Showing papers on "Rheometer published in 2008"

High shear rate viscometry

Abstract: We investigate the use of two distinct and complementary approaches in measuring the viscometric properties of low viscosity complex fluids at high shear rates up to 80,000 s−1. Firstly, we adapt commercial controlled-stress and controlled-rate rheometers to access elevated shear rates by using parallel-plate fixtures with very small gap settings (down to 30 μm). The resulting apparent viscosities are gap dependent and systematically in error, but the data can be corrected—at least for Newtonian fluids—via a simple linear gap correction originally presented by Connelly and Greener, J. Rheol, 29(2):209–226, 1985). Secondly, we use a microfabricated rheometer-on-a-chip to measure the steady flow curve in rectangular microchannels. The Weissenberg–Rabinowitsch–Mooney analysis is used to convert measurements of the pressure-drop/flow-rate relationship into the true wall-shear rate and the corresponding rate-dependent viscosity. Microchannel measurements are presented for a range of Newtonian calibration oils, a weakly shear-thinning dilute solution of poly(ethylene oxide), a strongly shear-thinning concentrated solution of xanthan gum, and a wormlike micelle solution that exhibits shear banding at a critical stress. Excellent agreement between the two approaches is obtained for the Newtonian calibration oils, and the relative benefits of each technique are compared and contrasted by considering the physical processes and instrumental limitations that bound the operating spaces for each device.

Viscosity and Specific Heat of Vegetable Oils as a Function of Temperature: 35°C to 180°C

Abstract: The viscosities and specific heat capacities of twelve vegetable oils were experimentally determined as a function of temperature (35 to 180° C) by means of a temperature controlled rheometer and differential scanning calorimeter (DSC). Viscosities of the oil samples decreased exponentially with temperature. Out of the three models (modified WLF, power law, and Arrhenius) that were used to describe the effect of temperature on viscosity, the modified WLF model gave the best fit. The specific heat capacity of the oil samples however increased linearly with increase in temperature. The equations developed in the study could be valuable for designing or evaluating handling and processing systems and equipment that are involved in the storage, handling and utilization of vegetable oils.

The rheology of shear thickening fluid (STF) and the dynamic performance of an STF-filled damper

Abstract: This paper presents a study of the rheological properties of shear thickening fluid (STF) and its application as a damper. The STF samples, with different weight fractions, were prepared by dispersing nanosized silica particles in a solvent. By using a parallel-plate rheometer, both steady-state and dynamic experiments were carried out to investigate the rheological properties of STFs. Experimental results indicated that these suspensions show an abrupt increase in complex viscosity beyond a critical dynamic shear rate, as well as this increase being reversible. Working with the fabricated STF materials, a prototype damper was fabricated and its dynamic performances were experimentally evaluated. An equivalent linear model through effective elastic stiffness and viscous damping was developed to address both the damping and the stiffness capabilities of the damper. Also, a mathematical model was developed to investigate working mechanisms of STF-based devices.

Analysis of the magnetic rod interfacial stress rheometer

Abstract: The magnetic interfacial needle stress rheometer is a device capable of sensitive rheological interfacial measurements. Yet even for this device, when measuring interfaces with low elastic and viscous moduli, the system response of the instrument contributes significantly to the measured response. To determine the operation limits of the magnetic rod rheometer, we analyze the relative errors that are introduced by linearly subtracting the instrument contribution from the measured response. An analysis of the fluid mechanics demonstrates the intimate coupling between the flow field at the two-dimensional interface and in the bulk at low Boussinesq number. A nonzero Reynolds number is observed to have a similar order of magnitude effect. The resulting nonlinear interfacial deformation profiles lead to an error, which depends on the magnitude of the interfacial modulus, as well as on the phase angle. The conditions under which reliable measurements can be obtained are identified. Based on the analysis of the...

