Volume 37, Issue 1, January 1993
Index of content:
Direct observation of particle microstructure in concentrated suspensions during the falling‐ball experiment37(1993); http://dx.doi.org/10.1122/1.550462View Description Hide Description
It is demonstrated in this study that suspension microstructure and mean fall velocity can be obtained simultaneously in real time during the falling‐ball experiment using nuclear magnetic resonance imaging(NMRI). When the size of the suspending particles is significantly different from that of the falling ball (at least by a factor of 2), the motions of both the falling ball and the surrounding neutrally buoyant particles can be tracked by monitoring the proton NMR signals from the suspending liquid. Using a 50 vol % suspension of approximately 700 μm diameter spheres and a 9 mm diameter cylinder, the suspension microstructure around a 4.76 mm (3/16 in.) diameter falling ball was found to be significantly perturbed by the motions of the ball, whereas the fall velocity decreases with time. The suspension concentration increases at the leading edge and decreases behind the falling ball. These effects diminish as the diameter of the falling ball decreases to 1.59 mm (1/16 in.). Possible mechanisms to account for the observed changes in microstructure are discussed and include many‐body hydrodynamic and boundary effects.
37(1993); http://dx.doi.org/10.1122/1.550461View Description Hide Description
Transient viscoelastic experiments such as shear stress relaxation can provide direct measurement of the stress relaxationmodulusG(t). Since the strain establishment is not instantaneous, the measured stress must be regarded as the convolution product of the relaxationmodulus with respect to the shear rate. If a Maxwellian analogy is assumed for the melt stress relaxation, then the main difficulty arises from the deconvolution procedure and from the unknown number of relaxation modes. The Padé–Laplace method based upon Padé approximants and the Laplace transform has been applied to shear stress relaxation experiments performed with melt samples of polystyrene and polypropylene. The major interest of this method lies in the fact that it detects through its own stability the number of modes necessary to fit the signal without any assumption. Moreover, it allows the determination of the nrelaxation times τ i and their weights G i . Finally, the mathematical properties of the Laplace transform can be used to perform the deconvolution of the signal.
37(1993); http://dx.doi.org/10.1122/1.550435View Description Hide Description
The rheological behavior of a very concentrated suspension (76.5 vol %), which serves as a widely used solid rocket fuel simulant, was characterized employing both torsional and capillary flows. No comprehensive studies of the rheology of concentrated suspensions have been carried out previously at such a high solids content. The suspension exhibited shear thinning over the apparent shear rate range of 30–3000 s−1. Significant slip at the wall was observed in both torsional and capillary flows with the slip velocity increasing from about 0.001 mm/s at a shear stress of 4 Pa to as high as 60 mm/s at 100 kPa. A flow visualization technique was applied for the first time to determine the wall slip velocities in torsional flow directly, to also provide the true deformation rate and feedback on yielding. The contribution of the slip of the suspension at the wall to the volumetric flow rate in capillary flow was found to increase with decreasing shear stress, giving rise to plug flow at sufficiently low shear stress values. The observed plug flow is related to the shear‐thinning nature of the suspension and differs from the behavior of shear thickeningsuspensions, which may exhibit plug flow at high wall shear stress values, i.e., above a critical wall shear stress in capillary flow.
The influence of viscometer dynamics on the characterization of an electrorheological fluid under sinusoidal electric excitation37(1993); http://dx.doi.org/10.1122/1.550436View Description Hide Description
The dynamic response of a controlled‐strain, Couette viscometer employed for the characterization of the frequency response of an electrorheological (ER) fluid is studied both numerically and experimentally. In the numerical model, the ER fluid flow between the cup and bob elements of the viscometer is coupled with the mechanical response of the cup–torque sensor system. The Bingham model is used for describing the ER fluid, with various functional forms for relating the electric field strength to the Bingham stress. Variation in the shear‐rate dependency of the Bingham stress response is also represented. Dynamic resonance tends to dominate the cup rotation response and the shear rate of the fluid. The Bingham stress response contains higher harmonic components whenever it does not follow a second power‐law relationship exactly with the electric field strength. Higher harmonics induce their own resonances at relatively lower values of excitation frequency. Experimental results obtained with zeolite‐based ER fluids generally agree with those predicted through the numerical analysis. The characterization of an ER fluid will be reasonably accurate only if the excitation frequency of the electric field is low, say less than 0.1 times the natural frequency of the cup–torque sensor assembly.
