Influence of shear and orthogonal vibration on semisolid aluminum flow properties

Understanding flow properties and flow effects of liquid and semisolid aluminum became a key solution for know-how of the casting processes. It is essential to find and study a new solution of interactive and efficient structure control of processed aluminum suspension. This task was solved by an adaptation of an electromagnetic actuator and high-resolution tempering unit to a conventional rotational rheometer. Initially, the research reveals a precise detection of transition temperatures in steady and transient shear flow. It was found that superposition of mechanical vibration orthogonal to the shear flow radically decreases shear viscosity of semisolid slurry. However, liquid state rheological properties show structural behavior, but stayed insensitive to mechanical oscillations. Analysis of boundary conditions before fundamental experiment shows that no considerable side effects were present during the experiment under vibration. The study reveals transition of strongly shear-thinning concentrated aluminum suspension to almost Newtonian fluid under vibration in shear flow. It is recommended to relate such phenomenon to non-equilibrium between structure formation and structure break-up under vibration and hydrodynamic forces of shear flow. The results illustrate how sensitive the structure of slurry is to vibration in general and in particular during the solidification phase. The revealed results provide a solid basis for further fundamental investigations.     

Radial filtration in highly concentrated suspensions undergoing constant-force squeeze flow and its effect on the normal stress distribution

Liquid-phase migration and jamming of the suspended particles appear to be the cause of the previously reported drastic changes in the normal stress distribution in concentrated suspensions subjected to squeeze flow as the initial volume fraction is raised above a critical value. Liquid-phase migration was found to depend on the initial volume fraction of solids, the viscosity of the suspending fluid, and the size of the particles. Under some conditions, liquid-phase migration did not take place to any significant degree; however, under other conditions, the volume fraction of solids increased throughout the sample, but especially in the central region, as liquid was expelled from the test region in preference to the solids. Criteria for the occurrence of liquid-phase migration in suspensions undergoing squeeze flow are discussed in terms of dimensionless groups.     

Investigation of the abrupt contraction flow of fiber suspensions in polymeric fluids

Numerical simulations of the flow of rigid fibres through a 4:1 planar contraction, and the predicted flow pattern and fiber orientation are presented. Entirely new is the examination of the nature of the suspending matrix which may consist of either a Newtonian fluid or a polymer melt. In the case of a polymer matrix three rheological models, the Phan-Thien–Tanner, FENE-CR, and Carreau models have been used to investigate the effects of shear-thinning and elasticity on the flow and the orientation of the fibers. The effects of inertia are neglected, and the governing equations for the flow field, polymer stress, and fiber orientation are coupled and simultaneously solved. A parametric study is used to explore the effects of different dimensionless parameters on the velocity field, the fiber orientation, the pressure drop, as well as the vortex size measured by the dimensionless reattachment length. We particularly focus on the role of the fibers aspect ratio, volume fraction, and interaction coefficient which measures the intensity of fiber interaction in the suspension. Furthermore, we evaluate and compare the results of four different closure approximations: the quadratic, linear, hybrid A and T, and natural closures.     

Surface tension driven oscillations of a bubble in a viscoelastic liquid

Small amplitude surface tension driven oscillations of a spherical bubble in a dilute polymer solution are considered. The rheological properties of the liquid are modelled by using a 3-constant constitutive equation of the Oldroyd type. The Laplace transform of the solution of the initial value problem is inverted numerically. As in the Newtonian fluid case, both a discrete and a continuous spectrum occurs. In addition to the non-dimensional parameters in the corresponding problem for a Newtonian fluid, the results depend on two other parameters: the ratio of the relaxation time of the polymer solution and the time scale of the flow (the Deborah number) and the product of the polymer concentration and the intrinsic viscosity. For small bubbles in an aqueous solution having a small relaxation time, significant additional damping is found even for dilute solutions.     

