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1.
An Australian hard wheat flour–water dough has been characterised using parallel plate and capillary rheometers over an extensive
range of apparent shear rates (10 − 3–103 s − 1) relevant to process conditions. Torsional measurements showed that the shear viscosity of the dough increased with strain
to a maximum value and then decreased, suggesting a breakdown of the dough structure. Both torsional and capillary experiments
revealed the shear-thinning behaviour of the dough. The wall slip phenomenon in capillary rheometry was investigated and found
to be diameter dependent and occurred at a critical shear stress of approximately 5–10 kPa. A two-regime power law behaviour
was observed, with the power law index approximately 0.3 in the low shear rate range increasing to 0.67 in the high shear
rate range. Pressure fluctuation was observed in the capillary data and increased with shear rate, in particular, at shear
rates approaching 104 s − 1. The results demonstrate that capillary rheometry is a viable means of rheologically testing dough at high shear rates provided
pressure fluctuation is carefully monitored and capillary rheometry corrections, including wall slip, are accounted for. 相似文献
2.
The rheological and structural properties of perfluoropolyether (PFPE) lubricant films including viscosity, shear stress,
and birefringence were measured at relatively low to extremely high shear rates using a rotational optical rheometer. The
viscosity of various films with different thicknesses exhibit Newtonian behavior up to a shear rate 1 × 104 s−1, with a transition to shear-thinning behavior obvious at higher shear rates. Birefringence of these films was also measured
for the first time, and these results indicate chain alignment with shear in the shear-thinning regime. The shear rate at
which alignment occurs is similar to that of the onset of shear thinning. This correlation between chain alignment and shear
thinning provides direct evidence that the ability of PFPEs to lubricate hard drives at high shear rates is a direct consequence
of the ability of the applied shear field to align the molecules on a molecular level. 相似文献
3.
Drag correction factors are calculated for the creeping motion of spheres descending in various associative polymers of different
concentration with various sphere-container ratios and Weissenberg numbers. The simple-shear rheology and linear viscoelasticity
of these polymeric fluids have been previously presented and modeled with the BMP (Bautista–Manero–Puig) equation of state
(Mendoza-Fuentes et al., Phys Fluids 21:033104, 2009). The drag on the sphere is initially kept nearly constant for small Weissenberg numbers, We < 0.1. As the Weissenberg number
increases, We < 0.1, a reduction in drag is found. Experimental results show the presence of a critical Weissenberg number
at which a drag reduction occurs. The reduction in the drag correction factor is associated to the onset of extension-thinning,
which coincides with the formation of a negative wake. No increase in the drag correction factor was observed, due to the
simultaneous opposing effects of extension-thickening and shear-thinning viscosity. The shape of the drag correction factor
curve may be predicted considering the extensional properties of the solutions, as suggested elsewhere (Chen and Rothstein,
J Non-Newton Fluid Mech 116:205–215, 2004). 相似文献
4.
The effects of coincidence window and measuring volume size on two-component laser velocimeter measurement of turbulence
in an isothermal liquid flow through a concentric annular channel were studied. Three different coincidence windows (100–500 μs)
and three different measuring volume sizes (diameter, 5–9 wall units; spanwise length, 24–91 wall units) were used in a flow
of Reynolds number 31,500 and data density spanning the high end of intermediate to the low end of high (3–6). While no significant
effects of the coincidence window and measuring volume size were found on the time-mean velocity and turbulence intensities,
the streamwise Reynolds shear stress measured near a wall was found to be markedly affected by both. The smallest feasible
measuring volume along with an appropriate coincidence window provides good measurement of the shear stress.
Received: 8 September 1999/Accepted: 11 July 2000 相似文献
5.
The response of a magneto-rheological fluid (MRF) to a change of magnetic flux density is investigated by using a commercial
plate–plate magneto-rheometer MCR501 (Anton Paar GmbH) at constant shear rate. The instrument was modified to allow an online
determination of the transient flux density in the MRF. Both current and voltage imposition to the magneto-cell were applied
by using a power operational amplifier to drive the electromagnet. Assuming a Maxwell behavior with switching time λ and a linear increase in shear stress with flux density, analytic relations for the transient shear stress are derived for
sinusoidal and single exponential flux densities vs time. True switching times of a few milliseconds are only obtained if
the low pass filter in the original MCR501 torque signal is surpassed by a firmware allowing a sampling rate of 0.1 ms. For
a sinusoidal flux density, the switching time is derived from the modulation depth of the shear stress. An upper bound of
λ < 3 ms for a flux density of 0.8 T was found. For step coil current imposition of 1 T magnitude, switching times of 2.8 ms
(start-up) and 1.8 ms (shutdown) allowed to fit the transient torque signal more than 2/3 of the total change. Finally, the
effect of a sigmoidal characteristic on the switching time determination is addressed.
