Index of content:
Volume 52, Issue 4, July 2008
52(2008); http://dx.doi.org/10.1122/1.2930872View Description Hide Description
The stress in the startup of uniaxial elongational flow until steady state, followed by stress relaxation, has been measured for a narrow molar mass distribution polystyrene melt with a molecular weight of . The experiments are conducted on a filament stretching rheometer, where a closed loop control of the midfilament diameter ensures controlled uniaxial extension. The closed loop control algorithm is extended to apply to the stress relaxation part of the experiment. It ensures a constant midfilament diameter, by controlling the motion of the end plates. By dividing the measured stress with the theoretically predicted stress from the Doi and Edwards model during relaxation, the stretch factors corresponding to each imposed stretch rate are obtained. These stretch factors converge to a unique envelope and eventually converge to unity for long times for all measured elongational rates.
Effect of particle aggregation on the magnetic and magnetorheological properties of magnetic suspensions52(2008); http://dx.doi.org/10.1122/1.2931008View Description Hide Description
In this work, the magnetorheological behavior of concentrated (10% solid volume fraction) iron-based magnetorheological fluids was studied. Two different compounds were added to stabilize the suspensions against aggregation and settling processes: A surfactant (aluminum stearate, AlSt), and a gel-forming agent (silicananoparticles). The shear stress vs. shear rate flow curves of the suspensions were obtained in a wide range of applied magnetic fields with the aim of determining the intensity of the field-induced yield stress, that is, the strength of the so-called magnetorheological(MR) effect. The suspension stabilized against particle aggregation by surfactant addition reached the strongest MR response, while the opposite behavior (the weakest MR response) was found in the suspension that contained silicananoparticles added as anti-settling agent. The scaling between the yield stress and the magnetic field strength was calculated and compared with the predictions of theoretical structural models. The results demonstrated that such scaling depends on both the aggregation degree of iron particles in the suspensions and the ratio (, saturation magnetization of the iron powder), showing a continuous decrease in the exponent for high enough magnetic fields.
Double concentric cylinder geometry with slotted rotor to measure the yield stress of complex systems: A numerical study52(2008); http://dx.doi.org/10.1122/1.2936073View Description Hide Description
Based on the success of the slotted plate device [Zhu et al., “A slotted plate device for measuring yield stress,” J. Rheol.45, 1105–1122 (2001)] a numerical analysis is performed for a double concentric cylinder geometry with slotted rotor to measure the yield stress of complex systems such as suspensions. This method overcomes the wall-slip problem, it also takes advantage of the amenable features of commercial rheometers. A finite element model is developed to simulate the deformation and stress distribution in the double concentric cylinder geometry under various wall and material interactions (no-slip, 50% slip, and free slip conditions). Our numerical results show that this geometry greatly reduces the wall slip effect and it proves to be a promising technique for measuring the yield stress of complex systems.
52(2008); http://dx.doi.org/10.1122/1.2933352View Description Hide Description
The rheological behavior and especially the rheopexy phenomena of dilute self-assembled solutions in the presence of a counterion are examined in stationary and transient shear flows. In the first part, a continuous and gradually increasing strain is applied on the same sample. We study the effect of the initial shear rate, the temporal variation of the viscosity, and the hysterisis between charge and discharge curves. The results show that the properties of shear thickening are independent of the initial shear rate. In the second part, fresh samples are successively subjected to increasing shear strains; this method allows us to follow the evolution of the rheological characteristics during long measuring durations and gives us a distinct picture of the behavior per shear rate. In both cases, we confirm that the chain length has a strong influence on the emergence and amplitude of the shear thickening. It was also found by studying the start-up flow behavior that the structure at equilibrium is composed of bigger structures for longer chain lengths. The maximum of the viscosity in the shear thickening transition occurs in a range of lower shear rates when enough time is given to the system to undertake the formation of the shear induced structure. Considering this result, we introduce the concept of “temporal shear thickening transition.”
52(2008); http://dx.doi.org/10.1122/1.2930670View Description Hide Description
The rheology of semidilute suspensions of rigid polystyrene ellipsoids at rotational Peclet numbers greater than was studied for two different aspect ratios. The ellipsoid suspensions exhibit shear thinning behavior for both aspect ratios, similar to previous experiments on suspensions of rigid fibers. Possible mechanisms for the rate dependent rheology are discussed and evaluated through a comparison with the rheology of suspensions of spheres with the same material properties as the ellipsoids. For the largest Peclet number, the relative viscosity scales linearly with dimensionless number density and is independent of the aspect ratio.