Concentration dependence of the linear viscoelastic properties of particle suspensions

The response under small amplitude oscillatory deformations of a suspension of non-Brownian spheres dispersed in a viscoelastic fluid is investigated. The correspondence principle of linear viscoelasticity is used to derive a simple constitutive model from a model for a suspension in a Newtonian liquid. The theory predicts that for a specific particulate system the concentration dependence of the viscoelastic properties should collapse to a single master curve when the values are normalized with those of the carrier fluid alone. Measurements with the micro-Fourier rheometer using oscillatory squeeze flow are carried out on two suspensions of 60 and 80 μm sized particles dispersed in polymeric fluid and in silicon oil, and the master curve is verified. Received: 27 April 1999/Accepted: 15 October 1999     

Shear and squeeze rheometry of suspensions of magnetic polymerized chains

We present the first experimental results on the magnetorheology of suspensions of non-Brownian magnetic ellipsoidal particles. These particles are made of spherical iron particles linked by polymers and are called polymerized chains. Steady shear, oscillatory shear, and oscillatory squeeze rheological tests have been performed. The rheological properties of the suspension of polymerized chains have been compared with those of the suspension of spherical iron particles. In shear flow, both suspensions develop nearly the same yield stress, while in squeeze flow, the yield stress is several times higher for the suspension of polymerized chains. We show that the squeezing force of a suspension of spherical particles is an increasing function of the magnetic field intensity at low magnetic fields but decreases dramatically at higher fields. Surprisingly, this phenomenon, attributed to cavitation or air entrainment, does not occur in the suspension of polymerized chains.     

Sphere settling in an aging yield stress fluid: link between the induced flows and the rheological behavior

The flow pattern induced by the settling of a non-Brownian sphere in a fluid depends on the rheological properties of that fluid. For instance, at small Reynolds numbers, the pattern presents a fore–aft symmetry in a Newtonian fluid, whereas, in some viscoelastic polymer solutions, it can exhibit a negative wake, i.e., an upward flow in the sphere’s wake. This study is an experimental work on the settling of a sphere in a suspension of a synthetic colloidal clay, laponite. The fluid is a yield stress fluid that ages, i.e., whose rheological properties evolve over time. We show that the settling velocity of a given sphere, as well as the induced flow pattern, are strongly modified as the fluid ages. In particular, the flow pattern asymmetry increases with the age of the fluid, and a negative wake eventually forms. We relate those modifications to rheological measurements and suggest, in line with works dealing with polymer solutions, that it is the increase in the fluid viscoelasticity that is responsible for the formation of a negative wake. The flow field measurements are also compared with flow-induced birefringence measurements, and we show that very slow relaxation processes are involved in the sphere settling.     

Rheology of shear thickening suspensions and the effects of wall slip in torsional flow

The rheological characterisation of concentrated shear thickening materials suspensions is challenging, as complicated and occasionally discontinuous rheograms are produced. Wall slip is often apparent and when combined with a shear thickening fluid the usual means of calculating rim shear stress in torsional flow is inaccurate due to a more complex flow field. As the flow is no longer “controlled”, a rheological model must be assumed and the wall boundary conditions are redefined to allow for slip. A technique is described where, by examining the angular velocity response in very low torque experiments, it is possible to indirectly measure the wall slip velocity. The suspension is then tested at higher applied torques and different rheometer gaps. The results are integrated numerically to produce shear stress and shear rate values. This enables the measurement of true suspension bulk flow properties and wall slip velocity, with simple rheological models describing the observed complex rheograms.     

Migration of a sphere suspended in viscoelastic liquids in Couette flow: experiments and simulations