This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27-29, 2006. 相似文献
6.
The electrorheological (ER) behavior and stress transient response of dilute solutions of a side-chain liquid-crystalline
polysiloxane (LCP) in 4,4′-n-octylcyanobiphenyl (8CB) is studied. In the flow-tumbling regime of 8CB, i.e. from T = 34–38 °C,
the stress transients of both 8CB and LCP/8CB solutions show oscillatory responses, but with shorter oscillation periodicities
for the solution. In the flow-aligning regime of 8CB, i.e. at 39 and 40 °C, a transformation to flow-tumbling is observed
in the stress transients of the LCP/8CB solution. In both cases, analysis of the transient responses indicates that the change
in Leslie viscosity coefficients on dissolving the LCP are δα2 < 0 and δα3 > 0. The amplitude of the ER response, defined as the viscosity difference between the on and off states, Δη = ηon − ηoff, is only weakly affected by the dissolution of LCP. These rheological results can be interpreted consistently using a modified
version of a hydrodynamic theory by Brochard, provided an additional dissipation mechanism is included, which derives from
the presence of an elastic torque between director rotation and LCP orientation.
Received: 8 September 1999/Accepted: 13 December 1999 相似文献
7.
In the present work the effects produced by the presence of two different surfactants (Abil B 8842 and Triton N 101) on the
rheological properties of aqueous welan matrices are studied, both in steady and in oscillatory shear conditions. Welan is
an acidic microbial polysaccharide having high thermal, pH, and salt stability. At sufficiently low concentrations it forms
aqueous weak gel matrices which can be profitably used to regulate the rheological properties of disperse systems and improve
their stability. Different systems are examined, having the same polysaccharide concentration (0.25 wt%) and different surfactant
concentrations (up to 40 wt%, far beyond the range of practical interest for emulsion preparation).
All the systems exhibit marked shear-thinning properties which can be described quite satisfactorily by the Cross equation.
The concentration dependence of the zero-shear-rate viscosity as well as the mechanical spectra confirm that, in the concentration
range considered, the aqueous welan systems are typically weakly structured fluids. The influence of both surfactants is examined
in detail by comparing the behavior of the different classes of systems. Both surfactants reduce the polymer contribution
at low shear, whereas an opposite action is exerted at high concentration and shear. These contrasting effects are ascribed
to the different structural features of the polymer matrix under low stresses and high shear conditions, respectively.
Received: 6 February 2000 Accepted: 1 November 2000 相似文献
8.
Henning Storz Ulrich Zimmermann Heiko Zimmermann Werner-Michael Kulicke 《Rheologica Acta》2010,49(2):155-167
Ultra-high viscosity alginates were extracted from the brown seaweeds Lessonia nigrescens (UHVN, containing 61% mannuronate (M) and 2% guluronate (G)) and Lessonia trabeculata (UHVT, containing 22% M and 78% G). The viscoelastic behavior of the aqueous solutions of these alginates was determined in shear
flow in terms of the shear stress σ
21, the first normal stress difference N
1, and the shear viscosity η in isotonic NaCl solutions (0.154 mol/L) at T = 298 K in dependence of the shear rate [(g)\dot]\dot{\gamma} for solutions of varying concentrations and molar masses (3–10 × 105 g/mol, homologous series was prepared by ultrasonic degradation). Data obtained in small-amplitude oscillatory shear (SAOS)
experiments obey the Cox–Merz rule. For comparison, a commercial alginate with intermediate chemical composition was additionally
characterized. Particulate substances which are omnipresent in most alginates influenced the determination of the material
functions at low shear rates. We have calculated structure–property relationships for the prediction of the viscosity yield,
e.g., η–M
w–c–[(g)\dot]\dot{\gamma} for the Newtonian and non-Newtonian region. For the highest molar masses and concentrations, the elasticity yield in terms
of N
1 could be determined. In addition, the extensional flow behavior of the alginates was measured using capillary breakup extensional
rheometry. The results demonstrate that even samples with the same average molar mass but different molar mass distributions
can be differentiated in contrast to shear flow or SAOS experiments. 相似文献
9.