Magnetorheology of submicron diameter iron microwires dispersed in silicone oil

Abstract: We investigate the magnetorheological (MR) properties of suspensions containing iron microwires with 260 nm diameter and two distinct length distributions of 54 ± 52 µm and 76 ± 51 µm suspended in silicone oil (045 Pa s) The rheological properties of these fluids were determined using a parallel plate rheometer equipped with a variable strength electromagnet The shear stress was measured as a function of shear rate for increasing applied magnetic fields These results were modeled using the Bingham-plastic constitutive model to determine the apparent yield stress and viscosity as a function of increasing volume fraction and length of microwires At a saturated magnetic flux density, the yield stress using the 54 µm microwires was found to be 065, 223, and 476 kPa for the 2, 4, and 6 vol% suspensions, respectively For the 76 µm wires, the yield stress increases to 82 kPa for the 6 vol% suspension Compared with conventional MR fluids employing spherical particles, the degree of settling is markedly decreased in the microwire-based fluids At 6 vol%, conventional fluids display appreciable settling whereas the microwire-based fluids display no discernable settling Moreover, the rod-shaped microwires are shown to increase the yield stress of the fluids and enhance the MR performance

Thin film and high shear rheology of multiphase complex fluids

Abstract: We explore the high-shear and gap-dependent theological properties of multiphase complex fluids using narrow-gap parallel-plate rheometry. This technique has been developed to explore the apparent theological properties of such fluids when they are confined to length scales comparable to that of their underlying microstructure. This is particularly relevant to processes such as lubrication and microfluidics, whereby complex fluids are typically confined to length scales of below 100 mu m and subjected to shear rates well in excess of 1000 s(-1). We demonstrate that the parallel-plate geometry is capable of accessing extremely high shear rates (e.g. 10(5) s(-1)) using narrow gap heights (5-100 mu m) for Newtonian, shear-thinning, and elastic fluids. In order to obtain meaningful measurements, numerous errors that arise must be accounted for. The most apparent error is that the measured viscosity decreases with gap height at gaps below a few hundred microns. This results from an error in the gap that is typically 5-30 mu m and usually occurs due to misalignment, of the parallel plates, although there is also a contribution from the squeeze flow of air during the gap-zeroing procedure for very accurately aligned plates. The effect of microscale-confinement on the apparent viscosity and viscoelastic properties of microstructured fluids and suspensions is also considered, whereby confinement to gaps that are approaching that of the characteristic microstructure length scale causes a solid-like response with a substantially enhanced storage modulus and apparent yield stress. Despite confinement and jamming effects at low stresses, at high stress the multiphase fluids flow with a viscosity similar to that of bulk fluid and continuous phase even when the gap height is similar to the particle size. Slip and depletion effects are particularly apparent at narrow gaps and must be considered in order to obtain reliable rheological measurements. It is anticipated that the utilisation of these techniques to explore the dynamics of confined microstructures will lead to new insights into the behaviour of such systems under the extreme conditions of narrow gaps and/or high shear rates that are experienced during many processes and/or applications. (c) 2007 Published by Elsevier B.V.

Simulations of extensional flow in microrheometric devices

Abstract: We present a detailed numerical study of the flow of a Newtonian fluid through microrheometric devices featuring a sudden contraction–expansion. This flow configuration is typically used to generate extensional deformations and high strain rates. The excess pressure drop resulting from the converging and diverging flow is an important dynamic measure to quantify if the device is intended to be used as a microfluidic extensional rheometer. To explore this idea, we examine the effect of the contraction length, aspect ratio and Reynolds number on the flow kinematics and resulting pressure field. Analysis of the computed velocity and pressure fields show that, for typical experimental conditions used in microfluidic devices, the steady flow is highly three-dimensional with open spiraling vortical structures in the stagnant corner regions. The numerical simulations of the local kinematics and global pressure drop are in good agreement with experimental results. The device aspect ratio is shown to have a strong impact on the flow and consequently on the excess pressure drop, which is quantified in terms of the dimensionless Couette and Bagley correction factors. We suggest an approach for calculating the Bagley correction which may be especially appropriate for planar microchannels.