The intrinsically coupled effects of the curvature of the flow-field and of the viscoelastic nature of suspending medium on the cross-stream lateral migration of a single non-Brownian sphere in wide-gap Couette flow are studied. Quantitative videomicroscopy experiments using a counterrotating device are compared to the results of 3D finite element simulations. To evaluate the effects of differences in rheological properties of the suspending media, fluids have been selected which highlight specific constitutive features, including a reference Newtonian fluid, a single relaxation time wormlike micellar surfactant solution, a broad spectrum shear-thinning elastic polymer solution and a constant viscosity, highly elastic Boger fluid. As expected for conditions corresponding to Stokes flow, migration is absent in the Newtonian fluid. In the wormlike micellar solution and the shear-thinning polymer solution, spheres are observed to migrate in the direction of decreasing shear rate gradient, i.e. the outer cylinder, except when the sphere is initially released close to the inner cylinder, in which case the migration is towards it. The migration is enhanced by faster relative angular velocities of the cylinders. Shear-thinning reduces the migration velocity, showing an opposite behavior as compared to previous results in planar shear flow. In the Boger fluid, within experimental error no migration could be observed, likely due to the large solvent contribution to the overall viscosity. For small Deborah numbers the migration results are well described by an heuristic argument based on a local stress balance.     

Investigation on effect of drag models on flow behavior of power-law fluid–solid two-phase flow in fluidized bed

In this study, a Eulerian-Eulerian two-fluid model combined with the kinetic theory of granular flow is adopted to simulate power-law fluid–solid two-phase flow in the fluidized bed. Two new power-law liquid–solid drag models are proposed based on the rheological equation of power-law fluid and pressure drop. One called model A is a modified drag model considering tortuosity of flow channel and ratio of the throat to pore, and the other called model B is a blending drag model combining drag coefficients of high and low particle concentrations. Predictions are compared with experimental data measured by Lali et al., where the computed porosities from model B are closer to the measured data than other models. Furthermore, the predicted pressure drop rises as liquid velocity increases, while it decreases with the increase of particle size. Simulation results indicate that the increases of consistency coefficient and flow behavior index lead to the decrease of drag coefficient, and particle concentration, granular temperature, granular pressure, and granular viscosity go down accordingly.     

Rheological characterization of a solder paste for surface mount applications

Solder pastes used in surface mount soldering techniques (SMT) are very complex suspensions containing high volumes of metallic powder in a carrier fluid. The rheological complexity results largely from the carrier fluid itself, which is a suspension of colloidal particles. In this work, we have characterized the rheological properties of a typical carrier fluid and its solder paste containing 64 vol.% metallic powder. A six-blade vane geometry was used to avoid wall slip and sample fracture. All measurements were carried out following pre-shearing and rest time in order to obtain reproducible results. Steady shear experiments showed that the solder paste was highly shear-thinning and thixotropic. In oscillatory shear, the linear viscoelastic domain was found to be very narrow for both the suspending fluid and the paste. Frequency sweep tests in the linear domain revealed a gel-like structure with a nearly constant G′ for the suspending fluid and a slightly increasing G′ for the solder paste. From creep experiments, a yield stress of about 40 Pa was determined for the suspending fluid at temperatures between 25 and 40°C, and of 100 Pa at 4°C. A much larger yield stress, 480 Pa, was determined for the solder paste at 25°C.     

Flowing colloidal suspensions in non-Newtonian suspending fluids: Decoupling the composite birefringence

The rheology of dilute, colloidal suspensions in polymeric suspending fluids can be studied with simultaneous dichroism and birefringence measurements. The dichroism provides a direct measure of the particle dynamics, but the birefringence is a composite property with independent contributions from the suspended particles and the polymer molecules. For suspensions where the contribution from the particles is significant, the composite birefringence must be decoupled in order to analyze the dynamics of the polymeric suspending fluid. A method to perform the decoupling is derived and then demonstrated through transient shear flow experiments with dilute suspensions ofFeOOH particles in semi-dilute, xanthan gum suspending fluids. The birefringence of the xanthan gum suspending fluid is calculated from experimental measurements of the composite birefringence and the dichroism of the suspension. To gather information on particle/polymer interactions, the calculated birefringence is compared to the birefringence of xanthan gum solutions containing no suspended particles and the dirchoism is compared to that of a suspension in a Newtonian fluid.     

