Showing papers in "Rheologica Acta in 2012"

Scaling and mesostructure of Carbopol dispersions

Abstract: Rheological measurements were performed on aqueous dispersions of two commercial crosslinked polymer microgels, Carbopol Ultrez 10 and Carbopol ETD 2050, prepared over a wide range of concentration and pH. For all concentrations studied, both the yield stress and the elastic modulus initially increased dramatically with pH and displayed broad peaks at intermediate pH. This is consistent with the onset of jamming of the Carbopol particles due to a rapid increase in particle size caused by osmotic swelling in the presence of NaOH. Scaling of both yield stress and elasticity with concentration was observed only at higher concentrations, which we believe indicates a change from a percolated structure at low volume fractions to a space filling network of compressed particles at high volume fractions. This model is supported by confocal microscopy of fluorescently dyed Carbopol dispersions.

Wall slip and melt fracture of poly(lactides)

Abstract: The wall slip and melt fracture behaviour of several commercial polylactides (PLAs) as well as their rheological properties under shear and extensional have been investigated. The PLAs have had weight-average molecular weights in the range of 104–105 g/mol and studied in the temperature range of 160–200°C. The solution properties and linear viscoelastic behaviour of melts indicate linear microstructure behaviour. PLAs with molecular weights greater than a certain value were found to slip, with the slip velocity to increase with decrease of molecular weight. The capillary data were found to agree well with linear viscoelastic envelope once correction for slip effects was applied. The onset of melt fracture for the high molecular weight PLAs was found to occur at about 0.2 to 0.3 MPa, depending on the geometrical characteristics of the dies and independent of temperature. Addition of 0.5 wt.% of a polycaprolactone (PCL) into the PLA that exhibits melt fracture was found to be effective in eliminating and delaying the onset of melt fracture to higher shear rates. This is due to significant interfacial slip that occurs in the presence of PCL.

Microsecond relaxation processes in shear and extensional flows of weakly elastic polymer solutions

Abstract: In this paper, we introduce an experimental protocol to reliably determine extensional relaxation times from capillary thinning experiments of weakly elastic dilute polymer solutions. Relaxation times for polystyrene in diethyl phthalate solutions as low as 80 μ s are reported: the lowest relaxation times in uniaxial extensional flows that have been assessed so far. These data are compared to the linear viscoelastic relaxation times that are obtained from fitting the Zimm spectrum to high frequency oscillatory squeeze flow data measured with a piezo-axial vibrator (PAV). This comparison demonstrates that the extensional relaxation time reduced by the Zimm time, λ ext/λ z, is not solely a function of the reduced concentration c/c*, as is commonly stated in the literature: an additional dependence on the molecular weight is observed.

Rheological and mechanical properties of silica colloids: from Newtonian liquid to brittle behaviour

Abstract: Rheological and mechanical properties of aqueous mono-disperse silica suspensions (Ludox® HS40) are investigated as a function of particle volume fraction (ϕ p ranging from 0.22 to 0.51) and water content, using shear rate tests, oscillatory methods, indentation and an ultrasonic technique. As the samples are progressively dried, four regimes are identified; they are related to the increasing particle content and the existence and behaviour of the electrical double layer (EDL) around each particle. For 0.22 ≤ ϕ p ≤ 0.30), the suspensions are stable due to the strong electrostatic repulsion between particles and show Newtonian behaviour (I). As water is removed, the solution pH decreases and the ionic strength increases. The EDL thickness therefore slowly decreases, and screening of the electrostatic repulsion increases. For 0.31 ≤ ϕ p ≤ 0.35, the suspensions become turbid and exhibit viscoelastic (VE) shear thinning behaviour (II), as they progressively flocculate. For 0.35 ≤ ϕ p ≤ 0.47, the suspensions turn transparent again and paste-like, with VE shear thinning behaviour and high elastic modulus (III). At higher particle concentration, the suspensions undergo a glass transition and behave as an elastic brittle solid (IV, ϕ p = 0.51).