Study on magnetorheological shear thickening fluid

Abstract: In this paper, a magnetic-field-controlled and speed-activated magnetorheological shear thickening fluid (MRSTF) is presented. We fabricated a kind of shear thickening fluid (STF) which was composed of nanosize silica particles suspended in a solvent, ethylene glycol, at high concentrations. Then the micron-size carbonyl iron particles with different volume fractions were added to the STF to fabricate the MRSTF. Their dynamic properties in different shear strain rates and magnetic fields were tested by using a rheometer. The suspension shows an abrupt increase in complex viscosity beyond a critical dynamic shear rate and a magnetic-field-controllable characteristic, as well as being reversible.

Filament stretching rheometry and break-up behaviour of low viscosity polymer solutions and inkjet fluids

Abstract: This paper reports experimental observations on the way certain low viscosity Newtonian, polymer and inkjet fluids respond to filament stretching experiments that have been carried out using a variant of a multipass rheometer (MPR). A series of experiments were conducted where the opposing pistons of an MPR were used to provide controlled separation of two flat surfaces. Using 1.2 and 5.0 mm diameter pistons, a small quantity of test fluid was positioned between the pistons and the two pistons were moved apart at an equal and opposite velocity, thereby enabling optical interrogation of the central position of the filament that formed between the pistons faces. High speed photography followed the way the resulting fluid filament stretched and relaxed when the pistons movement had stopped. Different piston diameters, piston velocities and final piston separation were explored and the filament stretching and break-up was classified into regimes of behaviour. Approximate extensional viscosity parameters were obtained from the results. In some cases it was possible to correlate the filament stretching behaviour with the inkjet printing behaviour of a particular fluid.

Integration approach of the Couette inverse problem of powder type self-compacting concrete in a wide-gap concentric cylinder rheometer

Abstract: For powder type self-compacting concrete (SCC) mixes, commonly used in Belgium, a shear thickening (Herschel–Bulkley) flow behaviour of the fresh mixes is quite often observed. A longstanding problem in rheometry is the so-called “Couette inverse problem”, where one tries to derive the flow curve τ ( γ ˙ ) from the torque measurements T(N) in a (wide-gap) concentric cylinder (Couette) rheometer, with T the torque registered at the inner, stationary cylinder and N the rotational velocity of the outer, rotating, cylinder. In this paper, the Couette inverse problem is approached by means of the integration method in order to convert T(N) into τ ( γ ˙ ) for a wide-gap (Ro/Ri = 1.45) concentric cylinder rheometer. The approach consists in the decoupling of the flow resistance and the power-law flow behaviour after exceeding the flow resistance. The integration approach is validated by experimental verification with different powder type SCC mixtures. By means of illustration, the results of one limestone powder type SCC mixture with different superplasticizer contents are shown in this paper.

Waxy Oil Gel Breaking Mechanisms: Adhesive versus Cohesive Failure

Abstract: Highly paraffinic (or waxy) crude oil can cause significant problems in subsea pipelines due to wax–oil gel blockage resulting from the precipitation of the wax components. Once blockage of the pipeline occurs and flow ceases, the pipeline flow cannot be restarted with the original steady state operating pressure but instead requires significantly higher pressures to restart the flow. In order to understand the wax–oil gel restarting phenomenon in the field pipelines, we have investigated the gel breaking mechanism at various temperatures and cooling rates using a model pipeline and also a controlled stress rheometer. This study has revealed that the controlled stress rheometer can successfully predict the required gel breaking pressure of a gelled pipeline when the cooling rate is low and breakage occurs between the gel and the pipe wall (adhesive failure). Furthermore, we have experimentally shown that there exists a delineation point between cohesive and adhesive failures when the gel strength is measu...

A simple-shear rheometer for linear viscoelastic characterization of vocal fold tissues at phonatory frequencies

