Volume 35, Issue 1, January 1991
Index of content:
35(1991); http://dx.doi.org/10.1122/1.550205View Description Hide Description
In this paper, we demonstrate that the boundary element method can be used effectively to simulate the sedimentation of a pair of nonspherical particles. Results are reported for tetrahedra, octahedra, cubes, and icosahedra. It was found that the rate of sedimentation of these particles is hardly affected by their shape and orientation. For octahedra, accurate results can be obtained with 24 elements on the surface of each particle. This allows more particles to be modeled and trajectories to be calculated; examples for eight and nine octahedra sedimenting in an unbounded fluid are given.
35(1991); http://dx.doi.org/10.1122/1.550207View Description Hide Description
We examine the multiplicity of steady state solutions of polymeric liquid crystals in shear flow regimes. We study flow configurations exhibiting two or more phases arranged as bands parallel to the flow. We discuss stability of such texture configurations within a range of shear rates.
35(1991); http://dx.doi.org/10.1122/1.550208View Description Hide Description
The response of a nematic liquid crystal to oscillating shear flow has been calculated. In both low and high frequency limits, the macroscopic response is dominated by viscouseffects, under the influence of dominant boundary‐induced and hydrodynamic torques, respectively. At intermediate frequencies, distortional (Frank) elasticity lends the macroscopic response viscoelastic character, exhibiting a significant effective storage modulus. The possible influence of distortional elasticity on the oscillatory response of liquid crystal polymers is qualitatively discussed.
Influence of electron beam irradiation on the microrheology of incompatible polymer blends: Thread break‐up and coalescence35(1991); http://dx.doi.org/10.1122/1.550209View Description Hide Description
The microrheology of polymer blends as influenced by crosslinks induced in the dispersed phase via electron beamirradiation, is systematically investigated for the model system polystyrene/low density polyethylene (PS/LDPE). Both break‐up of threads and coalescence of particles are delayed to a large extent, but are not inhibited completely and occur faster than would be expected for a nonirradiated material with a comparable viscosity. Small amplitude, dynamic rheological measurements indicated that in the irradiated materials a yield stress could exist. In contrast, direct microrheological measurements showed that this yield stress, which would prevent both break‐up and coalescence, could not be realized by EB irradiation. Apparently, the direct study of the microrheology of a blend system is important for the prediction of the development of its morphology and it is not possible to rely only on rheological data obtained via other methods.
35(1991); http://dx.doi.org/10.1122/1.550210View Description Hide Description
The pressure gradient due to the normal stress effect gives rise to a distribution of concentration in the polymer solution flowing in a rotating rheometer. The gradient of concentration in steady state was estimated with a thermodynamic method. It was shown to be proportional to the normal stress difference and inversely proportional to the osmotic compressibility.
35(1991); http://dx.doi.org/10.1122/1.550211View Description Hide Description
A finite element model for the analysis of inflation of a thick polymer sheet has been developed. The major objective was the creation of an algorithm for polymer thermoforming without the use of the thin membrane approximation. The model incorporates axisymmetric geometry, total Lagrangian description, and isothermally hyperelastic behavior (Mooney model) with incompressibility. It has the capabilities to deal with the free inflation of single layer or multilayerpolymeric sheets of finite thickness. Comparison with experimental data available in the literature for single layer sheets of rubber (Treloar, 1944) and PMMA (Lai and Holt, 1975) indicates very good agreement.
Laminar and turbulent flow of cement slurries in large diameter pipe: A comparison with laboratory viscometers35(1991); http://dx.doi.org/10.1122/1.550223View Description Hide Description
The primary objective of this paper is to illustrate the inherent problems of measuring the apparent viscosity of cement slurries with laboratory viscometers and using these measurements to estimate friction factors and flow regimes in large‐scale pipes. Experimental measurements obtained with Couette and tube viscometers are shown to exhibit an apparent wall slip that reaches a limiting value of 100% at shear stresses near the yield value. Nevertheless, comparable apparent viscosities are obtained after these data are corrected for slip. Experimental results for the laminar, transitional, and turbulent flows of clay and cement slurries in a large‐diameter pipe are also presented. It is shown that the onset of turbulence occurs at a critical Reynolds number that is in good agreement with the predicted value, provided that rheological properties are based on large‐scale pipe‐flow data. However, for yet unexplained reasons, samples of cement taken from the large‐scale facility had much higher apparent viscosities (as determined from a Couette viscometer) than viscosities calculated from laminar pipe‐flow data.
