applied magnetic field strength at fixed shear rate is reported. For small applied field strengths, the shear stress increases rapidly to a steady value. Above a critical field strength, the rapid initial increase in shear stress is followed by a slow, transient increase in stress. The critical Mason number corresponding to the critical magnetic field strength at the onset of this transient depends on the particle volume fraction as well as the shear rate. This is in contrast to a previous analysis where the critical Mason number was predicted to depend on only the particle volume fraction. The discrepancy is attributed to colloidal forces that are significant in our experimental system, but were not included in the analysis. Further comparison with the previous analysis requires either including the effects of colloidal forces, or performing experiments with systems in which colloidal forces are not important.

The electrostatic dipole moment distribution in heterogeneous suspensions is determined via a self-consistent, point-dipole model, which incorporates nonlocal electrostatics. Predictions agree qualitatively with previous asymptotic results for discontinuous concentration profiles. For small fluctuations in concentration, the dipole strength can be expressed as an expansion in gradients of the concentration. This expansion is incorporated into a linearized continuum model for structure evolution in sheared electrorheological suspensions. Prior stability analysis of a fully local continuum model predicts the formation of concentrated particle stripes oriented in the flow direction, in agreement with experimental observations. Incorporating nonlocal electrostatics suppresses the growth of high wave number fluctuations, providing a more realistic finite rate of growth of fluctuations. Incorporating nonlocal electrostatics in the full nonlinear continuum model produces a single particulate stripe at steady state.

We have developed a particle-level, dynamic simulation technique to probe the structural behavior of non-Brownian fiber suspensions in simple shear flow. The model incorporates a variety of realistic features including fiber flexibility, irregular equilibrium fiber shapes, and fiber interactions. Simulated suspensions exhibit heterogeneous structures, or flocculation, when the model fibers are flexible, have deformed equilibrium shapes, and interact through static friction forces, even in the absence of attractive forces between fibers. The addition of kinetic friction and weak attractive forces has little effect on flocculation behavior, while anisotropic fiber bending tends to shift the onset of flocculation to larger coefficients of friction.

A two-fluid continuum model is developed to describe mass transport in electro- and magnetorheological suspensions. The particle flux is related to the field-induced and hydrodynamic stresses. Solutions of the resulting mass balance show column formation in the absence of flow and stripe formation when a suspension is subjected simultaneously to an applied electric field and shear flow.

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.

A two-fluid continuum model is developed to describe mass transport in electro- and magnetorheological suspensions. The particle flux is related to the field-induced stresses. Solutions of the resulting mass balance show column formation in the absence of flow, and stripe formation when a suspension is subjected simultaneously to an applied electric field and shear flow.

Particle and electrode surfaces were altered to examine the effects on the electrorheological response of barium titanate/silicone oil suspensions. Unmodified suspensions exhibited nonlinear conduction. The dynamic yield stresses scaled as En, where nn, where n≥2 at some frequencies and increased with coating thickness. With coated electrodes, the current harmonic contents were significantly decreased. These results suggest that nonlinear conduction in ER suspensions is associated with charge injection at the electrode/liquid interface.

An equilibrium thermodynamic approach is employed to derive a continuum-level expression for the field-induced stress in uniaxial anisotropic materials, such as electro- and magnetorheological suspensions. This model introduces new electro- and magnetostriction coefficients, which are material parameters that describe the strain dependence of the dielectric and permeability tensors as well as the field-induced stresses. An idealized microscopic model illustrates the relationships between microscopic parameters and the macroscopic magnetostriction coefficients. The model is used to determine the stresses in common applications; predictions from the continuum approach agree with direct calculations of the normal stress and static shear modulus of magnetorheological suspensions.

Colloidal probe microscopy was employed to study sliding friction between model cellulose surfaces in aqueous solutions. Regardless of scan size, friction exhibits irregular stick–slip behavior related to surface roughness. At small scan sizes (∼10 nm), the coefficient of friction decreases with increasing load. Above a critical scan size of about 100 nm — corresponding to the average size of asperities on one of the model surfaces — friction forces are independent of scan size, but depend on the load. Hydrodynamic forces contribute little to friction. Small amounts of high molecular weight, polyelectrolytes decrease significantly sliding friction between cellulose surfaces.

Non-Brownian fibers commonly flocculate in flowing suspensions at relatively low concentrations (

Non-Brownian fibers commonly flocculate in flowing suspensions. A particle level simulation technique modeling fibers as chains of rods connected by hinges is developed to probe flocculation. Simulations show that flocculation can be induced solely by interfiber friction-attractive forces between fibers are not necessary. Simulated mechanical floc characteristics are consistent with experimental observations. In contrast, simulations of flocs formed by attractive forces behave qualitatively differently.

Suspensions of iron particles in media with yield stresses were investigated to determine the effect of the continuous phase yield stress on the magnetorheological (MR) response. The steady-shear MR response was independent of the continuous phase yield stress for yield stresses in the range 0.9–37 Pa. The field-induced suspension yield stress increased sub-quadratically with the flux density. The small amplitude oscillatory shear response exhibited history dependence. The storage modulus depended not only on the magnitude of the applied magnetic field, but also on its history. This history dependence can be explained in terms of the field-dependent evolution of the suspension microstructure.

An equilibrium thermodynamic approach is employed to derive a continuum-level expression for the electric field-induced stress in uniaxial anisotropic materials. Although this model is developed specifically to describe electrorheological and electrostrictive behavior of suspensions, it also applies to other uniaxial materials such as nonpolar nematic liquid crystals, biaxially oriented polymer films, and paper. This model introduces new electrostriction coefficients, which are material parameters that describe the strain dependence of the dielectric tensor as well as the field-induced stresses. An experimental technique for measuring the electrostriction parameters is outlined. An idealized microscopic model is presented to illustrate the relationships between microscopic parameters and the macroscopic electrostriction coefficients. The model is used to determine the stresses in common applications; predictions from the continuum approach agree with direct calculations from a microscopic approach of the ...

A model of a paramagnetic isotropic elastic material with small Young's modulus is proposed, wherein the stress tensor contains additional terms involving products of strain and field components. The new `cross’ terms produce an effective Young's modulus which depends on the magnetic field, whereas the classical model does not predict an influence of a magnetic field on deformation.

Electrorheological (ER) fluids are fascinating materials that undergo dramatic reversible changes in their rheological properties upon the application of electric fields. In many proposed applications, the fluids will be subjected to a dynamic stimulus with finite deformation. We use a particle-level simulation method to investigate the dynamic behavior of monolayer ER fluids. ER fluids are linear viscoelastic for only very small strain amplitudes. The transition to nonlinear deformation arises from very slight rearrangements of unstable structures. At large strain amplitudes, the behavior is viscoplastic, while at large dimensionless frequencies (∝ ω/E02, where ω is the oscillation frequency and E0 is the electric field strength), the response is Newtonian for all strain amplitudes. Simulation results agree qualitatively with experiments. The dependence of the flow behavior on the strain amplitude and dimensionless frequency is summarized in the form of a Pipkin diagram.

The phenomenon of electrorheology (ER), described in the guest editors' introduction to this issue, can largely be explained by electrostatic interactions between particles by an externally applied electric field. The purpose of this article is to review particle-polarization mechanisms active in ER suspensions and to highlight some poorly understood electrostatic phenomena that inhibit the commercialization of ER technology.Figure 1 plots the apparent viscosity of a 20-wt% suspension of alumina particles in silicone oil as a function of shear rate for different electric field strengths. The viscosity changes are most pronounced at small deformation rates. At small shear rates the viscosity is proportional to E2 where E is the electric field strength.