Abstract: Previous studies reporting the linear viscoelastic shear properties of the human vocal fold cover or mucosa have been based on torsional rheometry, with measurements limited to low audio frequencies, up to around 80 Hz. This paper describes the design and validation of a custom-built, controlled-strain, linear, simple-shear rheometer system capable of direct empirical measurements of viscoelastic shear properties at phonatory frequencies. A tissue specimen was subjected to simple shear between two parallel, rigid acrylic plates, with a linear motor creating a translational sinusoidal displacement of the specimen via the upper plate, and the lower plate transmitting the harmonic shear force resulting from the viscoelastic response of the specimen. The displacement of the specimen was measured by a linear variable differential transformer whereas the shear force was detected by a piezoelectric transducer. The frequency response characteristics of these system components were assessed by vibration experiments with accelerometers. Measurements of the viscoelastic shear moduli (G' and G") of a standard ANSI S2.21 polyurethane material and those of human vocal fold cover specimens were made, along with estimation of the system signal and noise levels. Preliminary results showed that the rheometer can provide valid and reliable rheometric data of vocal fold lamina propria specimens at frequencies of up to around 250 Hz, well into the phonatory range.

Modeling the frictional boundary condition in friction stir welding

Abstract: Material flow and frictional heating in friction stir welding are investigated using a three-dimensional numerical model. Two mechanical boundary conditions are investigated, including a sticking constant velocity and a slipping variable shear stress model. For the constant velocity model, material in contact with the tool is set at a velocity equal to some fraction of the tool rotational speed. The variable shear stress model is formulated such that it degenerates to Coulomb friction under low forces, but as forces at the tool increase, shear stresses at the tool/material boundary are limited according to Tresca friction. The variable shear condition model permits areas of significant slip, while shear stresses at other regions approach the flow stress of the material. The boundary models are compared with experimental data from flow visualization experiments and thermocouple measurements from plasticine welds. Results show that the variable shear model is superior to a sticking condition. With the variable shear model, the peak temperature in the weld compares well with thermocouple measurements. The variable shear model predicts that material simply extrudes around the tool, while the constant velocity model shows a region of material that rotates with the tool. Maximum velocities with the variable shear stress model are only 9% of the rotational speed of the tool and agree well with experimental findings. Additionally, the variable shear model shows a region of diminishing shear stress, velocity, and pressure at the back advancing side of the pin, suggesting formation of an internal void. The limited deformation, low velocities, and suggestion of void formation agree well with flow visualization studies using plasticine under identical operating parameters.

The measurement of mechanical properties of glycerol, m-toluidine, and sucrose benzoate under consideration of corrected rheometer compliance: an in-depth study and review.

Abstract: Determination of the mechanical response of materials can be fraught with error if rheometer compliance is not properly taken into account. The resulting inaccuracies in the determined mechanical properties of the materials of interest can result in mistakes in material modeling, design, and theory. In the present work, we build on our previous report [K. Schroter, S. A. Hutcheson, X. Shi, A. Mandanici, and G. B. McKenna, J. Chem. Phys. 125, 214507 (2006)] and investigate the effects of instrument compliance that result from use of a commercial rotary rheometer and its fixtures on the determination of the dynamic shear and stress relaxation responses of glycerol, m-toluidine, and sucrose benzoate near to the glass transition regime. We revisit the procedure for compliance corrections presented in earlier work and correct dynamic shear data for these materials. We also present a new correction procedure to obtain shear stress relaxation curves from data that was obtained using this instrument. In addition, we broaden our consideration of compliance effects to materials, such as polymer melts, that have lower moduli than the simple glass formers previously considered. We discuss the possible errors for the viscoelastic response of glass-forming liquids and polymer melts or rubber networks that have been reported in the literature. A major purpose of the present work is to alert the community to possible problems in modulus values and relaxation functions obtained for a large number of materials for which rotary rheometers were used in a range where material stiffness (modulus and geometry effects) was comparable to the rheometer stiffness. Finally, we include recommendations for both experimental protocol and instrument design to avoid, minimize, or correct for compliance effects.

The Viscosity Properties of Sodium Silicate Solutions

Abstract: Using a NDJ-1 rotational viscometer and an AR500 rheometer, both static and dynamic viscosities of sodium silicate solutions were measured with changes of concentration, temperature, modulus (molar ratio of SiO2 to Na2O), shear rate and chemical additives. Static results show that viscosity increases monotonously with concentration varying from 15 to 55%, decreases with temperature rising from 15 to 70 °C, and has a minimum value at a modulus of about 1.8. Measured data can be fitted quantitatively either by the Krieger-Dougherty expression or the Arrhenius equation with good agreement. This fact suggests that the sodium silicate solutions exhibit the properties of a suspension, in which the silicate anions, mainly constructed of Q 1 and Q 2 groups, act as a binder; the colloidal particles mainly constructed of Q 3 and Q 4 groups and small cations, act as effective rigid particles. Dynamic results show a shear thickening property in the high shear-rate regime, and a Newtonian property in the low shear-rate regime.

