摘要:

It is possible, in experimental flows of electrorheological (ER) fluids or in actual ER devices based on shear flows, that the electric field may have a component normal to and/or parallel to the fluid velocity. An analytical study of this possibility is presented here. The analysis is carried out using a constitutive equation for the three‐dimensional response of ER fluids [Rajagopal and Wineman, Acta Mechanica91(1992)] in which the electric field influences the response in two ways: (a) by affecting the material parameters, and (b) by contributing to stress components because of the interaction of the electric field vector and the shearing. It is shown that the viscosity is altered by a component of the electric field in the direction of the fluid velocity. Moreover, the viscosity is changed if the shear direction is reversed. The model is used in a study of the flow between parallel plates. It is found that the velocity field need not be symmetric about the midplane and the tractions on the plates may differ.

ISSN:0148-6055    

DOI:10.1122/1.550469

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The viscoelastic properties of model stable colloidal dispersions are calculated using Brownian dynamics simulations with a free‐draining model for the hydrodynamic interactions. In this report we describe a number of alternative routes to the dynamic moduli, making a comparison of the relative merits of the approaches. These include the use of the Green‐Kubo formulas (used widely by the molecular simulation community but little used so far in colloid modeling), which gives thelinearstress relaxation function from stress fluctuations in anunshearedmodel colloidal liquid. We also consider the direct application of an oscillating shear strain, as used in experiment, changing the frequency and strain amplitude in discrete jumps. This method is developed to consider a continuously varying sweep through frequency (constant strain amplitude) or strain amplitude (constant frequency). This route avoids problems associated with equilibration at each frequency. We avoid errors associated with truncation of the Fourier transform by cutting it off smoothly with a broad Gaussian. We concentrate on the real and imaginary parts of the complex storage modulusG*, the storage modulusG’, and loss modulusG‘, being the most frequently cited viscoelastic functions. For very large amplitudes, we find oscillatory‐shear‐induced crystallization to a fcc structure.

ISSN:0148-6055    

DOI:10.1122/1.550470

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The motion of a viscoelastic liquid, modeled via the Chilcott–Rallison version of the FENE dumbbell model, is studied numerically for a co‐rotating two‐roll mill. Solutions are obtained for values of the Deborah number up to 13.6, and for the dimensionless contour length 300≤L2≤2000 at three different values of the concentration parameterc≤1. The emphasis of the study is on start‐up flow from a state of rest. Of particular interest is the onset of a complicated overshoot and undershoot of the dumbbell configuration at very large values of De andL. This phenomenon is due primarily to the fact that the flow is significantly modified as the polymer contribution to the stress increases, but there is also a contribution due to the closed nature of the flow system which means that all streamlines of the flow are closed, and the deformation history experienced by a polymer molecule is therefore periodic in time.

ISSN:0148-6055    

DOI:10.1122/1.550471

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

This paper reports on the rheological properties of a series of concentrated poly(ethylene oxide) solutions. The polymers were obtained from two sources and dissolved in two solvents. Data on viscosity, primary normal stress differences, and dynamic rigidity were obtained using two Weissenberg rheogoniometers, a Rheometrics stress rheometer and a Bohlin VOR instrument. All the viscosity and normal stress data could be correlated using two key parameters: the zero shear viscosity and a characteristic time constant. Master curves are obtained which are almost independent of concentration, solvent, and molecular weight. The Cox–Merz rule is found to be valid for all solutions; however, the analogy between the primary normal stress coefficient and the storage modulus is verified only at low shear rates or frequencies. In addition, a modified rheological model based on the conformation tensor is shown to well represent the shear data.

ISSN:0148-6055    

DOI:10.1122/1.550472

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

Linear viscoelastic properties of polymer solutions depend on the solvent power even at high polymer concentrations. In this work, solutions of a commercial nitrile rubber in a homologous series of plasticizers were used. The rubbery plateau modulus is higher for the better solvent, the solvent power being assessed from the dilute solution viscosity.

ISSN:0148-6055    

DOI:10.1122/1.550473

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The rheological behavior of two low molar mass nematogens (LMMN), pentylcyanobiphenyl (5CB) and octylcyanobiphenyl (8CB), is studied. Over a major part of its nematic phase, 8CB shows shear stress oscillations in flow startup and step flow, characteristic of director tumbling, whereas pure 5CB has no oscillatory response but, over its entire nematic range, shows a stress overshoot, typical of flow alignment. For 8CB, the oscillation periodicity is a strong function of temperature, particularly near the SmA–N transition, and near the N–I transition where the crossover from tumbling to flow‐aligning response occurs. The transient shear response functions of each material are analyzed at different temperatures via Ericksen’s transversely isotropic fluid model to extract the Leslie viscosity coefficients α1, α2, and α3and the Miesowicz viscosity ηb. Our results are in good agreement with reported literature values.

