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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.

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.

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

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.

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.

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.

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.

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.

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.

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.

Barium titanate/insulating oil suspensions were investigated to determine the dielectric polarization mechanisms that govern their electrorheological (ER) behavior. The dynamic yield stress of 19.3 vol % suspensions exhibited Maxwell–Wagner-like frequency dependence, with the dynamic yield stress increasing significantly with ac electric field frequency, as expected for suspensions composed of particles with large dielectric constants. The dynamic yield stress at a given frequency was proportional to Em, where E is the applied electric field strength and the exponent m increased with frequency. For electric field strengths of 2 kV/mm at 1 kHz, dynamic yield stresses were approximately 500 Pa. Rheological experiments in which particle surface and oil conductivities were varied suggest that the dielectric relaxation of these suspensions is controlled by the particles’ bulk conductivity. The dielectric relaxation depended strongly on field strength, becoming broader with increasing field strength. Harmonic a...

The structures of nonaqueous suspensions subjected to electrical fields and continuous shear are described. Based on observations that particle strands and columns formed upon application of an electric field behave as elastic solids for small shear strains but yield above a critical strain, models for the yield stress and continuous-shear response of these electrorheological (ER) suspensions are developed. Comparison of model calculations with experimental results indicates that the yield stress is dominated by the work required to overcome electrically induced interparticle forces.

Colloidal probe microscopy was employed to study normal forces and sliding friction between model cellulose surfaces in aqueous solutions. Hydrodynamic interactions must be accounted for in data analysis. Longrange interactions are governed by double layer forces, and once surfaces contact, by osmotic repulsive forces and visco-elasticity. Increasing the ionic strength decreases surface potentials and increases adhesive forces. Polyelectrolytes cause strong steric repulsion at high surface coverage, where interactions are sensitive to probe velocity. Polymer bridging occurs at low coverage.

Anisotropic solid-state magnetorheological (MR) composites are prepared by applying magnetic fields to isotropic suspensions in polymer melts and preserved by cross-linking the melts. The composities are studied through a continuum-level perspective at various magnetic field strengths under oscillatory shear. The minimum number of constraints are applied in the derivation of the magnetic and rheological invariants, which relate the measured macroscopic magnetostriction coefficients to the discrete parameters of any microscopic model. The field-induced stress is defined by magnetostriction coefficients, which are material parameters that describe the strain dependence of the magnetization tensor. A comparison of the independently measured rheological and magnetic properties allow for the determination of the magnetostriction coefficients.

Theory and experiment suggest that the flocculation of non-Brownian, flexible fibers in flowing viscous media arises from the mechanical entanglement of fibers. We probe the role of frictional and attractive interfiber forces on fiber entanglement using a recently developed particle level simulation technique. With frictional and repulsive interparticle forces only, simulations capture flocculation in 0.125% by volume suspensions of fibers with properties similar to those of softwood pulp fibers. Here, model fibers elastically interlock-flocculation tendency and floc coherency are directly linked to elastic energy storage in fibers. By applying only repulsive and attractive interparticle forces of various strengths, sheared suspensions attain similar degrees of aggregation, and similar floc strengths, as in suspensions with interfiber friction. However, in the absence of interfiber friction, model fibers do not elastically interlock. We find friction to be an essential element in elastic entanglement of fibers. Simulations suggest that effective formation aids in papermaking should reduce interfiber friction under normal force loadings of 1-10mgr;N. Copyright 2000 Academic Press.

Electrostriction is the elastic deformation of a material caused by an electric field. Achievable deformations can be much larger than those of piezoelectric polymers and inorganic materials. Our research has focused on understanding electrostriction in thin polymer films - determining the underlying mechanism or mechanisms and the properties that control observed behavior. We have shown that experimental observations can be described as elastic deformation due to electrostatic forces. Critical to this conclusion has been correctly identifying the dielectric properties that govern electrostriction, and developing techniques to measure them. We have also developed a microscopic model for electrostriction of amorphous, polarizable solids. Model predictions agree very well with measured electrostriction parameters for a variety of polymer films.

Here we investigate the combined effects of carbon nanotube (CNT) properties such as aspect ratio, curvature, and tunneling length and shear rate on the microstructure and electrical conductivities of CNT/polymer composites using fiber-level simulations. Electrical conductivities are calculated using a resistor network algorithm. Results for percolation thresholds in static systems agree with predictions and experimental measurements. We show that imposed shear flow can decrease the electrical percolation threshold by facilitating the formation of conductive aggregates. In agreement with previous research, we find that lower percolation thresholds are obtained for nanotubes with high aspect ratio. Our results also show that an increase in the curvature of nanotubes can make more agglomeration and reduce the percolation threshold in sheared suspensions.

Effects of nonmagnetic interparticle forces on the on- and off-state behavior of magnetorheological fluids are investigated experimentally and with particle-level simulations. Suspensions of iron particles in an aliphatic oil are modified by surface-active species. The modifications significantly alter the off-state properties, but have little impact on the field-induced stresses. Simulations show similar behavior. Off-state rheological properties are strongly influenced by van der Waals forces and modifications of the short-range repulsive forces. Field-induced stresses are less sensitive to the nonmagnetic forces.

The stress transferred by many non-aqueous suspensions can be altered reversibly by the application of large (ca. 1 kV mm–1) electric fields. In the presence of an electric field, the suspension at rest is observed to rearrange to a state where the particles form fibres or columns that span the electrode gap. Application of a shear stress to the suspension degrades these structures and the added work required to overcome the electrically induced particle interactions is observed as an increase in the apparent viscosity. In this paper, the origin of the electrorheological response is reviewed and results indicate that, at intermediate shear rates and field strengths, the electrically induced structural alterations give rise to a direct contribution to the apparent viscosity and an indirect contribution, resulting from viscous interactions between particles in the altered structure. At low shear rates, the indirect contribution to the shear stress decreases with decreasing shear rate, while at high shear rates, the shear field is capable of destroying the electrically induced structures and the indirect contribution is again negligible.