The flow-induced microstructure of a mesophase pitch was studied within custom-made dies for changing wall shear rates from
20 to 1,100 s − 1, a flow scenario that is typically encountered during fiber spinning. The apparent viscosity values, measured at the nominal
wall shear rates ranging from 500 to 2,500 s − 1 using these dies, remain fairly constant. The microstructure was studied in two orthogonal sections: r–θ (cross section) and r–z (longitudinal mid plane). In these dies, the size of the microstructure gradually decreases toward the wall (to as low as
a few micrometers), where shear rate is highest. Furthermore, as observed in the r–θ plane of the capillary, for a significant fraction of the cross section, discotic mesophase has a radial orientation. Thus,
the directors of disc-like molecules were aligned in the vorticity (θ) direction. As confirmed from the microstructure in the r–z plane, most of the discotic molecules remain nominally in the flow plane. Orientation of the pitch molecules in the shear
flow conditions is consistent with that observed in controlled low-shear rheometric experiments reported earlier. Microstructral
investigation suggests that the radial orientation of carbon fibers obtained from a mesophase pitch originates during flow
of pitch through the die. 相似文献
10.
H. Henning Winter 《Rheologica Acta》2009,48(3):241-243
A slight rearrangement of the classical Cox and Merz rule suggests that the shear stress value of steady shear flow, , and complex modulus value of small amplitude oscillatory shear, G ∗ (ω) = (G′2 + G″2)1/2, are equivalent in many respects. Small changes of material structure, which express themselves most sensitively in the steady
shear stress, τ, show equally pronounced in linear viscoelastic data when plotting these with G ∗ as one of the variables. An example is given to demonstrate this phenomenon: viscosity data that cover about three decades
in frequency get stretched out over about nine decades in G ∗ while maintaining steep gradients in a transition region. This suggests a more effective way of exploiting the Cox–Merz rule
when it is valid and exploring reasons for lack of validity when it is not. The τ −G ∗ equivalence could also further the understanding of the steady shear normal stress function as proposed by Laun. 相似文献
11.
Christelle Grein Markus Gahleitner Bernhard Knogler Susanne Nestelberger 《Rheologica Acta》2007,46(8):1083-1089
One of the key factors to a better understanding of the phase morphology of high-impact ethylene–propylene (EP) copolymers
is the knowledge of the rheological behaviour of the various phases in these complex systems. Next to the molecular weight
distribution, the comonomer content also affects the viscosity, as could be demonstrated in a systematic study of a wide range
of compositions. More specifically, this means that for a given average molecular weight, the zero shear viscosity of the
disperse phase consisting of amorphous ethylene–propylene ‘rubber’ (EPR) and crystalline polyethylene (PE) of such an impact
copolymer are up to orders of magnitude higher than the respective polypropylene (PP) phase. This has a significant effect
on the phase morphology and ultimately the mechanical performance of these compositions.
Paper presented at the Annual European Rheology Conference (AERC) 2006, April 27–29 2006, Hersonisos, Greece. 相似文献
12.
Thomas Schweizer 《Rheologica Acta》2007,46(5):629-637
Creep experiments with a solution of polystyrene (M
w = 2.6 MDa, 16 vol.%, 25 °C) in diethyl phthalate are reported for stresses between 100 and 2,500 Pa (≈ 3G
N
0/4). The aim was to look for a flow transition as reported for strongly entangled poly(isobutylene) solutions. The experiments
with the polystyrene solution were repeated for cone angles of 2, 4, and 6° (radius 15 mm) and showed no dependence on cone
angle. The Cox–Merz rule was not fulfilled for stresses beyond about 800 Pa. The tangential observation with a CCD camera
showed that the edge took a concave shape because of the second normal stress difference. Beyond 1,000 Pa, the concave edge
develops into a crevice, thus substantially reducing the effective cross-section. This leads to runaway in a constant torque
experiment. At p
21 = 800 Pa, head-on particle tracking confirms that the originally linear velocity profile takes a gooseneck shape, thus revealing
shear banding. When the creep stress is stepped down to 100 Pa, this velocity profile evolves back to a linear one. The conclusion
from this work is that even if nonlinear creep experiments are reproducible and a steady state is reached, this does not mean
that the flow field is homogeneous.
This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006. 相似文献
13.
