Showing papers on "Rheometer published in 2005"

Capillary break-up rheometry of low-viscosity elastic fluids

Abstract: We investigate the dynamics of the capillary thinning and break-up process for low viscosity elastic fluids such as dilute polymer solutions. Standard measurements of the evolution of the midpoint diameter of the necking fluid filament are augmented by high speed digital video images of the break up dynamics. We show that the successful operation of a capillary thinning device is governed by three important time scales (which characterize the relative importance of inertial, viscous and elastic processes), and also by two important length scales (which specify the initial sample size and the total stretch imposed on the sample). By optimizing the ranges of these geometric parameters, we are able to measure characteristic time scales for tensile stress growth as small as 1 millisecond for a number of model dilute and semi-dilute solutions of polyethylene oxide (PEO) in water and glycerol. If the final aspect ratio of the sample is too small, or the total axial stretch is too great, measurements are limited, respectively, by inertial oscillations of the liquid bridge or by the development of the well-known beads-on-a-string morphology which disrupt the formation of a uniform necking filament. By considering the magnitudes of the natural time scales associated with viscous flow, elastic stress growth and inertial oscillations it is possible to construct an operability diagram characterizing successful operation of a capillary break-up extensional rheometer. For Newtonian fluids, viscosities greater than approximately 70 mPas are required; however for dilute solutions of high molecular weight polymer, the minimum Viscosity is substantially lower due to the additional elastic stresses arising from molecular extension. For PEO of molecular weight 2.10(6) g/mol, it is possible to measure relaxation times of order 1 ms in dilute polymer solutions with zero-shear-rate viscosities on the order of 2-10 mPas.

Measuring the transient extensional rheology of polyethylene melts using the SER universal testing platform

Abstract: We use a new extensional rheology test fixture that has been developed for conventional torsional rheometers to measure the transient extensional stress growth in a number of different molten polyethylene samples including a linear low density polyethylene (Dow Affinity PL 1880), a low density polyethylene (Lupolen 1840H), and an ultrahigh molecular weight polyethylene (UHMWPE). The transient uniaxial extensional viscosity functions for the linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) samples have both been reported previously in the literature using well-established instruments and this allows us to benchmark the performance of the new test fixture. Transient stress growth experiments are carried out over a range of Hencky strain rates from 0.003 to 30s−1 and the data show excellent agreement with the published material functions. At deformation rates greater than 0.3s−1 a true steady state extensional viscosity is not obtained in the LDPE samples due to the onset of neckin...

Correlation between yield stress and slump: Comparison between numerical simulations and concrete rheometers results

Abstract: Results of numerical flow simulations for two slump geometries, the ASTM Abrams cone and a paste cone, are presented. These results are compared to experimental results in the case of a cone filled with cement pastes in order to validate the proposed numerical method and the chosen boundary conditions. The correlation between slump and yield stress obtained numerically for the ASTM Abrams cone is then compared to the experimental correlations obtained by testing concrete with different rheometers during comparative studies that were organized at LCPC Nantes (France) in 2000 and MB Cleveland (USA) in 2003.

Thixotropy : Build-up and breakdown curves during flow

Abstract: Thixotropic systems are usually characterized by measuring the stress transients resulting from sudden changes in shear rate. By using a procedure that takes into account the transfer function of the transducer on a strain-controlled rheometer, the initial part of the transients could be accessed. In this manner complete transient stress curves and accurate constant-structure curves could be recorded on a model system consisting of fumed silica particles dispersed in a Newtonian medium. In agreement with previously obtained stress jump data [Dullaert and Mewis (2005b)], an initial viscoelastic response is detected that precedes the structural time effects in either build-up or breakdown experiments. The effect of flow history on the characteristic times and on the shape of the transient curves is investigated. It is shown that common assumptions in thixotropic models, such as a structure that can be characterized by a single parameter or the existence of exponential stress transients, do not generally hold. Although the material response is quite complex, the time constants of the stress transients obey a simple scaling law. The present data set is very suitable for model evaluation. Here, two representative models, those of Houska and of Coussot, have been tested. The initial parts of the curves are not well described. The time constants for the thixotropic structure build-up are reasonably well predicted, although shear history effects are not properly modeled. For the breakdown curves larger errors are recorded.