The shear‐distorted microstructure of adhesive hard sphere dispersions: A small‐angle neutron scattering study37(1993); http://dx.doi.org/10.1122/1.550437View Description Hide Description
In a small‐angle neutron scatteringexperiment, we measured the shear‐distorted microstructure of a concentrated model dispersion of adhesive hard spheres in the low shear rate regime (Pe≤1) using a couette shear cell. The nonequilibrium structure factor was probed in the velocity–vorticity plane. By changing the temperature, the interaction potential of the particles was varied from a hard sphere repulsion to a strong attraction. The particle distribution which is isotropic at rest, is anisotropically disturbed by the shear flow. At high temperatures, the dispersion is effectively a hard sphere system. In this case the main distortion of the scattering pattern is in the direction of the fluid flow, the shear flow shifts the first maximum in the structure factor toward smaller scattering angles. In the vorticity direction, no significant distortion of the equilibrium structure is observed. Qualitative agreement is found with theoretical predictions for the shear‐distorted structure factor. With decreasing temperature the short ranged attractions between the particles become stronger. At rest, the growing attractions shift the position of the first maximum in the structure factor toward larger scattering angles. When a flow field is applied, the peak tends to shift back toward smaller angles with increasing shear rate in the direction of the flow. In the vorticity direction the position of the peak is rather independent of the shear rate. The influence of the shear flow in the neighborhood of the peak is in both directions small compared to the changes induced in the structure factor at small scattering angles. At rest the structure factor shows an upswing at small scattering angles when interparticle attractions become stronger. This upswing is anisotropically decreased when the dispersion is subjected to flow; the flow diminishes the influence of the direct attractive forces. The measured shear‐distorted structure factor disagrees significantly with theory at low temperatures. Contrary to the theoretical predictions, a large distortion is measured in the vorticity direction, when attractions are strong. This discrepancy is probably due to the neglect of higher‐order correlations in the available theoretical expressions.
Rheological and morphological studies of a thermotropic liquid crystalline polymer with low temperature transitions37(1993); http://dx.doi.org/10.1122/1.550438View Description Hide Description
The rheology and morphology of thermotropic liquid crystalline (TLC) polyesters are greatly dependent on the shear and thermal history of the sample. In this paper, the results of the time‐dependent melt rheology of a main chain, low‐melt TLC polymer with two flexible spacers are presented. Factors which have an effect on the viscoelastic properties such as temperature, frequency, and strain amplitude are discussed. It is shown that the increase in η* at constant strain amplitude and temperature, depends only on time and not shear. X‐ray diffraction patterns indicate that shear causes the polymer molecules to orient in the shear direction. Both frequency and strain amplitude seem to induce a gradient of crystallinity in the bulk of the samples and the crystalline fraction is predominant near the surface.
37(1993); http://dx.doi.org/10.1122/1.550459View Description Hide Description
The effective longitudinal and transverse shear viscosities are derived for an aligned fiber suspension. The solutions are valid under very concentrated conditions for a hexagonal arrangement of the single size fibers. The results compliment the classical dilute suspension forms at the other extreme of concentration. Empirical forms are constructed to cover the full range of volume fraction of the fiber phase. Also, single size spherical particle suspensions are given a similar treatment to that of the fiber case.
37(1993); http://dx.doi.org/10.1122/1.550460View Description Hide Description
The addressed problem consists of finding the average number of fibers whose centerlines intersect a test tube circumscribing a test fiber, given the orientation distribution and concentration of the fibers. Approximate solutions to this problem have been published in the past. This note presents an exact solution for an arbitrary test volume and arbitrary fiber length distribution. The solution is then specialized to the above problem and compared with the previous approximate solutions.