Mechanical and structural investigation of isotropic and anisotropic thermoplastic magnetorheological elastomer composites based on poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS)

Abstract: Novel smart thermoplastic magnetorheological elastomer composites containing micron-sized magnetic carbonyl iron (CI) particles were prepared with a poly(styrene-ethylene-butylene-styrene) (SEBS) triblock copolymer utilized as the thermoplastic matrix rubber, and the structures and properties of the CI-SEBS composites were examined. The CI particles were uniformly dispersed in the composites prepared in the absence of the magnetic field at high temperatures T (>T\(_{\rm g}^{\rm S})\), and this isotropic composite exhibited a larger storage modulus G′ compared to the SEBS matrix at room temperature ( T\(_{\rm g}^{\rm S})\) contained a chain structure of CI particles. This chain structure became longer and better aligned on an increase of ψ up to a saturation of the particle magnetization and on an increase of the time interval of applying the field (that allowed the particles to move and equilibrate their aligned structure). The modulus G′ of this “pre-structured” composite measured for both cases of ψ = 0 and ψ > 0 in the direction perpendicular to the chain structure at room temperature was enhanced compared to G′ of the isotropic composites. This difference of the filler effect (for ψ = 0) and the magnetorheological effect (for ψ > 0) between the pre-structured and isotropic composites was enhanced when the chain structure of the CI particles in the pre-structured composites became longer and better aligned. A mechanism(s) of this enhancement was discussed in relation to the morphologies (particle distribution) in the composites with the aid of a filler model and a molecular expression of the stress due to magnetically interacting particles.

Use of ram extruder as a combined rheo-tribometer to study the behaviour of high yield stress fluids at low strain rate

Abstract: We propose in this work to provide an efficient and simple extruder device able to evaluate the rheological and tribological behaviour of high yield stress fluids, such as extrudible materials. An extruder able to measure simultaneously both the friction force acting on the extruder wall and the total extrusion force is developed. Based on previous studies, an efficient and accurate method of data analysis is then proposed and applied in order to obtain both a flow curve and a tribological law. Experimental tests are performed on soft modelling clay, kaolin paste and cement-based materials. Results are compared to conventional rheometry measurements. This comparison helps to evaluate the accuracy of the proposed experimental device and procedure.

Rheo-PIV of a shear-banding wormlike micellar solution under large amplitude oscillatory shear

Abstract: We explore the behavior of a wormlike micellar solution under both steady and large amplitude oscillatory shear (LAOS) in a cone–plate geometry through simultaneous bulk rheometry and localized velocimetric measurements. First, particle image velocimetry is used to show that the shear-banded profiles observed in steady shear are in qualitative agreement with previous results for flow in the cone–plate geometry. Then under LAOS, we observe the onset of shear-banded flow in the fluid as it is progressively deformed into the non-linear regime—this onset closely coincides with the appearance of higher harmonics in the periodic stress signal measured by the rheometer. These harmonics are quantified using the higher-order elastic and viscous Chebyshev coefficients e n and v n , which are shown to grow as the banding behavior becomes more pronounced. The high resolution of the velocimetric imaging system enables spatiotemporal variations in the structure of the banded flow to be observed in great detail. Specifically, we observe that at large strain amplitudes (γ 0 ≥ 1), the fluid exhibits a three-banded velocity profile with a high shear rate band located in-between two lower shear rate bands adjacent to each wall. This band persists over the full cycle of the oscillation, resulting in no phase lag being observed between the appearance of the band and the driving strain amplitude. In addition to the kinematic measurements of shear banding, the methods used to prevent wall slip and edge irregularities are discussed in detail, and these methods are shown to have a measurable effect on the stability boundaries of the shear-banded flow.