Role of organic coating on carbonyl iron suspended particles in magnetorheological fluids

Abstract: Carbonyl iron (CI) has been widely used as a suspended particle in magnetorheological (MR) fluids. However, pristine CI-based MR fluids have several drawbacks, including severe sedimentation of the CI particles due to the large density difference with the carrier liquid, difficulties in redispersion after caking, abrasion of device surfaces during long-term operation, and rust of iron by oxidation. To overcome these shortcomings, we coated the CI particles with a poly(vinyl butyral) (PVB) shell. CI and CI-PVB particles were suspended in mineral oil and their MR characteristics were examined via a rotational rheometer in a parallel plate geometry equipped with a magnetic field supplier. Yield stress and flow response (shear stress and shear viscosity) were investigated at magnetic field strengths ranging from 0to343kA∕m. Although the MR properties, such as yield stress and shear viscosity of CI-PVB based MR fluids, changed slightly compared with those of the pristine CI based MR fluid, the dispersion quali...

Liquid-liquid membrane dispersion in a stirred cell with and without controlled shear

Abstract: Oil was passed through membranes into a continuous water phase containing a surfactant (Tween 20) to form oil dispersions with drop diameters between 40 and 400 μm. Two types of stirred equipment were used: a Weissenberg rheometer (cone and plate geometry) providing constant shear stress at all radial positions which was modified to include a membrane instead of the plate and a simple stirred cell, with a paddle rotating above the membrane, providing variable shear with radial position. Experiments show that the simple paddle-stirred cell provided an oil drop dispersion that was as monosized as that produced by the controlled shear device, if not better. An analysis indicated that only the section of the membrane close to the radius of the highest shear under the paddle stirred membrane produced oil drops. The membranes used in the experiments contained a regular array of nontortuous pores uniformly spaced and provided oil injection rates up to 1000 L m-2 h-1, which is much higher than reported fluxes for the alternative tortuous pore channel membranes made by sintering.

Characterization of the viscoelastic behavior of complex fluids using the piezoelastic axial vibrator

Abstract: The piezoelectric axial vibrator (PAV) is a squeeze-flow rheometer working at frequencies f between 1 and 4000Hz. It can be used to measure the storage modulus G′ and the loss modulus G″ of complex fluids in this frequency range. Using polymer solutions with known G′ and G″ it is shown that the PAV gives reliable mechanical spectra for frequencies between 10 and 3000Hz. The measurements done with the PAV are combined with a conventional mechanical rheometer (10−3⩽f⩽15Hz) and a set of torsional resonators (f=13, 25, and 77kHz) to obtain G′ and G″ between 10−3Hz and 77kHz. Using this combination we present the first analysis of the viscoelasticity of an aqueous suspension of thermosensitive latex particles in this range of frequency. It is demonstrated that the combination of the three devices gives the entire mechanical spectra without resort to the time-temperature superposition principle.

High shear microfluidics and its application in rheological measurement

Abstract: High shear rheology was explored experimentally in microchannels (150×150 μm). Two aqueous polymer solutions, polyethylene oxide (viscoelastic fluid) and hydroxyethyl cellulose (viscous fluid) were tested. Bagley correction was applied to remove the end effect. Wall slip was investigated with Mooney’s analysis. Shear rates as high as 106 s−1 were obtained in the pressure-driven microchannel flow, allowing a smooth extension of the low shear rheological data obtained from the conventional rheometers. At high shear rates, polymer degradation was observed for PEO solutions at a critical microchannel wall shear stress of 4.1×103 Pa. Stresses at the ends of the microchannel also contributed to PEO degradation significantly.

Constitutive models of electrorheological and magnetorheological fluids using viscometers

Abstract: A key aspect of application of electrorheological (ER) and magnetorheological (MR) fluids is the characterization of rheological properties. In this study, two rotational viscometers to measure the field-dependent flow behavior (shear stress versus shear rate) of ER/MR fluids are theoretically analyzed. One is a rotational coaxial cylinder viscometer, and the other is a rotational parallel disk viscometer. The equations between shear stress and torque as well as shear rate and angular velocity are derived on the basis of the Bingham-plastic, biviscous, and Herschel–Bulkley constitutive models. The shear stress for the rotational coaxial cylinder viscometer can be straightforwardly calculated from the measured torque. However, in order to determine the shear rate, three approximation methods are applied. Meanwhile, the shear stress and shear rate in the rotational parallel disk viscometer can be obtained directly from the torque and angular velocity data. In order to comprehensively understand the flow behavior of ER/MR fluids with respect to the constitutive models, nondimensional analyses are undertaken in this study.