We investigated the deformation of a strong shear thinning droplet undergoing simple shear flow in a Newtonian liquid. The
droplet was an aqueous solution of poly(ethylene oxide) end capped with an alkyl group that forms spherical micelles in aqueous
solution. At high concentrations and below a critical temperature, the jammed micelles showed strong shear thinning behaviour,
and neither a yield stress nor a Newtonian viscosity was observed. At small shear rates, the droplet rotated and aligned in
the flow, but did not deform or only very weakly. At high shear rates, the droplet deformation increased with increasing shear
rate. The deformed droplet did not relax after the shear was stopped except for a modest rounding of the edges. For each shear
rate, an apparent viscosity, η
ad, of the equivalent Newtonian droplet was calculated assuming affine deformation. η
ad showed a power law dependence on the capillary number Ca with an exponent of − 1.8 and was larger than the shear viscosity
of the micelle suspension at the same shear rates. The results were explained by the existence of a strong gradient of the
viscosity inside the droplet leading to a very low viscosity fluid layer near the droplet/matrix interface. 相似文献
14.
Takehiro Yamamoto Yousuke Nakamura Atsushi Yamashita Takamasa Hashimoto Noriyasu Mori 《Rheologica Acta》2006,45(3):250-259
Viscous fingering in surfactant solutions in a rectangular Hele–Shaw cell was investigated. Test fluids were aqueous solutions
of cetyltrimethylammonium bromide (CTAB) with sodium salicylate (NaSal) as a counter ion, and the ratio of mole concentration
of CTAB and that of NaSal was 1–7.7. Two fluids that had a mole concentration different from that of CTAB were used. Air was
injected into the cell and the growth of the interface between air and a CTAB/NaSal solution was observed. The fingertip grew
similar to the finger growth in shear-thinning fluids at low pressure gradients. It took a cuspidate shape at the intermediate
pressure gradient, and a sudden protrusion at a critical shear rate occurred. In high shear rate regions, the finger behaved
as in a less shear-thinning fluid. These phenomena relate to rheological properties of the test fluids. Comparison with flow
curves for CTAB/NaSal systems showed that the critical shear rate related to the shear rate at which a bending point appeared
in the flow curve. 相似文献
15.
S. V. Stebnovskii 《Journal of Applied Mechanics and Technical Physics》2006,47(2):208-213
Instability of the structure of viscous fluids in the Couette flow regime (spontaneous formation of bands, which are tangential
discontinuities in terms of viscosity) is experimentally found. This process is demonstrated to be analogous to formation
of shear bands in polymethylmethacrylate (Plexiglas) under plastic shear strains.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 2, pp. 70–76, March–April, 2006. 相似文献
16.
The penetration of long gas bubble through a viscoelastic fluid in a capillary tube has been studied in order to investigate
the influence of viscoelastic material properties on the hydrodynamic coating thickness and local flow kinematics. Experiments
are conducted for three tailored ideal elastic (Boger) fluids, designed to exhibit similar steady shear properties but substantially
different elastic material functions. This allows for the isolation of elastic and extensional material effects on the bubble
penetration process. The shear and extensional rheology of the fluid is characterized using rotational and filament stretching
rheometers (FSR). The fluids are designed such that the steady-state extensional viscosity measured by the FSR at a Deborah
number (De) greater than 1 differs over three orders of magnitude (Trouton ratio = 103–106). The experiment set up to measure the hydrodynamic coating thickness is designed to provide accurate data over a wide range
of capillary numbers (0.01 < Ca < 100). The results indicate that the coating thickness in this process increases with an
increase in the extensionally thickening nature of the fluid. Experiments are also conducted using several different capillary
tube diameters (0.1 < D < 1 cm), in order to compare responses at similar Ca but different flow De. Suitable scaling methods and nonlinear viscoelastic
constitutive equations are explored to characterize the displacement process for polymeric fluids. Bubble tip shapes at different
De are recorded using a CCD camera, and measured using an edge detection algorithm. The influence of the mixed flow field
on the bubble tip shape is examined. Particle tracking velocimetry experiments are conducted to compare the influence of viscoelastic
properties on the velocity field in the vicinity of the bubble tip. Local shear and extension rates are calculated in the
vicinity of the bubble tip from the velocity data. The results provide quantitative information on the influence of elastic
and extensional properties on the bubble penetration process in gas-assisted injection molding. The bubble shape and velocity
field information provides a basis for evaluating the performance of constitutive equations in mixed flow.
Received: 19 January 1999 Accepted: 30 June 1999 相似文献
17.