Extensional-flow-induced crystallization of isotactic polypropylene

Abstract: A filament stretching extensional rheometer with a custom-built oven was used to investigate the effect of uniaxial flow on the crystallization of polypropylene. Prior to stretching, samples were heated to a temperature well above the melt temperature to erase their thermal and mechanical histories and the Janeschitz-Kriegl protocol was applied. The samples were stretched at extension rates in the range of $0.01\,\mbox{s}^{-1}\le \dot{{\varepsilon }}\le 0.75\,{\rm s}^{-1}$ to a final strain of e = 3.0. After stretching, the samples were allowed to crystallize isothermally. Differential scanning calorimetry was applied to the crystallized samples to measure the degree of crystallinity. The results showed that a minimum extension rate is required for an increase in percent crystallization to occur and that there is an extension rate for which percent crystallization is maximized. No increase in crystallization was observed for extension rates below a critical extension rate corresponding to a Weissenberg number of approximately Wi = 1. Below this Weissenberg number, the flow is not strong enough to align the contour path of the polymer chains within the melt and as a result there is no change in the final percent crystallization from the quiescent state. Beyond this critical extension rate, the percent crystallization was observed to increase to a maximum, which was 18% greater than the quiescent case, before decaying again at higher extension rates. The increase in crystallinity is likely due to flow-induced orientation and alignment of contour path of the polymer chains in the flow direction. Polarized light microscopy verified an increase in number of spherulites and a decrease in spherulite size with increasing extension rate. In addition, small angle X-ray scattering showed a 7% decrease in inter-lamellar spacing at the transition to flow-induced crystallization. Although an increase in strain resulted in a slight increase in percent crystallization, no significant trends were observed. Crystallization kinetics were examined as a function of extension rate by observing the time required for molten samples to crystallize under uniaxial flow. The crystallization time was defined as the time at which a sudden increase in the transient force measurement was observed. The crystallization time was found to decrease as one over the extension rate, even for extension rates where no increase in percent crystallization was observed. As a result, the onset of extensional-flow-induced crystallization was found to occur at a constant value of strain equal to e c = 5.8.

Rheo-PIV of a yield-stress fluid in a capillary with slip at the wall

Abstract: An analysis of the yielding and flow behavior of a model yield-stress fluid, 0.2 wt% Carbopol gel, in a capillary with slip at the wall has been carried out in the present work. For this, a study of the flow kinematics in a capillary rheometer was performed with a two-dimensional particle image velocimetry (PIV) system. Besides, a stress-controlled rotational rheometer with a vane rotor was used as an independent way to measure the yield stress. The results in this work show that in the limit of resolution of the PIV technique, the flow behavior agrees with the existence of a yield stress, but there is a smooth solid–liquid transition in the capillary flow curve, which complicates the determination of the yield stress from rheometrical data. This complication, however, is overcome by using the solely velocity profiles and the measured wall shear stresses, from which the yield-stress value is reliably determined. The main details of the kinematics in the presence of slip were all captured during the experiments, namely, a purely plug flow before yielding, the solid–liquid transition, as well as the behavior under flow, respectively. Finally, it was found that the slip velocity increases in a power-law way with the shear stress.

Who conceived the "complex viscosity"?

Abstract: In small amplitude oscillatory shear flow, we now universally represent the response of a viscoelastic fluid as a complex quantity, which we call “complex viscosity.” This short piece deepens our understanding of the origins of the complex viscosity and of the man who coined this term, now widely used in rheology.

Rheological modeling of the diffusion process and the interphase of symmetrical bilayers based on PVDF and PMMA with varying molecular weights

