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Journal of RheologyVolume 41, Issue 6 (November-December 1997) |
Contents
NMR Visualisation of Anomalous Flow in Cone-and-Plate RheometryMelanie M. Britton and Paul T. Callaghan AbstractWe demonstrate the use of NMR microscopy to image the velocity distribution for fluids sheared within the gaps (4º and 7º) of cone-and-plate rheometers. These measurements employ a specially-constructed rheogoniometer which fits within the NMR probe system. While the uniform shear rate assumption is verified in the case of simple Newtonian and non-Newtonian fluids, a range of anomalous behaviour (apparent slip, shear-banding and fracture) is observed in other systems, including wormlike surfactants, semi-dilute solutions of 18 MDa polyacrylamide, and dispersed silica in silicone grease. A Model of Adaptive Links in Nonlinear ViscoelasticityAleksey D. Drozdov AbstractA new constitutive model is derived for the nonlinear viscoelastic behavior of polymers which do not possess the separability property. The model is based on the concept of adaptive links (a version of the theory of transient networks). It is assumed that the response of a link is essentially nonlinear, and the rates of breakage and reformation depend on the strain intensity. With reference to the Eyring concept of thermally activated processes, a new equation is proposed to describe the effect of strains on reformation of adaptive links. Adjustable parameters of the model are found by using experimental data for polycarbonate and an epoxy glass in the standard relaxation tests. Comparison of experimental data in dynamic tests with results of numerical simulation shows that the model correctly predicts the nonlinear viscoelastic response. Simulating Complex Flows of Liquid Crystalline Polymers Using the Doi TheoryJ. Feng and L.G. Leal AbstractWe simulate the startup flow of lyotropic liquid crystalline polymers (LCPs) in an eccentric cylinder geometry. The objectives are to explore the mechanisms for the generation of disclinations in a non-homogeneous flow and to study the coupling between the flow and the polymer configuration. The Doi theory, generalized to spatially varying flows and approximated by the quadratic closure, is used to model the evolution of LCP configurations. This, along with the equations of motion for the fluid, is solved by a finite element method. The flow modification by the polymer stress is mild for the parameters used, but the LCP exhibits complex orientational behavior in different regions of the flow domain. For relatively weak nematic strength, a steady state is reached in which the director is oriented either along or transverse to the streamline depending upon local flow conditions and the deformation history. A pair of disclinations, with strength ±1/2, are identified in the steady state and the LCP configuration at the disclinations confirms the model of a structured defect core proposed by Greco & Marrucci (1992). For strong nematic strength, director tumbling occurs in the more rotational regions of the flow field, giving rise to a polydomain structure. The boundary of the tumbling domain consists of two disclinations of ±1/2 strength, a structure very similar to previous experimental observations of LCP domains. Dynamic Viscosity of Macroscopic Suspensions of Bimodal Sized Solid SpheresPhilippe Gondret* AbstractIn this paper, we present experimental measurements for the dynamic viscosity of macroscopic (non-Brownian and non-colloidal) suspensions of bimodal sized spheres when submitted to an oscillating plane Couette flow. The measured viscosity is what we call the dynamic viscosity at finite frequency. Concerning the viscosity of such systems, numerous experimental studies have been done under steady flow conditions, i.e., at zero frequency, but few ones concern the dynamic case. Our measurements have been performed for different values of the three relevant parameters, namely the size ratio l, the fraction x of small spheres to total solids and the total solid volume fraction F. Our results show a viscosity reduction upon mixing which increases as the total solid volume fraction F is increased. We analyze our results by a model that takes into account the volume fraction F and the maximum volume fraction Fm, which depends itself on the two parameters l and x. On the other hand, we compare our experimental results with recent numerical simulations performed by Chang and Powell (1993, 1994a) by Stokesian dynamics and Monte Carlo method, which lead respectively to viscosity at zero and infinite frequency. Our experimental results lie between these two different simulation results. Interfacial Phenomena in the Capillary Extrusion of Metallocene PolyethylenesSavvas G. Hatzikiriakos* and Igor B. Kazatchkov AbstractMetallocene catalysts are known to produce homogeneous random polyolefin copolymers with narrow molecular weight distribution and controlled long/short chain branching. Two such linear low-density polyethylenes were studied by using both constant stress and capillary rheometry, in order to assess their rheological and processing behaviour, as well as to identify critical conditions for the onset of flow instabilities. It was found that these polymers are thermorheologically complex liquids, apparently due to the presence of long chain branching. Compared with conventional linear low-density polyethylenes, these metallocene polyethylenes exhibit quite unusual behaviour in capillary flow, not