Rheometric measurement of dough rheological characteristics and factors affecting it.

Abstract: Bread is one of the most important foods consumed all over the world. This review focuses on the use of rheometer for the measurement of dough rheological properties and factors affecting them. Rheological properties of dough are very important in bread baking quality. Knowledge of the rheological behavior of bread dough is very important to understand mechanical properties of the dough and control finished products. Small amplitude oscillatory shear (SAOS) measurements afford the measurement of dynamic rheological functions, without altering the internal network structure of materials tested and are far more reliable than steady shear measurement. Viscoelasticity of dough is related to many factors such as nature of flour, dough ingredient, temperature, water uptake, air incorporation and type of mixing. There are many models to predicate dough rheology. In this work some of these models such as power low, linear Maxwell model, Lethersich's model, Peleg’s model and etc., were presented. The instruments such as farinograph, mixograph, Rheomixer, Extensigraph, Alveograph, Amylograph, Maturograph, Oven Rise Recorder, Fermentometer, Dynamic oscillatory, Concentric cylinders, Parallel plates, which are used for the measurement of dough rheological properties (due to viscoelastic behavior of dough) were also described. Key Word: Bread; Dough; Rheology; Flour; Rheology models

Processing the Couette viscometry data using a Bingham approximation in shear rate calculation

Abstract: This paper presents an approach to computing the shear flow curve from torque–rotational velocity data in a Couette rheometer. The approximation techniques in shear rate calculation are generally dictated by the radius ratio between coaxial cylinders and the rheological behaviour of fluid tested. Here, the approach consists in analysing the sheared material as a Bingham fluid and computing an average shear rate when the fluid in the cylindrical gap is partially and fully sheared. We focus in particular on the applicability of the Bingham approximation in shear rate calculation. First, the approach is assessed by examining synthetic data generated with Newtonian, non-Newtonian and yield stress materials with known properties, varying the gap radius ratio. The results, which are compared with commonly used techniques in shear rate calculation, prove the relevance of the proposed approach. Finally, its efficiency is examined by applying it to process Couette data of yield stress fluids taken from published works.

Primary and secondary normal stress differences of a magnetorheological fluid (MRF) up to magnetic flux densities of 1 T

Abstract: First and second normal stress differences of a 50 vol.% magnetorheological fluid (MRF) are investigated by using a commercial plate–plate magneto-rheometer (Anton Paar GmbH) with plate–plate and cone–plate geometry. The manufacturer modified the instrument to achieve higher normal force (60 N) and torque (295 mNm) capacity. An additional modification by us allows an online determination of the true magnetic flux density B in the MRF by means of a Hall probe. FEM Maxwell 2D simulations quantitatively verify the Hall probe results and give detailed insight into the radial flux density profile within the MRF sample. Without shear, the static normal force FN for plate–plate increases as a power law: FN ∝ B2.4. A similar magnitude is found for cone–plate geometry, in contrast to the expectation. For steady shear at 10 s−1, the plate–plate normal force built-up limits the experiments at high flux densities rather than the torque generated. The normal forces increase linearly with the shear stress at high flux density. The first normal stress difference N1 is positive and about five times larger than the shear stress. The second normal stress difference N2 is also positive. The experimentally derived N2/N1 ratio of 1/4 distinctly deviates from theoretical predictions (N2/N1 = −1) for a semi-dilute MRF. Improvements of the radial flux density profiles are required to verify the normal stress difference ratio and to support the conjecture that the positive but small N2 is a consequence of the densely packed MRF, which does not allow to create extended chain-like structures. As shown in the appendix, the experimentally determined N2/N1 ratio is favorable to stabilize concentricity in concentric cylinder arrangements, relevant for the MRF application in clutches.