Morphological and Rheological Properties of Polyamide 6/Poly(propylene)/Organoclay Nanocomposites

Abstract: Summary: Polyamide 6 (PA6)/Poly(propylene) (PP)/organoclay (octadecylamine intercalated montmorillonite) systems with and without compatibilizer (maleated poly(propylene) and ethylene/propylene-based rubber (PPgMA and EPRgMA, respectively)) were produced by extrusion melt blending and the clay dispersion was characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The rheological behaviour of the nanocomposites was determined by various methods, viz melt flow index (MFI), capillary and plate/plate rheological measurements. Attempts were made to trace the rheological parameters that reliably reflect the observed changes in the clay dispersion. It was found that some parameters in the viscoelastic range derived from frequency sweep measurements using a plate/plate rheometer are a suitable indicator for changes in the clay dispersion. High initial viscosity with a strong reduction as a function of frequency was attributed to improved clay exfoliation. The latter was also reflected by a high initial shear storage modulus (G′) along with its small change with the frequency. Variation of the storage modulus (G′) as a function of frequency for the PA6/PP nanocomposites. Note: this figure also indicates how the slopes at low and high shear rates were determined.

Viscosity overshoot in the start-up of uniaxial elongation of low density polyethylene melts

Abstract: The transient uniaxial elongational viscosity of BASF Lupolen 1840D and 3020D melts has been measured on a filament stretch rheometer up to Hencky strains of 6–7. The elongational viscosity of both melts was measured at 130°C within a broad range of elongational rates. At high elongation rates, an overshoot or maximum in the transient elongational viscosity followed by a steady viscosity was observed. The steady elongation viscosity was about 40%–50% less than the maximum at high strain rates. The steady elongational viscosity as a function of the elongation rate, ϵ, decreases approximately as ϵ−0.6 in both melts at high strain rates. The transient elongational viscosity, measured at a specific elongation rate at 170°C on the BASF Lupolen 3020D melt, did not follow the time temperature superposition principle based on linear viscoelasticity during the decrease in the transient elongational viscosity towards the steady state.

Analysis of the rheological behavior and stability of 316L stainless steel–TiC powder injection molding feedstock

Abstract: An experimental rheological study has been performed to evaluate the influence of TiC addition on the rheological behavior and stability of 316L stainless steel powder injection molding (PIM) feedstock. The effects of TiC concentration, solid loading, shear rate and temperature were investigated via capillary rheometer method. The stability of feedstocks was evaluated quantitatively using “instability index”, which describes the threshold beyond which the variation of viscosity becomes unacceptable for PIM purposes. The results show that the rheological behavior of PIM feedstocks highly depends on the blend composition. The addition of TiC particles to the stainless steel powder increases the viscosity of feedstock at relatively low shear rates, i.e. 2000 s−1, the viscosity of composite feedstocks was lower than that of the mono-component SS feedstock due to the better particle packing efficiency. This article presents the rheological behavior of bimodal powder mixture of stainless steel and TiC powders prepared for PIM application. The influences of blend composition, i.e. the TiC concentration and solid loading, and the processing parameters, i.e. temperature and shear rate, are addressed.

Rheological properties of long glass fiber filled polypropylene

Abstract: Long glass fiber-filled polypropylene (PP) composites are produced by pultrusion, and the extrudate is cut at different lengths producing composites containing long fibers of controlled length. The rheological properties of such composites in the molten state have been studied using different rheometers. A capillary rheometer has been constructed and mounted on a molding-injection machine. The shear viscosity of filled PP determined from the capillary rheometer, after corrections for entrance effects, was found to be very close to that of unfilled PP. However, large excess pressure losses at the capillary entrance were observed and these data have been used to obtain an apparent elongational viscosity. The apparent elongational viscosity was shown to be considerably larger than the shear viscosity for PP and filled PP, and it increased markedly with fiber length and fiber content. Rotational rheometers with a parallel-plate geometry were used to investigate the viscoelastic properties of these composites and their behavior was found to be non-linear, exhibiting a yield stress. A model is proposed to describe the shear viscosity from a solid-like behavior at low stresses to fluid-like behavior at high shear stresses taking into account fiber content and orientation. A modified model, proposed for elongational flow, describes relatively well the apparent elongational data.

Sorbitan tristearate layers at the air/water interface studied by shear and dilatational interfacial rheology.

Abstract: The shear and dilatational rheology of condensed interfacial layers of the water-insoluble surfactant sorbitan tristearate at the air/water interface is investigated. A new interfacial shear rheometer allows measurements in both stress- and strain-controlled modes, providing comprehensive interfacial rheological information such as the interfacial dynamic shear moduli, the creep response to a stress pulse, the stress relaxation response to a strain step, or steady shear curves. Our experiments show that the interfacial films are both viscoelastic and brittle in nature and subject to fracture at small deformations, as was supported by in-situ Brewster angle microscopy performed during the rheological experiments. Although any large-deformation test is destructive to the sample, it is still possible to study the linear viscoelastic regime if the deformations involved are controlled carefully. Complementary results for the dilatational rheology in area step compression/expansion experiments are reported. The dilatational behavior is predominantly elastic throughout the frequency spectrum measured, whereas the layers exhibit generalized Maxwell behavior in shear mode within a deformation frequency regime as narrow as two decades, indicating the presence of additional relaxation mechanisms in shear as opposed to expansion/compression. If the transient rheological response from stress relaxation experiments is considered, then the data can be described well with a stretched exponential model both in the shear and dilatational deformations.

