Index of content:
Volume 54, Issue 4, July 2010
54(2010); http://dx.doi.org/10.1122/1.3429067View Description Hide Description
Numerical simulations are performed for two rough spheres in nonlinear shear flow by the traction-corrected boundary element method with an added electrostatic-like force. It has been shown previously that the interaction of two rough spheres in nonlinear shear flow will result in a net permanent migration of the particle pair toward the low shear rate region of the flow field. In the current research, it is shown that a repulsive static force will augment the migration and that an attractive static force can diminish and even reverse the direction of migration. For oscillatory flows with an attractive static force between the spheres, the migration direction can change depending on the amplitude of the oscillation.
54(2010); http://dx.doi.org/10.1122/1.3380851View Description Hide Description
Usually, a bed of solid particles fluidized by a gas is inherently unstable. Gas bubbles are rapidly formed at the onset of fluidization, which hinders the efficiency of gas-solid contact. In the case of magnetizable particles, gas bubbles may be suppressed by means of an externally applied field that magnetizes the particles. In general, magnetized particles are assumed to behave as point dipoles that organize in chainlike structures oriented along field lines due to dipole-dipole attraction. The physical mechanism responsible for stabilization is, however, unclear. In particular, rheological characterization of magnetically stabilized beds (MSBs) has been a subject of controversy and there is no widely accepted explanation to the empirical fact that magnetofluidized beds (MFBs) can be stabilized by a horizontal field. Several experimental approaches have been used mainly aimed to observe the fluidity of MFBs. Generally, MFBs are reported to behave as a fluid up to a critical magnetic field strength at which there appears an appreciable yield stress. Most of these techniques are however invasive, which sheds doubts on the mechanism responsible for the appearance of the yield stress. In this work, we have measured the yield stress of MFBs of fine magnetic powders by means of a noninvasive technique that uses gas flow to put the bed under tension. It is shown that the MFB behaves as a plastic solid. The yield stress of the MFB, which is developed just at marginal stability, arises as a consequence of the magnetic attraction between particles at contact. Fine magnetic powders of different aggregative nature in the absence of applied magnetic field have been used in our work. It has been seen that the yield stress of MSBs of naturally aggregated particles is significantly larger than the yield stress of MSBs of naturally nonaggregated particles. Moreover, the MFB is stabilized at smaller field intensities in the former case. Noninvasive visualization of the MFB surface shows that quasivertical chainlike structures are stable despite that the magnetic field is applied in the horizontal direction, which sheds doubts on the validity of the widely used dipolar assumption.
54(2010); http://dx.doi.org/10.1122/1.3380852View Description Hide Description
The two-dimensional flow of a foam confined in a Hele–Shaw cell through a contraction is investigated. Its rheological features are quantified using image analysis, with measurements of the elastic stress, rate of plasticity, and velocity. The behavior of the velocity strongly differs at the contraction entrance, where the flow is purely convergent, and at the contraction exit, where a velocity undershoot and a re-focusing of the streamlines are unraveled. The yielded region, characterized by a significant rate of plasticity and a maximal stress amplitude, is concentrated close to the contraction. These qualitative generic trends do not vary significantly with the flow rate, bubble area, and contraction geometry, which is characteristic of a robust quasistatic regime. Using surfactants with a high surface viscoelasticity, a marked dependence of the elastic stress on the velocity is exhibited. The results show that the rate of plasticity does not only depend on the local magnitude of the deformation rate, but also crucially on the orientation of both elastic stresses and deformation rate. It is also discussed how the viscous friction controls the departure from the quasistatic regime.