previously reported to our knowledge. Specifically, we have encountered long transients in start-up of capillary experiments, and in some cases the capillary reservoir had to be loaded several times before a steady-state pressure was obtained. In addition, we found that these polymers slip at shear stresses higher than about 0.05 MPa. This critical value is much smaller than the critical value of 0.10 MPa reported for other conventional polyethylenes. A simple qualitative model is proposed to rationalise these findings. It suggests that the molecular mechanism of slip is dominated by a competition between flow-induced chain detachment from the wall and disentanglement of the chains in the bulk from those attached to the wall. Shear Induced Textures of Thermotropic AcetoxypropylcelluloseJ.B. Ritil, M.T. Cidade2, M.H. Godinho2,
A.F. Martins2, and P. Navardl* AbstractAcetoxypropylcellulose is a thermotropic cellulose derivative which is cholesteric from below room temperature up to 170ºC. Its textures during and after shear flow were studied by optical microscopy and SALS light scattering. The main features are similar to those occurring in hydroxypropylcellulose solutions, i.e. a polydomain texture evolving into a striated texture, as seen by optical microscopy. Elliptical or else four lobes with a streak are seen in the SALS patterns. Defects appearing as short dark lines not aligned with the flow direction are shown to be responsible for the existence of the four lobes of the SALS patterns. In addition these defects seem to be the location where the band texture which develops during relaxation. Testing the Structure of Magnetic Paints With and Without Superimposed ShearAndrei A. Potanin*, Suresh M. Shrauti, David W. Arnold, Alan M. Lane AbstractThe structure development in dispersions of magnetic barium ferrite particles in cyclohexanone with PVC wetting resin was tested by oscillatory rheological measurements and orthogonal superposition of steady and oscillatory shear. The optimum dispersion is achieved at the resin concentration c = c0, which corresponds to a minimum in the viscoelastic modulii. At c < c0 the system is highly elastic, brittle (G' drops sharply and G" goes through a strong maximum at high strain), thixotropic and its relaxation spectrum, H(t), is dominated by long relaxation times, t, which indicates the existence of a strong network. At c > c0 the system becomes less elastic, more flexible, less thixotropic and H(t) is dominated by short t, which indicates that the structure formation in this case is dominated by small aggregates rather than a network. By superimposing steady and oscillatory shear we show that the steady shear dramatically break the structure and shifts H(t) to very short t. Direct Measurement of Adhesion between Viscoelastic Polymers: A Contact Mechanical ApproachAfshin Falsafi, Pascal Deprez, Frank S. Bates, and Matthew Tirrell* AbstractWe have measured the surface energies of several viscoelastic polymers, using contact mechanics methods. The materials studied were diblock copolymers of poly(ethylene)-poly(ethylene-propylene) (PE-PEP) having different PE volume fractions and molecular weights. The materials exhibit differing viscoelastic relaxation functions in the bulk. Surface energies were determined from contact mechanics experiments in the loading mode, analyzed by incorporating the bulk viscoelastic effects into the Johnson Kendall-Roberts (JKR) theory of adhesive elastic contacts. The samples were made of spherical caps of the materials formed by first melting, and then cooling to room temperature. The measured values of the surface energies are close to the reported value for that of the surface-active PEP block, based on prior contact angle measurements, indicating that reliable surface energy values can be extracted from contact mechanics experiments, suitably analyzed to account for viscoelastic behavior. We believe that this method may be generalizable to surface and interfacial energy studies of a wide range of viscoelastic polymers. A Viscoelastic Model for Dense Granular FlowsD.Z. Zhang and R.M. Rauenzahn AbstractIn traditional kinetic theory for a granular flow, it is usually assumed that particle interactions are instantaneous and binary. For a dense granular system these assumptions are usually invalid. In this paper, we use an ensemble averaging technique to examine the effects of finite particle interaction time and multi-particle collisions. The main objectives of this paper are to develop a method and to provide a tool to study dense granular materials. As an example, we study flows of granular particles coated with thin layers of resin. To model particle elasticity and resin viscosity, the force between a pair of particles is approximated by a serial connection of a linear spring and a dashpot. Subsequently, a viscoelastic model is developed from the averaging method. In order to determine coefficients in the constitutive model, direct numerical simulations are performed. When the particle concentration is relatively low, the shear stress is quadratically proportional to the shear rate, in agreement with kinetic theories. At a high particle concentration, the shear stress depends linearly on the rate of strain. The transition between this quadratic and linear dependence is similar to a phase transition. In a dense system, when the shear rate exceeds a critical value, shear band formation is also observed. |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |