共查询到20条相似文献,搜索用时 31 毫秒
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.
Mahmoud Rajabian Ghassem Naderi Charles Dubois Pierre G. Lafleur 《Rheologica Acta》2010,49(1):105-118
Transient elongational rheology of two commercial-grade polypropylene (PP) and the organoclay thermoplastic nanocomposites
is investigated. A specifically designed fixture consisting of two drums (SER Universal Testing Platform) mounted on a TA
Instruments ARES rotational rheometer was used to measure the transient uniaxial extensional viscosity of both polypropylene
and nanoclay/PP melts. The Hencky strain rate was varied from 0.001 to 2 s − 1, and the temperature was fixed at 180°C. The measurements show that the steady-state elongational viscosity was reached at
the measured Hencky strains for the polymer and for the nanocomposites. The addition of nanoclay particles to the polymer
melt was found to increase the elongation viscosity principally at low strain rates. For example, at a deformation rate of
0.3 s − 1, the steady-state elongation viscosity for polypropylene was 1.4 × 104 Pa s which was raised to 2.8 × 104 and 4.5 × 104 Pa s after addition of 0.5 and 1.5 vol.% nanoclay, respectively. A mesoscopic rheological model originally developed to predict
the motion of ellipsoid particles in viscoelastic media was modified based on the recent developments by Eslami and Grmela
(Rheol Acta 47:399–415, 2008) to take into account the polymer chain reptation. We show that the orientation states of the particles and the rheological
behavior of the layered particles/thermoplastic hybrids can be quantitatively explained by the proposed model. 相似文献
3.
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
4.
The effects of shear, uniaxial extension and temperature on the flow-induced crystallization of two different types of high-density
polyethylene (a metallocene and a ZN-HDPE) are examined using rheometry. Shear and uniaxial extension experiments were performed
at temperatures below and well above the peak melting point of the polyethylenes in order to characterize their flow-induced
crystallization behavior at rates relevant to processing (elongational rates up to 30 s − 1 and shear rates 1 to 1,000 s − 1 depending on the application). Generally, strain and strain rate found to enhance crystallization in both shear and elongation.
In particular, extensional flow was found to be a much stronger stimulus for polymer crystallization compared to shear. At
temperatures well above the melting peak point (up to 25°C), polymer crystallized under elongational flow, while there was
no sign of crystallization under simple shear. A modified Kolmogorov crystallization model (Kolmogorov, Bull Akad Sci USSR,
Class Sci, Math Nat 1:355–359, 1937) proposed by Tanner and Qi (Chem Eng Sci 64:4576–4579, 2009) was used to describe the crystallization kinetics under both shear and elongational flow at different temperatures. 相似文献
5.
Souzanna Sofou Edward B. Muliawan Savvas G. Hatzikiriakos Evan Mitsoulis 《Rheologica Acta》2008,47(4):369-381
Bread dough (a flour–water system) has been rheologically characterized using a parallel-plate, an extensional, and a capillary
rheometer at room temperature. Based on the linear and nonlinear viscoelastic and viscoplastic data, two constitutive equations
have been applied, namely a viscoplastic Herschel–Bulkley model and a viscoelastoplastic K–BKZ model with a yield stress.
For cases where time effects are unimportant, the viscoplastic Herschel–Bulkley model can be used. For cases where transient
effects are important, it is more appropriate to use the K-BKZ model with the addition of a yield stress. Finally, the wall
slip behavior of dough was studied in capillary flow, and an appropriate slip law was formulated. These models characterize
the rheological behavior of bread dough and constitute the basic ingredients for flow simulation of dough processing, such
as extrusion, calendering, and rolling. 相似文献
6.
7.
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
8.
The shear flow of mayonnaise is generally characterized by an apparent yield stress, shear thinning in steady flow, stress
overshoots upon inception of flow and other time-dependent effects. These observations are usually understood to be the result
of structural rearrangement within the material. Additionally and separately, the possibility that emulsions may exhibit apparent
wall slip on a microscopic scale at a solid-liquid boundary has been reported by some researchers. Thus, observed rheological
behavior is likely to be the result of the interplay between these two phenomena.
In the present work, it is demonstrated that when measurements are sought to be made on mayonnaise using rotational viscometers
visible wall slip occurs, rendering such instruments ineffective for the purpose of making viscosity measurements even at
shear rates as low as 10–3s–1. The factors that influence the onset and extent of slip are investigated with the help of parallel plate viscometers, and
it is concluded that the observed “yielding” of mayonnaise is actually an artifact of the onset of macroscopic slip. Slip
effects are also found in capillary flow but are ameliorated with increasing shear rate. To circumvent these problems, it
is proposed that extensional viscometry be employed for determining the flow behavior of mayonnaises.
Received: 18 August 1997 Accepted: 1 April 1998 相似文献
9.
A filament-stretching rheometer is used to measure the extensional viscosity of a shear-thickening suspension of cornstarch
in water. The experiments are performed at a concentration of 55 wt.%. The shear rheology of these suspensions demonstrates
a strong shear-thickening behavior. The extensional rheology of the suspensions demonstrates a Newtonian response at low extension
rates. At moderate strain rates, the fluid strain hardens. The speed of the strain hardening and the extensional viscosity
achieved increase quickly with increasing extension rate. Above a critical extension rate, the extensional viscosity goes
through a maximum and the fluid filaments fail through a brittle fracture at a constant tensile stress. The glassy response
of the suspension is likely the result of jamming of particles or clusters of particles at these high extension rates. This
same mechanism is responsible for the shear thickening of these suspensions. In capillary breakup extensional rheometry, measurement
of these suspensions demonstrates a divergence in the extensional viscosity as the fluid stops draining after a modest strain
is accumulated. 相似文献
10.
Saeid Kheirandish Ilshat Guybaidullin Wendel Wohlleben Norbert Willenbacher 《Rheologica Acta》2008,47(9):999-1013
We investigate the effect of hydrophobic aggregation in alkali-swellable acrylic thickener solutions on shear and extensional
flow properties at technically relevant polymer concentrations using the commercial thickener Sterocoll FD as model system.
Apparent molecular weight of aggregates in water is M
w
≈ 108 g/mol and decreases by more than an order of magnitude in ethanol. Zero shear viscosity η
0 is low and shear thinning is weak compared to the high molecular weight of the aggregates. Linear viscoelastic relaxation
is described by the Zimm theory up to frequencies of 104 rad/s, demonstrating that no entanglements are present in these solutions. This is further supported by the concentration
dependence of η
0 and is attributed to strong association within the aggregates. Extensional flow behavior is characterized using the capillary
break-up extensional rheometry technique including high-speed imaging. Solutions with ϕ ≥ 1% undergo uniform deformation and show pronounced strain hardening up to large Hencky strains. Elongational relaxation
times are more than one order of magnitude lower than the longest shear relaxation times, suggesting that aggregates cannot
withstand strong flows and do not contribute to the elongational viscosity.
相似文献
Norbert WillenbacherEmail: |
11.
Norhayani Othman Bashar Jazrawi Parisa Mehrkhodavandi Savvas G. Hatzikiriakos 《Rheologica Acta》2012,51(4):357-369
The wall slip and melt fracture behaviour of several commercial polylactides (PLAs) as well as their rheological properties
under shear and extensional have been investigated. The PLAs have had weight-average molecular weights in the range of 104–105 g/mol and studied in the temperature range of 160–200°C. The solution properties and linear viscoelastic behaviour of melts
indicate linear microstructure behaviour. PLAs with molecular weights greater than a certain value were found to slip, with
the slip velocity to increase with decrease of molecular weight. The capillary data were found to agree well with linear viscoelastic
envelope once correction for slip effects was applied. The onset of melt fracture for the high molecular weight PLAs was found
to occur at about 0.2 to 0.3 MPa, depending on the geometrical characteristics of the dies and independent of temperature.
Addition of 0.5 wt.% of a polycaprolactone (PCL) into the PLA that exhibits melt fracture was found to be effective in eliminating
and delaying the onset of melt fracture to higher shear rates. This is due to significant interfacial slip that occurs in
the presence of PCL. 相似文献
12.
Jonathan S. Schulze Timothy P. Lodge Christopher W. Macosko Jens Hepperle Helmut Münstedt Heike Bastian Dino Ferri David J. Groves Yong Hoon Kim Mike Lyon Thomas Schweizer Terry Virkler Erik Wassner Wim Zoetelief 《Rheologica Acta》2001,40(5):457-466
The transient uniaxial extensional viscosity η
e
of linear low density polyethylene (LLDPE) has been measured using the commercial Rheometric Scientific RME and the Münstedt
Tensile Rheometer in an effort to compare the performance of available extensional rheometers. The RME indicated a significant
strain hardening of the LLDPE, especially at a strain rate of 1 s−1. In contrast, the Münstedt rheometer showed the LLDPE to be only slightly strain hardening. This artificial strain hardening
effect in the RME resulted from the strain rate applied to the sample, determined from the sample deformation, being up to
20% less than the set strain rate. These results initiated a round-robin experiment in which the same LLDPE was tested on
several RMEs in various locations around the world. All but one of the RMEs indicated a deviation between set and applied
strain rates of at least 10%, especially at strain rates above 0.1 s−1. The strain rate deviation was found to depend strongly on the value of the basis length L
0
, and may result from the upper pair of belts not properly gripping the sample during extension. Thus visual inspection of
the sample deformation is necessary to determine the applied strain rate. The most accurate measurements of η
e
with respect to the strain rate deviation were obtained when the correct L
0
value and belt arrangement were used. A list of recommendations for running an RME test is provided. Future work focusing
on the fluid mechanics during the test may identify fully the cause of the strain rate deviation, but from a practical point
of view the problem can be corrected for in the determination of η
e
.
Received: 27 September 2000/Accepted: 5 February 2001 相似文献
13.