Rheological investigation of interactions between sorbitol and polyhedral oligomeric silsesquioxane in development of nanocomposites of isotactic polypropylene54(2010); http://dx.doi.org/10.1122/1.3439695View Description Hide Description
Sorbitol-type nucleating agents can be used to aid dispersion of polyhedral oligomeric silsesquioxane (POSS) molecules in isotactic polypropylene (iPP) and to promote self-assembly of POSS molecules into nanoparticles. In this paper, the interactions between a sorbitol-type nucleating agent and POSS molecules were investigated using polarized optical microscopy and rotational rheometry. Dibenzylidene sorbitol (DBS), a nucleating agent of iPP, was used in conjunction with two non-reactive POSS—trisilanolphenyl-POSS (ph-TPOSS) and octaisobutyl-POSS (bu-MPOSS). DBS formed three-dimensional fibrillar networks in iPP upon cooling from a homogeneous state. These fibrillar networks were characterized as gels using oscillatory shear rheometry. Trisilanolphenyl-POSS was incompatible with iPP but offered strong hydrogen bonding possibility with DBS; bu-MPOSS, on the other hand, was compatible with iPP but did not produce significant interactions with DBS. It was found that as low as 0.5 wt % of ph-TPOSS molecules in a ternary compound with 0.7 wt % DBS subdued fibrillar network formation by DBS and altered the rheological properties of the compounds. No such effect was seen in compounds of bu-MPOSS, although viscosity showed strong reduction. The storage modulus vs loss modulus data provided much better indication of changes of morphology in iPP/POSS and iPP/DBS/POSS compounds than loss tangent vs oscillatory frequency plots.
The rheological characterization of linear viscoelasticity for ink jet fluids using piezo axial vibrator and torsion resonator rheometers54(2010); http://dx.doi.org/10.1122/1.3439696View Description Hide Description
This paper is concerned with the experimental ability to measure viscoelasticity of low viscosity ink jet fluids and demonstrates the capability of both a piezo axial vibrator and torsion resonator rheometer to capture high frequency rheological data for both model and commercial ink jet fluids. Results are presented for polymer and particle laden suspensions together with a commercial ink. The data demonstrate that high frequency linear viscoelastic rheology can be captured using both rheometers and that both the presence of polymer and particles can induce viscoelasticity within the fluid. It is believed that the physical origin of viscoelasticeffects produced by the presence of polymer or particles is different, and this results in a different high frequency limiting slope for the data.
54(2010); http://dx.doi.org/10.1122/1.3439732View Description Hide Description
This work aims at studying the role of interface properties on the rheological behavior of non-compatibilized and compatibilized polymer blends. Blends of polymethylmetacrylate (PMMA) with polystyrene (PS) and PS functionalized with oxazoline (PSOX) with concentrations of up to 20 w/w of the dispersed phase were used. It was observed that until a critical concentration is reached the increase in PSOX content leads to a significant increase in (a) the elasticity at low frequencies and (b) the relaxation time after cessation of flow, both in shear and extension. This points to a likely significant role played by interface elasticity. Since no chemical reactions occur between PMMA and the oxazoline groups of PSOX, the latter is probably caused by the partial miscibility between PMMA and PSOX. Beyond this critical concentration, the amount of PSOX does not have a significant influence on the rheological behavior of the blends. In order to gain an insight into the relaxation dynamics of the droplets and interface, and their relationship with the rheological behavior of the blends, small angle light scattering (SALS) was used in diluted blends (1 wt % of the dispersed phase) during step shear. SALS shows a slight deformation of dispersed phase in the vorticity direction for the 99PMMA/1PSOX blend while the droplets of the 99PMMA/1PS blend deforms in the flow direction only. This result confirms the large increase in the interfacialelasticity for the 99PMMA/1PSOX blend.
54(2010); http://dx.doi.org/10.1122/1.3439731View Description Hide Description
We study both experimentally and theoretically the rheological behavior of isotropic bidisperse suspensions of noncolloidal particles in yield stress fluids. We focus on materials in which noncolloidal particles interact with the suspending fluid only through hydrodynamical interactions. We observe that both the elastic modulus and yield stress of bidisperse suspensions are lower than those of monodisperse suspensions of the same solid volume fraction. Moreover, we show that the dimensionless yield stress of such suspensions is linked to their dimensionless elastic modulus and to their solid volume fraction through the simple equation of Chateau et al. [J. Rheol.52, 489–506 (2008)]. We also show that the effect of the particle size heterogeneity can be described by means of a packing model developed to estimate random packing of assemblies of dry particles. All these observations finally allow us to propose simple closed form estimates for both the elastic modulus and the yield stress of bidisperse suspensions: while the elastic modulus is a function of the reduced volume fraction only, where is the estimated random packing, the yield stress is a function of both the volume fraction and the reduced volume fraction.