Abstract: The diffusion process in the molten state at a polymer/polymer interface of symmetrical and model bilayers has been investigated using a small-amplitude oscillatory shear measurement. The polymers employed in this study were poly (vinylidene fluoride) (PVDF) and poly (methyl methacrylate) (PMMA) of varying molecular weights and polydispersities. The measurements were conducted in the linear viscoelastic regime (small deformations) so as to decouple the effect of flow from the diffusion. The focus of this paper has been to investigate the effects of healing time, angular frequency (ω), temperature, and molecular weight on the inter-diffusion and the triggered interphase between the neighboring layers. The kinetics of diffusion, based on the evolution of the apparent diffusion coefficient (D a) versus the healing time, was experimentally obtained. The transition from the non-Fickian to the normal Fickian region for the inter-diffusion at the interface was clearly observed, qualitatively consistent with the reptation model, but it occurred at a critical time greater than the reptation time (τ rep). In non-Fickian region, effects of frequency and temperature were studied with regard to the ratio of the apparent diffusion coefficient to the self-diffusion coefficient (D a/D s). The D s determined in the Fickian region was found to be consistent with Graessley’s model as well as with the literatures. And the dependence of the Ds on the frequency agreed well with the Doi–Edwards theory, in particular, scaling as $D_{\rm s} \sim \omega^{1/2}$ at ω > 1/τ e and $D_{\rm s} \sim \omega^{0}$ at ω < 1/τ rep. Our experimental results also confirmed that the dependence of the D s on the temperature for PMMA and PVDF can be well described by the Arrhenius law. Moreover, blends of PMMAs have been proposed in order to be able to change the $\overline M_\emph{w} $ . The rheological investigations of these corresponding bilayers rendered it possible to monitor the effect of $\overline M_\emph{w} $ on the diffusion process. The obtained results gave $D_{\rm s} \sim \overline M_\emph{w}^{-1}$ , thus corroborating some earlier studies and some experimental results recently reported by Time-Resolved Neutron Reflectivity Measurements. Lastly, the thickness of the interphase and its corresponding viscoelastic properties could be theoretically determined as a function of the healing time.

Variable viscosity and slip velocity effects on the flow and heat transfer of a power-law fluid over a non-linearly stretching surface with heat flux and thermal radiation

Abstract: The flow and heat transfer of a non-Newtonian power-law fluid over a non-linearly stretching surface has been studied numerically under conditions of constant heat flux and thermal radiation and evaluated for the effect of wall slip. The governing partial differential equations are transformed into a set of coupled non-linear ordinary differential equations which are using appropriate boundary conditions for various physical parameters. The remaining set of ordinary differential equations is solved numerically by fourth-order Runge–Kutta method using the shooting technique. The effects of the viscosity, the slip velocity, the radiation parameter, power-law index, and the Prandtl number on the flow and temperature profiles are presented. Moreover, the local skin friction and Nusselt numbers are presented. Comparison of numerical results is made with the earlier published results under limiting cases.

Flow-induced crystallization of high-density polyethylene: the effects of shear and uniaxial extension

Abstract: The effects of shear, uniaxial extension and temperature on the flow-induced crystallization of two different types of high-density polyethylene (a metallocene and a ZN-HDPE) are examined using rheometry. Shear and uniaxial extension experiments were performed at temperatures below and well above the peak melting point of the polyethylenes in order to characterize their flow-induced crystallization behavior at rates relevant to processing (elongational rates up to 30 s − 1 and shear rates 1 to 1,000 s − 1 depending on the application). Generally, strain and strain rate found to enhance crystallization in both shear and elongation. In particular, extensional flow was found to be a much stronger stimulus for polymer crystallization compared to shear. At temperatures well above the melting peak point (up to 25°C), polymer crystallized under elongational flow, while there was no sign of crystallization under simple shear. A modified Kolmogorov crystallization model (Kolmogorov, Bull Akad Sci USSR, Class Sci, Math Nat 1:355–359, 1937) proposed by Tanner and Qi (Chem Eng Sci 64:4576–4579, 2009) was used to describe the crystallization kinetics under both shear and elongational flow at different temperatures.

Magnetorheology in an aging, yield stress matrix fluid

Abstract: Field-induced static and dynamic yield stresses are explored for magnetorheological (MR) suspensions in an aging, yield stress matrix fluid composed of an aqueous dispersion of Laponite® clay. Using a custom-built magnetorheometry fixture, the MR response is studied for magnetic field strengths up to 1 T and magnetic particle concentrations up to 30 v%. The yield stress of the matrix fluid, which serves to inhibit sedimentation of dispersed carbonyl iron magnetic microparticles, is found to have a negligible effect on the field-induced static yield stress for sufficient applied fields, and good agreement is observed between field-induced static and dynamic yield stresses for all but the lowest field strengths and particle concentrations. These results, which generally imply a dominance of inter-particle dipolar interactions over the matrix fluid yield stress, are analyzed by considering a dimensionless magnetic yield parameter that quantifies the balance of stresses on particles. By characterizing the applied magnetic field in terms of the average particle magnetization, a rheological master curve is generated for the field-induced static yield stress that indicates a concentration–magnetization superposition. The results presented herein will provide guidance to formulators of MR fluids and designers of MR devices who require a field-induced static yield stress and a dispersion that is essentially indefinitely stable to sedimentation.