ISSN:0148-6055    

DOI:10.1122/1.550474

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The rheological properties of polypropylene melts were investigated in oscillatory shear flow, capillary rheometry, and uniaxial elongation at constant tensile stress as well as constant strain rate. At small stresses the steady‐state elongational viscosity of linear conventional polypropylene has the threefold value of the shear viscosity. With increasing stress both the shear and elongational viscosity decrease. The transient elongational viscosity at constant strain rate is equal to the threefold value of the linear viscoelastic stressing viscosity as calculated from the relaxation time spectrum. In contrast, long chain branched polypropylene shows a maximum in the steady‐state elongational viscosity and pronounced strain hardening in experiments at constant strain rate above an elongation of ε=1. These phenomena are obtained by less than three branches per molecule. The description of the strain hardening by means of the Lodge model underestimates the measured data at deformation rates less than ε̇=0.2 s−1. An improvement is obtained by adding a rubber‐like stress component to the tensile stress.

ISSN:0148-6055    

DOI:10.1122/1.550475

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

Steady shear rheological measurements are carried out on lyotropic aqueous solutions of hydroxypropylcellulose. The molecular weight is the main parameter investigated. Careful sealing of the sample permits very long time experiments, thus leading to steady‐state measurements at shear rates as low as 10−3s−1. At such low deformation rates, for two of the three molecular weights investigated, a Newtonian plateau is found before region I in the viscosity curve, thus ruling out the possibility of a yield stress in this material. A phenomenological model is proposed in order to interpret the effects of molecular weight on the rheological behavior. The polymer system is modeled as a structured fluid, in which the low shear rate response is dominated by the polydomain structure, whereas molecular dynamics dominates the high velocity gradient region. The model is able to predict the observed qualitative changes in the viscosity vs shear rate curve as a function of molecular weight, as well as the scaling of the first normal stress difference with a molecular time scale of the system. Finally, intrinsic viscosity measurements show that the molecular weight of the samples used in this work is systematically larger than the value indicated by the supplier.

ISSN:0148-6055    

DOI:10.1122/1.550476

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

Experimental results on the linear viscoelastic behavior of concentrated suspensions are presented. The materials studies were prepared by dispersing submicron silica spheres at volume fractions φ ranging from 0.3 to 0.6, in a highly viscous liquid. The response to oscillatory shearing was determined over a wide range of frequency, ω. The zero frequency viscosity η0and the limiting high‐frequency viscosity η’∞for all particle radii studied were essentially identical to results previously obtained for suspensions having hard sphere interactions. In this range of concentrations, the frequency dependence of the dynamic moduli,G’andG‘−ωη’∞, appears to be described by universal functions of ωτwwhere τwis the mean longest relaxation time proportional to a characteristic (Peclet) time defined as τp=a2/6Ds(φ). Hereinadenotes the particle radius andDs(φ) is a φ dependent short‐time self‐diffusion constant. We also found that at sufficiently high frequencies,G’has a plateau,G∞, that is given bykT/a3times a function of φ.

ISSN:0148-6055    

DOI:10.1122/1.550477

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

Thermotropic liquid crystalline polymers commonly develop a bubble texture at time scales necessary to perform low‐rate rheological experiments. We have fabricated a capillary rheometer capable of high‐sensitivity (relatively low‐rate) measurements under a hydrostatic pressure adequate to suppress formation of visible bubbles. Vectra A (a copolyester of 73% HBA and 27% HNA) exhibited a transient reduction in viscosity at low rates that is independent of strain history and dependent only on time in the melt; no change in inherent viscosity was observed despite a threefold change in viscosity. The transient, which was not observed at high rates on the time scale of the experiment, may reflect changes in the defect structure. The flow curve for a given thermal history (time in the melt) exhibited a flattening at low to intermediate rates, suggestive of the flow curves reported for lyotropic systems. This result is qualitatively in agreement with rotational rheometer measurements known to be contaminated by offgassing, which exhibit higher apparent viscosities but give the same flow curve shape. Rotational rheometer transients scaled with strain in a manner similar to that reported for lyotropic systems. Transient reduction in viscosity and flow curve flattening with decreasing rate was also observed in pressurized rheometer measurements of the fully nematic copolyester of 80% HBA and 20% PET; a decrease in inherent viscosity was observed for this polymer, but it appears to be too small to account for the observed decrease in viscosity with time.

ISSN:0148-6055    

DOI:10.1122/1.550478

出版商:The Society of Rheology    

年代:1994

数据来源: AIP