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1.
The shear orientation of hexagonal and lamellar liquid crystalline phases of polymeric surfactants was investigated by rheo-optical
techniques (flow birefringence (Δn), small-angle light scattering) as well as by nuclear magnetic resonance and optical microscopy. The evolution of birefringence
in the hexagonal phase is discussed for simple and oscillatory shear, and an alignment of rodlike micelles along the flow
direction was found. A shear induced formation of vesicles (“onions”) is observed with the lamellar phase. They displayed
a characteristic four-lobe pattern in depolarized light scattering. Above a critical shear stress vesicles were degraded and
perpendicularly aligned lamellae (i.e. with their normal along the vorticity direction) were obtained. A comparison of experiments
performed at constant stress and constant rate revealed that the vesicle to planar lamellae transition occurred above a critical
shear stress. The behavior of the polysoap lyotropic mesophases under shear, i.e. the strain dependent alignment in the hexagonal
phase, the shear induced formation of vesicles, and a transition to planar lamellae in the lamellar phase, is very similar
to the behavior of lyotropic mesophases formed by low molar mass surfactants or amphiphilic block copolymers. The geometrical
constraints that are introduced when amphiphilic side groups are fixed to a polymer backbone do not significantly alter the
response of the mesophase to a shear deformation.
Received: 4 May 1999 /Accepted: 19 July 1999 相似文献
2.
The Larson-Doi (LD) polydomain model is used to simulate orientation development along the centerline of slit-expansion and
slit-contraction flows of liquid crystalline polymers (LCPs). Orientation is computed using the LD structural evolution equations,
subject to an imposed velocity field that accounts for the spatial variation of both shear and extension rates characteristic
of this class of flows. Computed axial distributions of orientation averaged through the sample thickness are qualitatively
similar to birefringence and X-ray scattering measurements of molecular orientation in similar flows of lyotropic and thermotropic
LCPs. In slit-expansion flows, the simulations predict a 90∘ flip in orientation direction near the midplane due to transverse stretching in the expansion region. Far away from the midplane
where shear gradients dominate, orientation remains primarily along the flow direction. Within the LD model, tumbling and
flow aligning materials respond in a qualitatively similar manner to mixed shear and extension, although tumbling materials
are systematically more susceptible to the effects of extension.
Received: 22 October 1999/Accepted: 13 January 2000 相似文献
3.
The flow of a `model' lyotropic liquid crystal polymer, (hydroxypropyl)cellulose in water, through a rectangular channel
with a divergence in the channel width, is studied by in situ light microscopy. Microscopic texture observations are related
to measurements of the flow velocity field, in order to characterize the shear and elongational aspects of the flow and to
examine the effects of the divergence from a narrow channel to a wide channel. A strong dependence of flow-induced texture
on position in the channel is observed and is related to the interplay of shear and elongational strain. The divergence generates
both a perpendicular elongational strain due to the widening of the channel, and subsequently an elongational strain along
the flow direction due to the change in flow pattern from quasi-radial to unidirectional down the wide channel. Additionally
side wall structure is observed to be more complex than a simple strong alignment, displaying a fine birefringent texture.
Finally there is a marked dependence of the macroscopic structure on the strain history of the fluid prior to entry into the
channel, indicating that very different structures of, for instance, moulded parts, can result from differences in geometry
and fluid treatment prior to entry into the mould itself.
Received: 12 October 1999/Accepted: 29 October 1999 相似文献
4.