Investigation of the properties of fully reacted unstoichiometric polydimethylsiloxane networks and their extracted network fractions

Abstract: We investigated the linear dynamic response of a series of fully reacted unstoichiometric polydimethylsiloxane networks and of the two corresponding network fractions namely the sol and the washed network. The sol and the washed network were separated by a simple extraction process. This way, it was possible to obtain rheological data from the washed network without interference from the sol fraction and furthermore from the sol fraction without interference from the elastic washed network. When the stoichiometry increased towards perfectly reacted networks and beyond, we observed harder networks both qualitatively and by rheology, and the properties of the two fractions became more and more different. At the gel point, the sol fraction and the washed networks have more or less identical properties which our data also show. The storage and loss moduli, G′ and G″, were analysed with the gel equation as proposed by Winter and Chambon (J Rheol 30:367–382, 1986) and Chambon and Winther (J Rheol 31:683–697, 1987). We observed that one of the investigated samples which before the swelling experiment did not show any elastic response gave an elastic washed network after swelling; this was verified by analysis with the gel equation. We also calculated the weight fraction of the sol fraction by using the theory by Villar et al. (Macromolecules 29(11):4072–4080, 1996) and compared this with experimentally found values.

Viscoelastic behaviour and flow instabilities of biodegradable poly ( ε -caprolactone) polyesters

Abstract: The viscoelastic behaviour of a number of commercial and newly synthesized linear biodegradable polyesters—poly (e-caprolactone) (PCLs) with different molecular characteristics was investigated using both rotational and extensional rheometry. The variation of the zero-shear viscosity and relaxation spectrum with molecular weight was studied in detail. The damping function of these PCLs was also determined in order to model their viscoelastic behaviour. The classic Wagner constitutive equation was found to represent the rheology of all PCL polymers quite well. Finally, the PCL processing instabilities were studied by capillary extrusion using a number of capillary dies having various diameter and length-to-diameter ratios. Sharkskin and gross melt fracture was observed at different shear rates depending on the molecular characteristics of the resins and the geometrical details of the capillary dies.

Thixotropy, yielding and ultrasonic Doppler velocimetry in pulp fibre suspensions

Abstract: This paper reports thixotropy in concentrated pulp fibre suspensions and studies their transient flow behaviour using conventional rheometry coupled with a velocimetry technique. Specifically, an ultrasonic Doppler velocimeter is used in conjunction with a rate-controlled rheometer to deduce the local velocity profiles of pulp fibre suspensions. Pulp suspensions are found to exhibit a plateau in their flow curves where a slight increase in the shear stress generates a jump in the corresponding shear rate, implying the occurrence of shear banding. The velocity profiles were found to be discontinuous in the vicinity of the yielding radius where the Herschel–Bulkley model failed to predict the flow. Shear history and the time of rest prior to the measurement were found to play a significant role on the rheology and the local velocity profiles of pulp suspensions.

Detection of viscoelasticity in aggregating dilute protein solutions through dynamic light scattering-based optical microrheology

Abstract: This study illustrates the applicability of dynamic light scattering (DLS)-based optical microrheology in generating new insights into the rheological response of dilute protein solutions as they start to form insoluble aggregates under the influence of a thermal stress. The technique is also shown to provide a quick method for measuring the viscosity in protein solutions. The optical microrheological technique, which is based on DLS with improved single scattering detection, is shown here to capture the rich dynamics in these systems, where traditional mechanical rheometry cannot be effectively employed due to low torque generation and high sample volume requirements and the more widely known diffusing wave spectroscopy microrheology technique is not desirable due to the required high probe particle concentrations The study illustrates the careful consideration which must be given to the tracer particle surface chemistry, tracer particle concentration and tracer particle size in order to extract out rheological responses that are truly representative of the underlying protein dynamics and microstructure. We outline a procedure for ensuring that the pitfalls inherent to this type of measurement are avoided.