Steady state measurements in stress plateau region of entangled polymer solutions: Controlled-rate and controlled-stress modes52(2008); http://dx.doi.org/10.1122/1.2936869View Description Hide Description
Despite decades of efforts, reliable measurements of nonlinear flow behavior of well-entangled polymers in continuous shear have been challenging to obtain. The present work attempts to accomplish three important tasks: (A) overcome this challenge by adopting a strategy of decoupling rheological measurements from the outer meniscus region in a cone-partitioned plate (C/PP) setup; (B) determine whether well-entangled solutions indeed undergo a flow transformation under creep that can be taken to phenomenologically define an entanglement-disentanglement transition (EDT); (C) provide the velocity profiles of such solutions undergoing either controlled-stress or controlled-rate shear by carrying out in situ particle-tracking velocimetric (PTV) measurements. Upon removing any influence of edge fracture and sample loss, we are able to reach steady state during continual shear and elucidate more reliably the nonlinear flow behavior of well entangled polymer solutions with little ambiguity. Three well-entangled solutions with entanglements per chain exhibited overlapping continuous and monotonic flow curves both in controlled-rate and controlled-stress modes. The C/PP based experiments qualitatively confirm the report by Tapadia and Wang [Macromolecules37, 9083–9095 (2004)] that at a given applied shear stress the solution evolves over time from a state of high viscosity to that of substantially reduced viscosity, i.e., EDT occurs ubiquitously in absence of any edge effects. These results, made possible by a combination of C/PP and PTV, cause us to reevaluate the objectives of polymer rheology, and are expected to impact the future development in the field of rheometry for entangled polymers and other viscoelastic materials.
52(2008); http://dx.doi.org/10.1122/1.2930876View Description Hide Description
We show that a constitutive equation describing the extensional behavior of “soft” microcrystalline cellulose pastes [J. Rheology51, 493–516 (2007)] can be applied to dense granular suspensions of “hard” spheres. Biaxial extension of ballotini (size ) suspensions featuring a Newtonian liquid phase was investigated experimentally employing constant velocity squeeze flow with lubricated plates. All suspensions, featuring solid volume fraction , exhibited elasto-viscoplasticity: they deformed as quasi-linear elasticsolids prior to yielding and flowed as non-Newtonian (shear-thickening) liquids thereafter, with the transition occurring at a particular strain regardless of strain rate. The yield stress was dependent on the strain rate experienced by the material at yielding, the values being consistent with those obtained from separate limiting height experiments. The differences in the model parameters between hard and soft particle systems are discussed in terms of inter-particle interactions and particle deformability.
New method of forced-resonance measurement for the concentrated and large-viscous liquid in the low frequency range by torsion resonator52(2008); http://dx.doi.org/10.1122/1.2931695View Description Hide Description
A new method for measurement of the viscoelastic properties of the liquid with the torsion resonator on forced resonance is presented in this paper. Both theoretical analysis and experiments of different glycerol-water mixtures with our homemade torsion pendulum apparatus demonstrate that this new method avoids the restriction of the common method that the resonator should have the high quality factor both in air and in liquid sample and has the prominent advantage of measuring the concentrated and large-viscous liquid in the low frequency range with a better precision, which is usually unavailable by the common method. Employment of the new method can thus extend the accessible ranges of viscosity and frequency of the torsion resonator apparatus and close the gaps between conventional rheometers and the torsion resonators.
52(2008); http://dx.doi.org/10.1122/1.2933171View Description Hide Description
Many common materials display significant nonlinear rheological properties. Characterizing these properties can be done with a variety of methods. One such method uses inertio-elastic oscillations, which occur naturally in rotational rheometry as a consequence of a material’s elasticity and the inertia of the rheometer. These oscillations have primarily been used to characterize linear viscoelasticproperties. In addition to allowing for the imposition of stress-biased oscillations on short time scales, we demonstrate that extending this technique to nonlinear deformations provides accurate measurements of nonlinear material properties. Our experiments are performed on fibrin networks, which are well characterized and have dramatic nonlinear properties that are biologically significant. We compare the tangent modulimeasurements of inertio-elastic oscillations with three standard methods of nonlinear rheology: forced oscillations about a prestress, a geometric interpretation of large amplitude oscillatory shears, and an extension of the linear viscoelasticmoduli to the nonlinear regime. Inertio-elastic oscillations provide an accurate characterization of fibrin’s nonlinear properties, and further, our measurements suggest that inertio-elastic oscillations provide the most straightforward method of distinguishing between nonlinear elasticity and dissipation at any given stress. In fact, we find that inertio-elastic oscillations provide the most accurate measurement of the subdominant loss component of our networks.
52(2008); http://dx.doi.org/10.1122/1.2933436View Description Hide Description
The second normal stress difference experienced by non-Newtonian fluids flowing in a pipe may give rise to secondary flows in the transverse direction. As a result, one component tends to encapsulate the other in stratified flows. In multilayer coextrusion, such secondary flows tend to distort the interface and affect layer uniformity. This paper presents numerical simulations of the elastically driven encapsulation in two-component stratified viscoelastic fluids. The simulations are based on a phase-field theoretical model and use finite elements with adaptive meshing to resolve the moving interfaces. The results suggest two mechanisms for elastic encapsulation: One due to the mismatch of between the components and the other due to noncircular geometry of the cross section. In circular pipes, the more elastic fluid tends to protrude into the other component in the center of the pipe and become encapsulated. This produces the curtate cycloid interface shape commonly seen in experiments. If the cross section is noncircular, both the geometric effect and the elastic stratification are at work, and the interfacial motion is determined by the competition of these two mechanisms. This understanding provides an explanation for the anomalous encapsulation of the less elastic component by the more elastic one observed in multilayer coextrusion.