Rheology of concentrated suspensions containing mixtures of spheres and fibres

Abstract: Optimising flow properties of concentrated suspensions is an important issue common for many industries. The rheology of concentrated suspensions has therefore been studied intensively both experimentally and theoretically. Most studies have focused on monodisperse and polydisperse suspensions of either spheres or fibres. In practice, most suspensions contain particles that are polydisperse both in size and shape. A mixing rule for such systems is expected to be a powerful tool for engineers and product designers. Therefore in this work, suspensions of spheres, fibres and mixtures thereof were characterised using rotational shear rheometry and in-line image analyses. Thereby, total solids volume concentration and fibre fraction was varied. Results from transient and steady-state shear rheometry are discussed with respect to concentration, fibre fraction, and shear induced microstructure. Experimentally obtained viscosity data were accurately fitted using the model proposed by Farris (T Soc Rheol 12:281, 1968) for mixtures of monodisperse non-interacting spheres of different sizes.

Constriction flows of monodisperse linear entangled polymers: Multiscale modeling and flow visualization

Abstract: We explore both the rheology and complex flow behavior of monodisperse polymer melts. Adequate quantities of monodisperse polymer were synthesized in order that both the materials rheology and microprocessing behavior could be established. In parallel, we employ a molecular theory for the polymer rheology that is suitable for comparison with experimental rheometric data and numerical simulation for microprocessing flows. The model is capable of matching both shearand extensional data with minimal parameter fitting. Experimental data for the processing behavior of monodisperse polymers are presented for the first time as flow birefringence and pressure difference data obtained using a Multipass Rheometer with an 11:1 constriction entry and exit flow. Matching of experimental processing data was obtained using the constitutive equation with the Lagrangian numerical solver, FLOWSOLVE. The results show the direct coupling between molecular constitutive response and macroscopic processing behavior, and differentiate flow effects that arise separately from orientation and stretch.

Solving the Couette inverse problem using a wavelet-vaguelette decomposition

Abstract: This paper develops a new approach to computing the shear rate from the torque and rotational-velocity measurements in a Couette rheometer. It is based on wavelet-vaguelette decomposition (WVD) proposed by Donoho [Donoho, D., Appl. Comput. Harmon. Anal. 2, 101–126 (1995)]. This decomposition consists in expanding the shear rate into a truncated wavelet series, whose coefficients can be determined by computing the inner products of the wavelet functions with dual functions (vaguelette). Compared to other strategies used for recovering the shear rate such as Tikhonov regularization, the WVD method exhibits greater accuracy and faster convergence. Because of the spatial adaptivity of wavelets, it still performs well when the flow curve is irregular (yield stress, sudden behavior change, etc.) and thus no prior knowledge of the shear rate characteristics (e.g., existence of a yield stress, smoothness) is needed. Its efficiency is demonstrated by applying the method to two fluids (a polymeric gel and a granula...

Shear and extensional properties of short glass fiber reinforced polypropylene

Abstract: Shear and extensional properties of a commercial short glass fiber reinforced polypropylene were carefully investigated using commercial rheometers and a novel on-line rheometer. This on-line slit rheometer, installed on an injection molding press, has been designed to measure the steady shear viscosity, the first normal stress difference, and the apparent extensional viscosity of polymer melts and composites for high strain rates up to 105 s−1 in shear and 200 s−1 in extension. Our results show that the steady-state viscosity measurements using the on-line rheometer are in excellent agreement with those obtained using commercial rheometers. The steady-state and the complex viscosities of the composites were found to be fairly close to that of the matrix, but the Cox-Merz rule was not verified for the composites at high rates. The elasticity of the composites was found to be equal to that of the polypropylene matrix. The apparent extensional viscosity was obtained from the pressure drop in the planar converging die of the slit rheometer using the analyses proposed by Cogswell [1] and Binding [2]. The extensional viscosity of the polypropylene was found to be much larger than the shear viscosity at low strain rates with a Trouton ratio of about 40 that decreased rapidly with increasing strain rate down to the value of 4 at 200 s−1. The extensional viscosity of the composites was also found to be close to that of the matrix, with values 35 and 5% larger for the 30 and 10 wt% reinforced polypropylenes, respectively. These results are compared with the predictions of the Goddard model [3], which are shown to overpredict our experimental results. POLYM. COMPOS. 26:247–264, 2005. © 2005 Society of Plastics Engineers.