An intriguing empirical rule for computing the first normal stress difference from steady shear viscosity data for concentrated polymer solutions and melts

Abstract: The Cox–Merz rule and Laun’s rule are two empirical relations that allow the estimation of steady shear viscosity and first normal stress difference, respectively, using small amplitude oscillatory shear measurements. The validity of the Cox–Merz rule and Laun’s rule imply an agreement between the linear viscoelastic response measured in small amplitude oscillatory shear and the nonlinear response measured in steady shear flow measurements. We show that by using a lesser-known relationship also proposed by Cox and Merz, in conjunction with Laun’s rule, a relationship between the rate-dependent steady shear viscosity and the first normal stress difference can be deduced. The new empirical relation enables a priori estimation of the first normal stress difference using only the steady flow curve (i.e., viscosity vs shear rate data). Comparison of the estimated first normal stress difference with the measured values for six different polymer solutions and melts show that the empirical rule provides values that are in reasonable agreement with measurements over a wide range of shear rates, thus deepening the intriguing connection between linear and nonlinear viscoelastic response of entangled polymeric materials.

Measuring yield stress : a new, practical, and precise technique derived from detailed penetrometry analysis

Abstract: The displacement of an object through a yield stress is a complex process which involves the continuous deformation and transition of new regions from the solid to the liquid regime. We studied the force vs depth variations during the progressive penetration of a plate or a cylinder in a bath of simple yield stress fluids with negligible thixotropic character (Carbopol solutions, emulsions, and foams). Three regimes could be distinguished: elastic deformation, penetration (partially immersed object), and displacement through the fluid (fully immersed object). A detailed analysis of the force vs depth curves makes it possible to show that in the partially immersed regime the force is the sum of the critical force before penetration and a term associated with a uniform shear stress along the main plate surface, which is independent on the object geometry (plate dimensions and cylinder radius). This understanding can be used to precisely determine the yield stress as the critical shear stress along the plate at vanishing velocities. We also show that it is possible to measure accurately the yield stress from relaxation tests (stress vs time curve for motion stoppage): it indeed appears that the additional force term associated with penetration is negligible in that case so that the asymptotic average shear stress after stoppage is equal to the yield stress.

Stress and neutron scattering measurements on linear polymer melts undergoing steady elongational flow

Abstract: We use small-angle neutron scattering to measure the molecular stretching in polystyrene melts undergoing steady elongational flow at large stretch rates. The radius of gyration of the central segment of a partly deuterated polystyrene molecule is, in the stretching direction, increasing with the steady stretch rate to a power of about 0.25. This value is about half of the exponent observed for the increase in stress value σ, in agreement with Gaussian behavior. Thus, finite chain extensibility does not seem to play an important role in the strongly non-linear extensional stress behavior exhibited by the linear polystyrene melt.

Fractional viscoelastic models: master curve construction, interconversion, and numerical approximation

Abstract: We use fractional viscoelastic models that result from the application of fractional calculus to the linear viscoelastic theory to characterize thermorheologically simple linear viscoelastic materials. Model parameters are obtained through an optimization procedure that simultaneously determines the time–temperature shift factors. We present analytical interconversion based on the fractional viscoelastic model between the main viscoelastic functions (relaxation modulus, creep compliance, storage modulus, and loss modulus) and the analytical forms of the relaxation and retardation spectra. We show that the fractional viscoelastic model can be approximated by a Prony series to any desired level of accuracy. This property allows the efficient determination of the fractional viscoelastic model response to any loading history using the well-known recursive relationships of Prony series models.