35(1991); http://dx.doi.org/10.1122/1.550224View Description Hide Description
Influence of ionic strength on the steady flow and linear and nonlinear viscoelastic properties of concentrated monodisperse polystyrene latices has been measured by means of a cone–plate type rheometer. In the complete deionized state and in the presence of a small amount of the added salt NaCl, the system, which is notably iridescent, shows a marked non‐Newtonian flow having an evident yield stress and a dynamic modulus which is mostly independent of the angular frequency. This means that a crystal‐like structure of the dispersing particle develops fully in the system. As the salt concentration increases, the iridescence disappears and the system shows Newtonian flow. It is plausible that the crystal–liquid transition begins to occur in the ionic concentration range where the Yukawa potential between the particles is smaller than the thermal energy kT and D eff (=2a+2D l ) is close to or less than the distance between neighboring particles in the crystal‐like structure. Here a is the radius of the particle and D l is the Debye screening length. The crystal‐like ordered structure is kept in the nonlinear viscoelastic region of 40% dynamic strain, and it also suggests that the flow and the nonlinear deformation in the colloidal crystal are due to a slip in a lattice plane at a constant stress.
35(1991); http://dx.doi.org/10.1122/1.550225View Description Hide Description
The pH and nature of anions were found to have a very significant effect on the rheological properties of zirconia suspensions. The variation in rheological properties with pH and anion type correlated well with the change in surface properties of zirconia. Zirconia suspensions with maximum yield stress and viscosity occurred at and near the isoelectric point (IEP). At a pH where the magnitude of the electrophoretic mobility of the zirconia is high, a low viscosity dispersed suspension was obtained. The zirconia suspensions were dispersed with potassium hydroxide at high pH while hydrochloric acid or nitric acid was used for dispersion at low pH. Sulfuric acid however did not disperse the suspensions at low pH because of significant sulfate adsorption on the zirconia particle. Polycarboxylate additives (polyacrylate) shifted the pH of maximum yield stress of zirconia to a lower pH. The degree of pH shift increased with increasing concentration of polycarboxylate; an effect attributed to the adsorption of polycarboxylate on the zirconia particle and to the degree of adsorption which increased with increasing concentration of polycarboxylate.
The influence of materials of construction on biconical rotor and capillary measurements of shear viscosity of rubber and its compounds and considerations of slippage35(1991); http://dx.doi.org/10.1122/1.550226View Description Hide Description
An experimental study is presented of the flow of elastomers and their compounds in biconical and capillary geometries using rotors and capillaries produced from different materials including aluminum, brass,copper, steel, and stainless steel plus polytetrafluorethylene (PTFE). The rotational rheometer could be operated at various controlled pressure levels. A butadiene–styrene copolymer (SBR) was investigated as well as an SBR compound with 20 vol % carbon black and a second SBR compound with 7% zinc stearate added. It was found in the pressurized rheometer, when the pressure was greater than 0.2 MPa, that the data for SBR and SBR–carbon black was essentially the same for smooth and grooved rotors fabricated from different metals. However when the pressure was reduced the torques were substantially reduced and the data for the grooved rotors were higher than for the smooth rotors. Generally, the copper and brass gave rise to higher shear stresses than the steel or stainless steel. The PTFE yielded the lowest shear stresses. With the compounds containing zinc stearate, the grooved rotors gave higher stresses than the smooth rotors even in pressurized experiments. Experiments are also reported for capillary dies manufactured from different metals. The shear viscosity data obtained in general agree with the pressurized rotational rheometer and a sandwich rheometer, but there is a tendency for higher shear stresses, higher extrudate swell, and more distorted extrudates with the copper and brass dies. The results of the studies indicate that (i) slippage occurs with rubber and rubbercompounds at low applied pressure in all systems and at high pressures with certain additives (zinc stearate) and (ii) copper alloys exert higher stresses on unpressurized elastomers or rubbercompounds in shear flow than iron alloys. We critique different mechanisms of slippage in the flow of polymer fluids and suspensions.
Note: NMR imaging of shear‐induced diffusion and structure in concentrated suspensions undergoing Couette flow35(1991); http://dx.doi.org/10.1122/1.550227View Description Hide Description
This note describes nuclear magnetic resonance(NMR)imaging of two concentrated suspensions undergoing flow between rotating concentric cylinders (wide‐gap, annular Couette flow).Suspensions of both monodisperse (50% by volume) and bidisperse (60% by volume) spheres are studied. We find that particles migrate from the higher shear rate regions near the rotating inner cylinder to the lower shear rate regions near the stationary outer wall, establishing large concentration gradients after only a short time. In addition, the large particles in the bimodal suspension form concentric cylindrical sheets, parallel to the axis of the Couette device, which rotate relative to each other. In the Couette devices used in these studies, no significant axial migration of the particles is observed: the dispersion of particles is almost entirely in the radial direction. This particle migration and structure formation is believed responsible for torque reductions and other anomalous behavior witnessed during rheological testing of concentrated suspensions reported by Leighton and Acrivos (1987).