摘要:

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.

ISSN:0148-6055    

DOI:10.1122/1.550462

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

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 thickening suspensions, which may exhibit plug flow at high wall shear stress values, i.e., above a critical wall shear stress in capillary flow.

ISSN:0148-6055    

DOI:10.1122/1.550435

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

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.

ISSN:0148-6055    

DOI:10.1122/1.550436

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

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.  

ISSN:0148-6055    

DOI:10.1122/1.550438

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

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.

ISSN:0148-6055    

DOI:10.1122/1.550459

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

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.

ISSN:0148-6055    

DOI:10.1122/1.550460

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550466

出版商:The Society of Rheology    

年代:1993

数据来源: AIP