Linear and non-linear viscoelastic properties of ethylene vinyl acetate/nano-crystalline cellulose composites

Abstract: This paper reports on the melt rheological properties of ethylene vinyl acetate containing between 0 and 10 wt.% of nano-crystalline cellulose (NCC). A complete set of rheological tests including frequency sweeps, shear transients, and uniaxial elongations was performed. Frequency sweeps showed that at low frequencies, a pseudo solid-like behavior was obtained for NCC concentrations higher than 5%. This behavior was related to hydrogen bonding between NCC particles and the creation of particle networks as the result of particle–particle interactions. For transient shear tests, all compositions presented a stress overshoot at high shear rates before reaching a steady state. It was found that the amplitude of this overshoot depends on both NCC content and shear rate. On the other hand, the time to reach the maximum was found to be highly shear rate dependent but concentration dependence was rather weak. For uniaxial extensional flow, higher extensional viscosity was observed with increasing NCC content. On the other hand, strain hardening was found to decrease with increasing NCC content.

Shear thickening of chemical mechanical polishing slurries under high shear

Abstract: Chemical mechanical polishing (CMP) is a fundamental technology used in the semiconductor manufacturing industry to polish and planarize electronic materials. During the high shear (≥1,000,000 s − 1) polishing process, it is hypothesized that individual slurry particles begin to interact and collide with one another forming large agglomerates (≥0.5 μm). These agglomerates are suspected of causing defects such as scratches or gouges during polishing, which costs the semiconductor industry billions of dollars annually. We have developed a method for investigating the shear thickening behavior of fumed silica slurries (20–34 wt.%) under high shear using a parallel-plate geometry in a conventional rotating rheometer. The CMP slurries displayed irreversible thickening at shear rates exceeding 10,000 s − 1. Viscous heating and sample evaporation are shown to be inconsequential to the witnessed shear thickening behavior. Also, the observed thickening is not a result of a critical rheometer speed, as the thickening was independent of the experimental gap height. In agreement with previous work, the slurries thickened at lower shear rates as silica concentration was increased. The shear thickening of the fumed silica slurries is truly shear-induced, and therefore, the thickening of CMP slurries can be examined using a rotational rheometer at small gap heights (≤100 μm).

Electrical conductivity, phase behavior, and rheology of polypropylene/polystyrene blends with multi-walled carbon nanotube

Abstract: The electrical, rheological properties and phase change behavior of polypropylene (PP)/polystyrene (PS) blends filled with multi-walled carbon nanotube (MWNT) were investigated. Two kinds of masterbatch were used to prepare ternary blends of PP, PS, and MWNT, and the effects of the kinds of masterbatch were confirmed by phase morphology of ternary blends and the distribution of MWNT. From thermodynamic analysis, MWNT is expected to locate in PS phase and it shows a good agreement with the TEM observations. The ternary composites show the lowest conductive percolation threshold and fine morphologies when most MWNT particles are located at the interface. Time sweep test were carried out to monitor the phase coalescence of the ternary blends and MWNT migration and agglomeration in the PS phase during annealing. The enhancement of thermal properties of MWNT-filled blends was also investigated by DSC and TGA.

Axial laminar flow of viscoplastic fluids in a concentric annulus subject to wall slip

Abstract: The flow of non-Newtonian fluids in annular geometries is an important problem, especially for the extrusion of polymeric melts and suspensions and for oil and gas exploration. Here, an analytical solution of the equation of motion for the axial flow of an incompressible viscoplastic fluid (represented by the Hershel–Bulkley equation) in a long concentric annulus under isothermal, fully developed, and creeping conditions and subject to true or apparent wall slip is provided. The simplifications of the analytical model for Hershel–Bulkley fluid subject to wall slip also provide the analytical solutions for the axial annular flows of Bingham plastic, power-law, and Newtonian fluids with and without wall slip at one or both surfaces of the annulus.