Shifang Han 《Acta Mechanica Sinica》2007,23(2):149-158
A continuum constitutive theory of corotational derivative type is developed for the anisotropic viscoelastic fluid–liquid
crystalline (LC) polymers. A concept of anisotropic viscoelastic simple fluid is introduced. The stress tensor instead of
the velocity gradient tensor D in the classic Leslie–Ericksen theory is described by the first Rivlin–Ericksen tensor A and a spin tensor W measured with respect to a co-rotational coordinate system. A model LCP-H on this theory is proposed and the characteristic
unsymmetric behaviour of the shear stress is predicted for LC polymer liquids. Two shear stresses thereby in shear flow of
LC polymer liquids lead to internal vortex flow and rotational flow. The conclusion could be of theoretical meaning for the
modern liquid crystalline display technology. By using the equation, extrusion–extensional flows of the fluid are studied
for fiber spinning of LC polymer melts, the elongational viscosity vs. extension rate with variation of shear rate is given
in figures. A considerable increase of elongational viscosity and bifurcation behaviour are observed when the orientational
motion of the director vector is considered. The contraction of extrudate of LC polymer melts is caused by the high elongational
viscosity. For anisotropic viscoelastic fluids, an important advance has been made in the investigation on the constitutive
equation on the basis of which a series of new anisotropic non-Newtonian fluid problems can be addressed.
The project supported by the National Natural Science Foundation of China (10372100, 19832050) (Key project). The English
text was polished by Yunming Chen. 相似文献
5.
6.
The flow of isotropic and liquid crystalline (LC) hydroxypropylcellulose (HPC) aqueous solutions into an abrupt axisymmetric
contraction has been quantitatively measured by pulsed-field-gradient NMR techniques. Steady-state axial velocity profiles,
acquired upstream of the contraction, reveal a large contraction entry length for the LC solution. This entry flow field exists
over an order of magnitude change in flow rate and is attributed to elasticity that is associated with polydomain liquid crystallinity.
Pronounced, off-centerline velocity maxima (in an axisymmetric flow field) were present upstream of the contraction, in the
entry flow region. Apparently, a more viscous and elastic core of fluid was present along the centerline; this fluid resisted
elongational strain more than the fluid closer to the walls. Quantitative velocity profiles were extracted from displacement
distributions and corrected for elongational dispersion. The isotropic solution velocity profiles matched those obtained from
viscoelastic simulations using an approximate Doi-Edwards model, parameterized with independent rheological data.
Received: 29 April 1999/Accepted: 30 August 1999 相似文献
7.
We have studied by in situ microscopy the flow of a lyotropic liquid crystal polymer, hydroxypropylcellulose (HPC) in water,
around an obstacle placed in a rectangular flow channel. The obstacle separates the flow into two parts which rejoin downstream
of the obstacle, resulting in the formation of a `weld-line'. Measuring the velocity field in the vicinity of the weld-line
beyond the obstacle, we find as expected a positive elongational strain (acceleration) along the weld (parallel to the flow
direction). For an anisotropic (concentrated) HPC solution we observe in addition a significant shear strain in the weld-line
region, there being an important velocity gradient perpendicular to the plane of the weld line. Isotropic (lower concentration)
solutions of the same polymer demonstrate no visible weld line, a larger elongational strain rate near the obstacle, and no
shear component of strain downstream of the obstacle. These results are similar to observations reported for fluids reinforced
by macroscopic fibres. Polarised light observations of the anisotropic solution show that the strain field generates a generally
increased degree of orientation of the liquid crytalline polymer near the weld (generally reduced crossed-polariser transmitted
intensity when the polariser is parallel to the flow direction), however there is also a striking fine birefringent colour
variation in the weld-line region, reminiscent of the structure observed at the channel side walls in rectangular channel
flow (Haw and Navard 2000). The results show that the simple concept of weld-line structure as confined to an enhanced alignment
along the weld due to elongational strain is incomplete; the two-dimensional shear strain field must also be taken into account
for the anisotropic fluid.
Received: 22 December 1999/Accepted: 4 January 2000 相似文献
8.