54(2010); http://dx.doi.org/10.1122/1.3439774View Description Hide Description
In viscoelastic property measurements, material is subjected to time unsteady deformations using a rheometer. In step shear strain experiments, for example, the shear strain suddenly jumps to a steady value. In this paper, we develop a method to study the dynamic response of a shear stress transducer in a sliding plate rheometer for any time unsteady rheological test. This general method is developed by first considering a special case of step shear strain for a fluid sample with material ingress in the annular transducer gap. Both the fluid sample and its ingress obey the generalized Maxwell model. Our main mathematical trick is assumed periodicity where a single step in shear strain is treated as the first step in a reciprocating square wave. After solving the problem in the frequency domain, we recover our single step by taking the limit as the period goes to infinity. The transducer eccentricity following step shear is then determined analytically by solving the force balance on the transducer active face using bipolar cylindrical coordinates. Dimensionless graphs are provided for estimating transducer dynamic response in step shear strain. Finally, a worked example illustrates the application of these results.
54(2010); http://dx.doi.org/10.1122/1.3445064View Description Hide Description
Several attempts have been made recently to develop universal analysis techniques for the stress response to large-amplitude oscillatory shear (LAOS) in a variety of materials from viscous and viscoelastic to viscoplastic. All previous attempts have assumed, tacitly or otherwise, that such responses display a minimum number of symmetries in stress/strain. We present the stress response to LAOS of a glassy suspension of multiarm star polymers, which exhibits a complete lack of any type of symmetry during the experimental window. We also present new physical interpretations of previously defined parameters and conjecture that the lack of symmetry observed in the stress response is a consequence of the aging process, which can be viewed as slow cage rearrangement and evolution of heterogeneities.
Wormlike micellar solutions: II. Comparison between experimental data and scission model predictions54(2010); http://dx.doi.org/10.1122/1.3439729View Description Hide Description
Although many constitutive models for wormlike micellar solutions have been proposed, few quantitative comparisons have been made with detailed rheological measurements. The majority of comparative studies focus on the linear viscoelastic properties of micellar solutions, which are well described by monoexponential Maxwell-like behavior. In the present work we compare the predictions of a prototypical two-species reptation-reaction model [developed in Part 1, Vasquez et al., “A network scission model for wormlike micellar solutions: I. Model formulation and viscometric flow predictions,” J. Non-Newtonian Fluid Mech.144(2–3), 122–139 (2007)] with rheological measurements performed using a concentrated cetyl pyridinium chloride/sodium salicylate (CPyCl/NaSal) solution in a range of steady and transient shear flows. The model captures the continuous rupture and reformation of the long entangled chains that form a physically entangled viscoelastic network and the enhanced breakage rates that occur during imposed shearing deformations. In homogeneous shearing flows, the model describes numerous qualitative features of the linear and nonlinear rheologies, including a strongly strain-dependent damping function during large strains, agreement with the Lodge–Meissner rule at moderately large strains, large rate-dependent first normal stress coefficients in steady shear flow, and pronounced stress overshoots during start-up of steady shear. The present model cannot predict the second normal stress difference observed experimentally or the persistent agreement with the Lodge–Meissner rule observed experimentally at very large strains. Homogeneous flow calculations with this simplified two-species model cannot capture quantitatively the full range of transient dynamics observed experimentally. More complex time-dependent test protocols, including step-jumps (up and down) in deformation rate and applied stress, are used to reveal the slow temporal dynamics associated with evolution of the shear-banding plateau. Such experiments help to provide insight into additional features (such as diffusion coefficients for stress-microstructure coupling) that are required for fully quantitative rheological equations of state describing these concentrated wormlike micellar solutions.
54(2010); http://dx.doi.org/10.1122/1.3442901View Description Hide Description
We investigate the complex flow behavior of a well-characterized colloidal star glass using linear and nonlinear rheology. The results are integrated into a generic state diagram which specifies the states of the glass for different initial conditions and mechanical histories: viscoelastic liquid and solid, homogeneous shear-thinned solution, and shear-banded material.Aging takes different forms which can be interpreted as kinetic pathways through the state diagram. Shear-banding appears to be an intrinsic mechanical instability which occurs when the solid state of the glass is shear-melted. Our results provide a straightforward methodology to fingerprint the material behavior of soft glassy materials undergoing rheological transitions which can be used as predictive tool to design systems with a desired rheological response.