Random close packing and relative viscosity of multimodal suspensions

Abstract: Multimodal suspensions, consisting of non-colloidal spherical particles and a Newtonian matrix, are considered. A new differential (or multi-scale mean field approach) model for the relative viscosity of multimodal suspensions has been developed. The problem of the random close packing fraction of the solid phase is also investigated. We suppose that the multimodal suspension has a dominant large particle composition and that the smaller particles are embedded in the empty space between the larger particles. The relative viscosity model can therefore be based on the theory of monomodal suspensions. Experimental data of Eveson are compared to the predictions given by using three different models of monomodal suspensions respectively. The Maron–Pierce approach appears to give the best agreement with Eveson’s experiments. However, due to experimental uncertainties, we recommend that the Mendoza and Santamaria-Holek (MSH) formula be adopted.

Detecting very low levels of long-chain branching in metallocene-catalyzed polyethylenes

Abstract: The detection of long-chain branches (LCBs) is an issue of significant importance in both basic research and industrial applications, as LCBs provide excellent means to improve the processing behavior, especially in elongation-dominated processing operations. In this article, different methods for the detection of very low amounts of LCBs in metallocene-catalyzed polyethylene are presented and compared with respect to their sensitivity. Depending on the molar mass, the zero shear rate viscosity increase factor η 0/ $\eta_{0}^{\rm lin}$ , the steady-state elastic recovery compliance $J_{e}^{0}$ , the complex modulus functions G′(ω) and G″(ω), and the thermorheological complexity were found to be sensitive. In general, the higher the molar mass, the more important the transient quantities become and the easier finding the long-chain branches gets. Although rheology is very sensitive, rheological methods in combination with size exclusion chromatography proved to be the most sensitive combination to detect even very low amounts of LCBs. Especially methods involving the elastic properties (G′(ω), $J_{\rm e}^{0}$ , and J r(t)) react very sensitively, but these are also very distinctly influenced by the molar mass distribution.

On the validity of continuous media theory for plastic materials in magnetorheological fluids under slow compression

Abstract: In this manuscript, we address the long-standing question of whether a single theory for model plastic fluids is suitable to deal with the unidirectional compression problem in magnetorheological (MR) fluids. We present an extensive experimental investigation of the performance of MR fluids in slow-compression, no-slip, constant-volume squeeze mode under different magnetic field strengths (0–354 kA/m), dispersing medium viscosities (20–500 mPa·s) and particle concentrations (5–30 vol%). Normal force versus compressive strain curves reasonably collapse when normalizing by the low-strain normal force. Deviations from the squeeze flow theory for field-responsive yield stress fluids are associated to microstructural rearrangements under compression in good agreement with the so-called squeeze strengthening effect. Yield compressive stresses are found to scale as \(\sim \eta^{0.33\, }\phi ^{2.0\, }\)H2.0.

Numerical simulations of Boger fluids through different contraction configurations for the development of a measuring system for extensional viscosity

Abstract: This paper reports the flow behaviour of Newtonian and Boger fluids through various axisymmetric contraction configurations by means of numerical predictions. A principal aim has been to evaluate the geometrical design choice of the hyperbolic contraction flow. The FENE-CR model has been used to reflect the behaviour of Boger fluids, with constant shear viscosity, finite (yet large) extensional viscosity and less than quadratic first normal stress difference. Numerical calculations have been performed on six different contraction configurations to evaluate an optimized geometry for measuring extensional viscosity in uniaxial extensional flow. The influence of a sharp or rounded recess-corner on the nozzle has also been investigated. Few commercial measuring systems are currently available for measurement of the extensional rheology of medium-viscosity fluids, such as foods and other biological systems. In this context, a technique based on the hyperbolic contraction flow would be a suitable alternative. The pressure drop, the velocity field, the first normal stress difference and the strain rate across the geometry have each been evaluated for Newtonian and Boger fluids. This numerical study has shown that the hyperbolic configuration is superior to the other geometry choices in achieving a constant extension rate. In this hyperbolic configuration, no vortices are formed, the measuring range is broader and the strain rate is constant throughout the geometric domain, unlike in the alternative configurations tested. The difference between sharp and rounded recess-corner configurations proved to be negligible and a rise in excess pressure drop (epd) for increasing deformation rates has been observed.