Influence of Shear on the Templated Crystallization of Poly(butylene terephthalate)/Single Wall Carbon Nanotube Nanocomposites

Abstract: The templating effect due to single wall carbon nanotubes (SWCNT) and shear on polymer crystallization has been studied in films of nanocomposites based on poly(butylene terephthalate) (PBT). With the use of a rheometer, a step shear was applied to the molten polymer. After shear cessation, the sample was immediately cooled down to the crystallization temperature. Crystalline development, in real time, was investigated by small-angle X-ray scattering (SAXS) with a synchrotron radiation beam parallel to the film. SWCNT bundles template polymer lamellae to grow perpendicular to the SWCNT surfaces in a shish-kebab fashion even under quiescent conditions. Because of the power of SWCNT as nucleating agents, the shear rate has a minor effect on the crystallization kinetics. However, the fraction of oriented material increases significantly with shear rate. The results indicate that SWCNT act as nuclei stabilizers, providing surfaces which favor polymer crystallization.

Rheology of Non-Newtonian Fluids Containing Glass Fibers: A Review of Experimental Literature

Abstract: The objective of this review is to elucidate the rheological behavior of glass fiber suspensions whose suspending mediums are non-Newtonian fluids. In particular, this review focuses on determining the impact of fiber concentration, aspect ratio, orientation distribution, interaction with the suspending medium, and suspending medium viscoelasticity on the rheology of glass fiber composite fluids. The presence of glass fiber can induce a yieldlike behavior, causing shear thinning to occur at reduced shear rates. Glass fiber can impede the elastic properties of the suspending medium but enhance the first normal stress function. Large stress overshoots in both the shear and normal stress growth functions are observed that are associated with changes in fiber orientation. Upon cessation of flow, stress relaxation follows that of the suspending medium but fibers retain their orientation. The presence of glass fiber can induce extension rate thinning and suppress the strain thickening behavior of the suspending medium. 1. Introduction Glass fibers have been used for decades to improve the mechanical, thermal, and insulative properties of polymers. 1 These property improvements are highly dependent on the orientation distribution of the glass fiber. This makes it desirable to be able to predict not only the rheological behavior of the composite fluid but also the orientation of the fiber generated during processing. Understanding the rheological behavior of polymeric fluids containing glass fibers is essential to model development. With respect to the fiber, it is of interest to understand the role of concentration and aspect ratio and their relation to the degree of interparticle interaction (fiber -fiber) as well as orientation distribution and interaction with the suspending medium. With respect to the suspending medium, it is of interest to understand the role of viscoelasticity and how it is affected by the presence of the fibers. The rheological properties of glass fiber suspensions in Newtonian suspending mediums has been reviewed in detail by Ganani and Powell 2 and Zirnsak et al. 3 and will be referred to in this review for comparison purposes only. The primary focus of this review is to elucidate the rheological properties of glass fiber suspensions in non-Newtonian fluids of various degrees of viscoelasticity with an emphasis on composite fluids. Before reviewing the rheology, it is imperative to have a basic understanding of glass fiber suspensions, the rheometers used to characterize them, and any extra forces that can lead to mechanisms for changes in the fiber microstructure. For this reason, subsequently, we will briefly discuss the use of surface treatments to increase the interaction with the matrix. We then classify fiber suspensions by their concentration and length. This is followed by a review of the different rheometers and rheometer geometries used to characterize glass fiber suspensions, including a discussion of their strengths and weaknesses with respect to obtaining accurate measurements of rheological material functions. Last, we discuss and estimate the contribution of Brownian motion and gravity (relating to particle sedimentation) to changes in fiber orientation within the suspension. 1.1. Surface Modifications. The surface of a glass fiber is typically modified for two reasons: to minimize the selfdestructive abrasive contact between the fibers and to increase the fiber-matrix interaction of the melt and the adhesive strength of the composite. The use of “sizing” as a surface treatment has become an industrial standard and addresses both the abrasive contact and the surface interaction. Sizing is a functional coating that acts as an abrasive barrier that is tailored