摘要:

The polymer coil whose size is comparable in solution to particle diameter makes a flexible bridge between two particles. The suspensions flocculated by the bridging mechanism show shear thickening at high shear rates when both the particle and polymer concentrations are increased above some critical levels. The boundary for appearance of shear thickening is analyzed in terms of a percolation process. The shear thickening results from the three‐dimensional network structure consisting of particles bridged by flexible polymer coils. Since the intrinsic mechanism of shear thickening is the restoring forces of extended bridges in shear fields, the most important variable affecting the flow behavior is bridging conformation. The effects of particle and coil sizes on the shear‐thickening behavior are examined in relation to bridging conformation. The shear thickening takes place for suspensions prepared using particles with diameters of 80–125 nm and polymers with molecular weights of 2.5×105–4.5×105. The appearance of shear thickening is determined by a delicate balance between flexibility of bridge and adsorption affinity.

ISSN:0148-6055    

DOI:10.1122/1.550464

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Large amplitude oscillatory shear (LAOS) is a useful tool for the study of nonlinear viscoelasticity in polymeric liquids. To concisely describe the response of a material to such a test, it is desirable to make use of a constitutive equation. In this way, the response can be described in terms of the parameters of the rheological model. We have found a separable BKZ model, e.g., Wagner’s equation, to be useful in this regard for LDPE IUPAC X. The damping function determined in step shear experiments does not lead to accurate predictions of the LAOS response. For a given low frequency, however, it is possible to fit the parameters of a simple damping function equation to the LAOS response at one strain amplitude and to use these parameter values to reliably predict the response at other strain amplitudes. Thus, the Wagner equation provides a basis for the concise description of the response of LDPE IUPAC X to large amplitude oscillatory shear experiments at low frequencies. Conversely, it was found that the response of a HDPE melt to large amplitude oscillatory shear cannot be concisely described by the Wagner equation.

ISSN:0148-6055    

DOI:10.1122/1.550396

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Biaxial extensional flow behavior of polystyrene and polypropylene melts was measured utilizing the lubricated squeezing flow method under the condition of constant strain rate. The uniaxial extensional flow behavior of these samples was also measured using a Meissner‐type rheometer for comparison with the biaxial behavior. The biaxial stress growth coefficient ηB+grows more slowly than the uniaxial stress growth coefficient ηE+at long times. Deviation of ηB+from the low strain rate asymptote is not so prominent as that of ηE+. The deviation of ηB+and the upturn of ηE+from the low strain rate asymptotes are more apparent in broad distribution samples than in a narrow distribution sample. It is found that ηB+decreases with strain rate at long times near steady state. The upturn behavior of ηE+at high strain rates in a narrow distribution sample is discussed based on the Doi–Edwards theory and stress relaxation data. The upturn is clearly observed when the strain rate exceeds the contraction (retraction) rate τeq−1, where the contraction time τeqis evaluated from the relaxation data. Consideration on chain stretching in various multi‐axial extensional flows gives a prediction for deviation of stress growth coefficients from the low strain rate asymptotes in uniaxial, biaxial, planar, and ellipsoidal extensional flows.

ISSN:0148-6055    

DOI:10.1122/1.550397

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Flow experiments using a capillary rheometer with 0.2% xanthan aqueous solutions were carried out to give experimental evidence of the dependence of the slip velocity on the geometrical parameters of the rheometer. The Mooney method was used to evaluate the slip velocityVs. It was found thatVsis an increasing function of the wall shear stress and also of the length to diameter ratioL/Dnot considered in the Mooney theory. That is,Vs=Vs(τw,L/D), although, for each τwvalue,Vsbecomes independent ofL/Dat largeL/D. In addition, a method to determine the excess pressure drop due to end effects is outlined and experimental results presented. The differences between this method and the one by Bagley are highlighted.

ISSN:0148-6055    

DOI:10.1122/1.550399

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

The biaxial extensional viscosity of wheat flour doughs was measured using the lubricated squeeze film apparatus. Two disks of different diameters, 80 and 57.2 mm, were employed, with an extension rate (0.011 s−1) which remained approximately constant with both disks. Using polydimethylsiloxane a steady state lasted from 40 to 70 s. Obtaining steady extensional flow with wheat doughs, on the other hand, necessitated 10 to 200 s depending on the magnitude of normal stresses which ranged from 5.018 to 0.361 KPa. Doughs with different protein contents (13.2%, 16.0%, and 18.8% based on 14%MB) showed different biaxial extensional viscosities. These increased with increasing protein content. At an extensional rate of 7.3×10−5s−1the biaxial extensional viscosity approached 6η; Trouton’s ratio (ratio of ηEto η) increases with extension rate. The differences between biaxial extensional viscosities and shear viscosities become more profound when viscosities are plotted versus stresses.

ISSN:0148-6055    

DOI:10.1122/1.550400

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Measurements of hole pressure for shallow recesses were made for two Newtonian and two non‐Newtonian fluids. The study was conducted by mounting two pressure transducers opposite each other in the walls of a wide rectangular channel and measuring the pressures with one transducer flush and the other recessed. Data were obtained for recesses up to 60% of the hole diameter and for Reynolds numbers up to 400. For the Newtonian fluids, the hole pressure was negative and its magnitude increased with Reynolds number, reaching up to 90% of the wall shear stress. The magnitude also increased with recess depth, attained a maximum when the depth was 0.16 of the hole diameter, and then decreased for deeper recesses. The two non‐Newtonian fluids were polymer solutions, one was concentrated and shear thinning the other dilute and non‐shear‐thinning. For both fluids, the hole pressure was positive and the magnitudes were up to ten times those for the Newtonian fluids, which indicates that elastic effects were much larger than inertial effects. It was also found that the ratio of the hole pressure to the wall shear stress was roughly constant, i.e., roughly independent of the Reynolds numbers. For all fluids, the hole pressure was detectable for recesses as small as 2% of the diameter.

ISSN:0148-6055    

DOI:10.1122/1.550401

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

All three viscometric functions are investigated as a function of shear rate for lyotropic solutions of hydroxypropyl cellulose (HPC). Through the use of m‐cresol as the solvent, it was possible to investigate liquid crystal solutions at lower polymer volume fractions than have previously been studied for HPC. We find no evidence of a three‐region viscosity curve, as has been widely reported for HPC at higher polymer volume fractions. The first and second normal stress differences (N1andN2, respectively) both change sign twice as the shear rate is varied.

ISSN:0148-6055    

DOI:10.1122/1.550404

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Infinitely dilute polymer solutions undergoing elongational flows are studied by modeling the polymer molecules as bead‐spring chains. Monte Carlo integration is used to calculate the properties of chains of finitely extensible nonlinear elastic springs in steady uniaxial elongational flow from the exact solution for the distribution of configurations. The properties calculated include the mean square end‐to‐end distance of the chains and the elongational viscosity and intrinsic birefringence of the solution. These properties are compared to the predictions of the model under the Peterlin approximation. Whereas the models with and without the approximation show qualitatively similar results, there are substantial quantitative differences.

ISSN:0148-6055    

DOI:10.1122/1.550405

出版商:The Society of Rheology    

年代:1993

数据来源: AIP