Shear and elongational viscosity measurements were performed on low-density polyethylene/phosphate glass (LDPE/Pglass) hybrid
materials in the liquid state. Under shear deformation, the hybrids with low concentrations of Pglass showed a Newtonian region
at low frequencies, followed by shear-thinning behavior at high frequencies. High Pglass concentrations displayed shear-thinning
behavior over the whole range of frequencies studied. Deviations from the log-additivity rule for viscosity were found to
be compositionally dependent and generally indicated an immiscible mixture. The elongational viscosity of the hybrids increased
at very low Pglass concentrations (1–2 vol.% Pglass) and then was drastically reduced at higher concentrations (i.e., >10 vol.%
Pglass). In addition, elongational flow was found to induce the formation of Pglass fibrils in hybrids containing at least
10 vol.% Pglass. This was correlated to the elongational capillary number; the critical elongational capillary number was
estimated to be 0.22. The elongational deformation was also found to greatly increase the overall crystallinity of the system
due to molecular orientation of the LDPE polymer chains as confirmed by wide angle X-ray diffraction. A critical composition
of 5 vol.% Pglass was found to be the point at which LDPE hybrid rheological properties, molecular orientation, and morphology
changed drastically. 相似文献
18.
José Inés Escalante Daniel Escobar Emma Rebeca Macias Juan Humberto Pérez-López Fernando Bautista Eduardo Mendizábal Jorge E. Puig Octavio Manero 《Rheologica Acta》2007,46(5):685-691
Low-viscosity micellar aqueous solutions of cetyltrimethylammonium bromide (CTAB) undergo a major change in the presence of
the hydrotrope, potassium 1-phenylmethylsulfate (KPhMS), producing a highly viscoelastic entanglement network of polymer-like
micelles. The system studied here shows typical shear banding flow behavior, which tends to disappear with increasing the
hydrotrope-to-surfactant concentration ratio (C
H / C
S). The linear rheological response was analyzed with the model of Granek–Cates, whereas the nonlinear behavior was reproduced
with the Bautista–Manero–Puig (BMP) model. Both models introduce a kinetic equation to account for the breaking and reformation
of the micelles, and they predict the linear and nonlinear rheological data very well.
This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006. 相似文献
19.
Andréas Redl Marie Hélène Morel Joëlle Bonicel Stéphane Guilbert Bruno Vergnes 《Rheologica Acta》1999,38(4):311-320
The rheological behaviour of a gluten plasticized with glycerol has been studied in oscillatory shear. The mixing operation
in a Haake batch mixer leads to a maximum torque for a level of specific energy (500–600 kJ/kg) and temperature (50–60 °C)
quite independent of mixing conditions (rotor speed, mixing time, filling ratio). The gluten/glycerol dough behaves as a classical
gluten/water dough, with a storage modulus higher than the loss modulus over the frequency range under study. A temperature
increase induces a decrease of moduli, but the material is not thermorheologically simple. Glycerol has a plasticizing effect,
which can be classically described by an exponential dependence. Mixing conditions influence the viscoelastic properties of
the material, mainly through the specific mechanical energy input (to 2000 kJ/kg) and temperature increase (to 80 °C). Above
50 °C, specific mechanical energy highly increases the complex modulus. The aggregation of proteins, as evidenced by size-exclusion
chromatography measurements, occurs later as the dough temperature reaches 70 °C. The nature of network interactions and the
respective influence of hydrophobic and disulphide contribution is discussed. A general expression is proposed for describing
the viscous behaviour of a gluten/glycerol mix, which could seem simplistic for such a complex rheological behaviour, but
would remain sufficient for modelling the flow behaviour in a twin screw extruder.
Received: 24 November 1997 Accepted: 28 April 1999 相似文献
20.
This paper describes the changes in the orientation of multiwall carbon nanotubes (MWCNT) in polycarbonate as determined by
transient and oscillatory shear rheology. It is well known from rheological studies on composites with macroscopic fibers
that the overshoot in transient shear viscosity is caused by the change in orientation distribution of these fibers. This
study shows that although an overshoot in transient shear viscosity of MWCNT/polycarbonate is measured at shear rates as low
as 0.1 s − 1, the MWCNT network is disturbed only at considerably higher shear rates. Scanning electron microscopy micrographs and oscillatory
shear show that MWCNT in thermoplastic composites will only be oriented at high shear rates. Simultaneous measurements of
the electrical conductivity during rheological start-up shear and oscillatory measurements show large differences between
electrical and mechanical relaxation behaviors. The viscosity of the composite seems to depend strongly on the MWCNT network
density, whereas the proximity of the tubes at the network points seems to determine the electrical properties of the MWCNT
composite. 相似文献