Dissipation in a dilute suspension of spheres in a second-order fluid

We examine the effective medium properties of a dilute suspension of spheres in a second-order fluid under linear shear. Since the second-order fluid is the first step toward the general viscoelastic fluid, the results obtained may provide a qualitative feel for the problem in which the suspending fluid obeys a more complicated (and realistic) constitutive relation.The dissipation in the medium is calculated by determining the rate of working by surface forces; this is compared to the dissipation in a homogeneous fluid to give the effective properties. The results show that the term linear in volume fraction increases the corresponding rheological coefficient, just as in the Newtonian case. It is to be noted that the second-order dissipation is zero for simple shear and other weak flows, whereas for strong flows the small correction may increase or decrease the overall dissipation.     

Modeling radial filtration in squeeze flow of highly concentrated suspensions

Liquid-phase migration in highly concentrated suspensions undergoing constant-force squeeze flow is modeled numerically by taking into account the time and position dependence of the rheological properties due to changes in the volume fraction of solids. This is done by coupling the equation of motion for a non-Newtonian material that behaves approximately as a Bingham plastic with a continuity equation that includes diffusive flux. The developed model was first tested with experimental data and then used to study the effect of various parameters on liquid-phase migration.     

Shear rheometry and visualization of glass fiber suspensions

The nonlinear rheological behavior of short glass fiber suspensions has been investigated in this work by rotational rheometry and flow visualization. A Newtonian and a Boger fluid (BF) were used as suspending media. The suspensions exhibited shear thinning in the semidilute regime and weaker shear thinning in the transition to the concentrated one. Normal stresses and relative viscosity were higher for the BF suspensions than for the Newtonian ones presumably due to enhanced hydrodynamic interactions resulting from BF elasticity. In addition, relative viscosity of the suspensions increased rapidly with fiber content, suggesting that the rheological behavior in the concentrated regime is dominated by mechanical contacts between fibers. Visualization of individual fibers and their interactions under flow allowed the detection of aggregates, which arise from adhesive contacts. The orientation states of the fibers were quantified by a second order tensor and fast Fourier transforms of the flow field images. Fully oriented states occurred for shear rates around 20 s^− 1. Finally, the energy required to orient the fibers was higher in step forward than in reversal flow experiments due to a change in the spatial distribution of fibers, from isotropic to planar oriented, during the forward experiments.     

Spatially periodic suspensions of convex particles in linear shear flows. II. Rheology

Following the purely kinematical developments of Part 1, a rigorous analysis is presented of the “almost” time-periodic low Reynolds number hydrodynamics of a spatially periodic suspension of identical convex particles in a Newtonian liquid undergoing a macroscopically homogeneous linear shear flow. By considering the case of a single particle within a unit cell of the instantaneous spatially periodic configuration, the quasistatic dynamical analysis of this infinite-particle system is effected in much the same way as for a single particle suspended in an unbounded fluid. This is accomplished via the introduction of a partitioned hydrodynamic Stokes resistance matrix, linearly relating the force, couple and stresslet on the particle in the unit cell to the translational and rotational particle-(mean) suspension slip velocities and the mean rate-of-strain of the suspension. In contrast with the unbounded fluid case for a given geometry of the individual particles, the (purely geometric) elements of the resistance matrix depend upon the instantaneous lattice configuration.These dynamic quasistatic calculations for a given instantaneous lattice conformation, in particular that for the stresslet, are then appropriately averaged over both space and time for the class of almost time-periodic, lattice-reproducing, flows discussed in Part I. (In actually performing the time average, an important distinction is drawn between the ergodic and deterministic shear processes whose kinematical basis was laid in Part I.) In turn, this averaged dynamical information is translated into knowledge of the rheological properties of the macroscopically homogeneous suspension.A rigorous asymptotic, lubrication-theory analysis is performed during the course of an illustrative calculation of the rheological properties of a concentrated suspension of almost-touching spheres in a simple shear flow. Contrary to the findings of a previous heuristic treatment of this same lubrication-theory problem—one that ignores evolutionary variations in the instantaneous geometrical configuration of the spatially periodic suspension as the shear proceeds—the time-average properties of the suspension are found to be nonsingular in the limit.Finally, brief comments are offered on potential extensions of the scheme to include nonlinear phenomena, such as nonNewtonian fluids and inertial effects.     