A formulation describing the rheology of crystallising polymers is discussed. For some semi-crystalline polymers where spherulites
form as part of the crystallisation process, the use of a suspension-type model is appropriate. Whilst it is possible to so
describe simple shearing and elongational rheology during on-going crystallisation with such models, the flow through a capillary
tube is much more complex and numerical solution is usually necessary. To give some insight into this complex flow, a ‘step
function’ or ‘amorphous-frozen’ model of the viscosity changes due to crystallisation has been devised so that a semi-analytical
approach is feasible. We use this simple model and compare the results with recently published experiments in tubes and channels
at high (O(103 s − 1)) shear rates using poly(butene-1). A direct correlation between simple shear and tube flow crystallisation onset times is
found. 相似文献
14.
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
15.
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
16.
Transient elongational viscosities of aqueous polyacrylamide solutions measured with an optical rheometer 总被引:1,自引:1,他引:0
An anionic polyacrylamide solution was characterized in elongational flow by combining laser-Doppler velocimetry to determine
the strain rate in the flow direction and the two-color flow-induced birefringence method to measure the first normal stress
difference along the axial centerline of a hyperbolic die. The elongational rate was constant along the axial centerline of
the planar hyperbolic die as long as vortices at the die entrance did not occur. The transient elongational viscosity μ
+ was determined as a function of the elongational rate. The parameters varied are the Hencky strain rate and the polymer concentration.
μ
+ showed a pronounced increase over the linear viscoelastic behavior above critical Hencky strains of 1.2 to 1.5; that is, a significant strain hardening could be observed for polyacrylamide
solutions. This strain hardening is stronger the higher the elongational rate. A slight enhancement of strain hardening was
found by increasing the concentration from 0.5 to 1 g/l. The stress optical coefficient was determined as 1.8 × 10−7 Pa−1 (0.5 g/l) and 1.2 × 10−7 Pa−1 (1 g/l).
相似文献
Helmut MünstedtEmail: |
17.
T. S. Ng 《Rheologica Acta》1982,21(4-5):456-459
It is shown that the extensional viscosity measured by uniaxial stretch may give useful information as to the processability of rubber compounds. A simple apparatus was constructed to measure this rheological property at constant rate of strain. The influence of rubber type, filler, aging, mastication and degree of crosslinking on the reduced stress is represented by diagrams. No steady-state extensional viscosity was obtained for highly filled rubber compounds. It was found that the flow behaviour of rubber compounds can be better characterized with an extensional rheometer than with a high-pressure capillary rheometer. 相似文献
18.
19.
A filament stretching extensional rheometer with a custom-built oven was used to investigate the effect of uniaxial flow on
the crystallization of polypropylene. Prior to stretching, samples were heated to a temperature well above the melt temperature
to erase their thermal and mechanical histories and the Janeschitz-Kriegl protocol was applied. The samples were stretched
at extension rates in the range of 0.01 s-1 £ [(e)\dot] £ 0.75 s-10.01\,\mbox{s}^{-1}\le \dot{{\varepsilon }}\le 0.75\,{\rm s}^{-1} to a final strain of ε = 3.0. After stretching, the samples were allowed to crystallize isothermally. Differential scanning calorimetry was applied
to the crystallized samples to measure the degree of crystallinity. The results showed that a minimum extension rate is required
for an increase in percent crystallization to occur and that there is an extension rate for which percent crystallization
is maximized. No increase in crystallization was observed for extension rates below a critical extension rate corresponding
to a Weissenberg number of approximately Wi = 1. Below this Weissenberg number, the flow is not strong enough to align the contour path of the polymer chains within
the melt and as a result there is no change in the final percent crystallization from the quiescent state. Beyond this critical
extension rate, the percent crystallization was observed to increase to a maximum, which was 18% greater than the quiescent
case, before decaying again at higher extension rates. The increase in crystallinity is likely due to flow-induced orientation
and alignment of contour path of the polymer chains in the flow direction. Polarized light microscopy verified an increase
in number of spherulites and a decrease in spherulite size with increasing extension rate. In addition, small angle X-ray
scattering showed a 7% decrease in inter-lamellar spacing at the transition to flow-induced crystallization. Although an increase
in strain resulted in a slight increase in percent crystallization, no significant trends were observed. Crystallization kinetics
were examined as a function of extension rate by observing the time required for molten samples to crystallize under uniaxial
flow. The crystallization time was defined as the time at which a sudden increase in the transient force measurement was observed.
The crystallization time was found to decrease as one over the extension rate, even for extension rates where no increase
in percent crystallization was observed. As a result, the onset of extensional-flow-induced crystallization was found to occur
at a constant value of strain equal to ε
c
= 5.8. 相似文献
20.
Gerald Henry Meeten 《Rheologica Acta》2007,46(6):803-813
Various soft solid suspensions were squeezed at constant force between polished and roughened circular glass plates and the
time-dependence of the interplate separation was measured. The filterability of suspensions was quantified by their desorptivity
S obtained from measurements of capillary suction time. The squeeze flow (SF) of suspensions for which S < 2 μm s−1/2 was largely consistent with rheological theory, which neglected radial filtration: the relative motion between the liquid
and solid phases of the suspension in the radial pressure gradient. Suspensions having S > 2 μm s−1/2 showed SF behavior that was consistent with the presence of radial filtration. 相似文献