On the rheology of red blood cell suspensions with different amounts of dextran: separating the effect of aggregation and increase in viscosity of the suspending phase

We investigate the shear thinning of red blood cell-dextran suspensions. Microscopic images show that at low polymer concentration, aggregation increases with increasing concentration until it reaches a maximum and then decreases again to non-aggregation. This bell-shape dependency is also deduced from the rheological measurements, if the data are correctly normalized by the viscosity of the suspending phase since a significant amount of polymers adsorb to the cell surfaces. We find that the position of the maximum of this shear rate-dependent bell shape increases with increasing viscosity of the suspending phase, which indicates that the dynamic process of aggregation and disaggregation is coupled via hydrodynamic interactions. This hydrodynamic coupling can be suppressed by characterizing a suspension of 80 % hematocrit which yields good agreement with the results from the microscopical images.     

The rheological behavior of flexible-chain polymers in the region of high shear rates and stresses,the critical process of spurting,and supercritical conditions of their movement at T > Tg

Methods of capillary viscometry were used in studying the rheological properties and behavior of a broad range of rubbers, including polymers with narrow and wide molecular-weight-distribution as well as commercial rubber grades, at widely varying shear rates and stresses. As is shown, in full conformity with the previously conducted experiments, during transition from a fluid to highelastic (quasi-cross-linked) state, they are chracterized by spurting followed by sliding over the channel walls. This relaxation transition is characterized by a critical shear stress value invariant with respect to the molecular weight, molecularweight distribution and temperature. The parameters defining spurting of polymer flow as a function of molecular-weight characteristics, temperature, and channel geometry have been investigated in detail. It is shown for the first time that under supercritical conditions the rate of polymer flow through channels does not depend, in the first approximation, on the molecular weight of the polymer, its molecularweight distribution, temperature, and filling, but is determined only by the shear stress.     

A thermodynamic approach to the rheology of highly interactive filler-polymer mixtures: Part I – Theory

The rheological behavior of highly interactive filler-polymer mixtures is simulated utilizing a double network created by the entangled polymer matrix and the adsorbed polymer. Both networks are represented by a nonlinear viscoelastic constitutive equation. The dependence of rheological properties on filler concentration is taken into account through the bridging density resulting from polymer-filler interactions and a hydrodynamic reinforcement. The relative contribution of both networks is computed through the energy balance consistent with the thermodynamics of the polymer-filler chemical interactions and fluid mechanics. This self-consistent approach allows one to calculate the strain dependence of dynamic properties under oscillatory flow and shear rate dependence of stresses under steady simple shear flow and upon start up and cessation of shear flow. Received: 11 May 2000 Accepted: 8 March 2001     

Non-linear viscoelastic behavior of fumed silica suspensions

Suspensions of fumed silica exhibit a wide range of rheological properties depending on the nature and magnitude of the interparticle forces. In a non-polar fluid, the particles interact through hydrogen bonding and can form a three-dimensional network. The microstructure formation is responsible for the non-linear viscoelastic behavior of fumed silica suspensions, even at very small strain. These non-linear rheological properties have been studied in small amplitude oscillatory experiments as a function of particle size, surface treatment of particles, suspending medium polarity and solids concentration. The non-linear viscoelastic behavior is characterized by a non-sinusoidal waveform of the signal response. For suspensions in a non-polar fluid, both the elastic and the loss moduli are shown to be sensitive to the strain amplitude: the elastic modulus is decreasing with increasing strain whereas the loss moduli is initially increasing with strain. We have chosen to examine the dissipated energy which is clearly related to the breakup of the suspension structure. A comparison of model predictions and the experimental data shows the limitations of these models, recently proposed in the literature to describe the behavior of colloidal suspensions. Received: 9 March 1998 Accepted: 17 November 1998