The accurate calculation of the viscosity η as function of the shear rate &γdot; from capillary viscometry is still a matter of debate in the literature. In fact, this problem involves the inversion of
an integral equation, which leads to multiple solutions due to the unavoidable noise present in the experimental data. The
purpose of this work is to develop an efficient procedure to determine the viscosity function from experimental data of capillary
flow without presenting the difficulties inherent in other methods discussed previously in the literature. The system identification
procedure is used here to estimate the parameters of a viscosity model, which is appropriately selected for the fluid under
study through preliminary calculations involving the apparent shear rate – shear stress data. Once the model is chosen by
satisfying criteria for the fit goodness and its parameters are evaluated, a smooth and continuous function η(γdot;) is obtained in the range of experimental shear rates. The procedure proposed is also applicable to fluids in shear flow
that present two Newtonian plateaus, as it is typically found in macromolecular dilute solutions. The mean value theorem of
continuous functions is used to reduce significantly the computational time.
Received: 15 November 1999 Accepted: 7 November 2000 相似文献
9.
Planar contraction flows of non-Newtonian fluids with integral constitutive models are studied to investigate the problem
of numerical breakdown at high Weissenberg or Debrorah numbers. Spurious shear stress extrema are found on the wall downstream
of the re-entrant corner for both sharp and rounded corners. Moreover, a non-monotonic relation between shear stress and strain
rate is found when the Deborah number limit is approached, which correlates with these shear extrema. This strongly suggests
that non-monotonicity between shear stress and strain rate may be responsible for the Deborah number limit problem in contraction
flow simulations. This non-monotonicity is caused by the inaccuracy of the quadrature, using constitutive equations that do
not have shear stress maxima when exactly evaluated. This conclusion agrees with recent analytical findings by others that
inaccuracy of the integration along the streamlines – either by numerical integration or asymptotic approximation – makes
the problem ill-conditioned, with spurious growth occurring on the wall downstream of the re-entrant corner.
Received: 5 March 1999/Accepted: 1 September 1999 相似文献
10.
David C. Venerus 《Rheologica Acta》2000,39(1):71-79
The behavior of a low-density polyethylene melt in exponential shear strain histories is examined and compared to its behavior
in constant rate planar elongation. A new set of shear stress and first normal stress difference data in exponential shear
are presented and used in several different material functions that have been previously proposed. Viscosities composed of
principal stress differences for the two flows showed no correspondence suggesting that, contrary to previous assertions,
exponential shear and constant rate planar elongation flows are fundamentally different. It is further suggested that the
presence of vorticity makes exponential shear a weak, rather than strong, flow.
Received: 5 March 1999/Accepted: 1 September 1999 相似文献
11.
The equations of linear and angular momentum for nematic liquid crystals have been described with Ericksen's transversely
isotropic fluid [TIF] model and solved for start-up of shear flow at constant rate and varying initial alignment conditions.
An analytical solution for the rotation provides predictions of the nematic director which closely agree with experimental
results of Boudreau et al. (1999), supporting the validity of Ericksen's TIF model. The solution is limited to flows where
the effects of director gradients are negligible.
Received: 13 September 1999/Accepted: 24 January 2000 相似文献
12.
Edge fracture is an instability of cone-plate and parallel plate flows of viscoelastic liquids and suspensions, characterised
by the formation of a `crack' or indentation at a critical shear rate on the free surface of the liquid. A study is undertaken
of the theoretical, experimental and computational aspects of edge fracture. The Tanner-Keentok theory of edge fracture in
second-order liquids is re-examined and is approximately extended to cover the Criminale-Ericksen-Filbey (CEF) model. The
second-order theory shows that the stress distribution on the semi-circular crack is not constant, requiring an average to
be taken of the stress; this affects the proportionality constant, K in the edge fracture equation −N
2c = KΓ/a, where N
2c is the critical second normal stress difference, Γ is the surface tension coefficient and a is the fracture diameter. When the minimum stress is used, K = 2/3 as found by Tanner and Keentok (1983). Consideration is given to the sources of experimental error, including secondary
flow and slip (wall effect). The effect of inertia on edge fracture is derived. A video camera was used to record the inception
and development of edge fracture in four viscoelastic liquids and two suspensions. The recorded image was then measured to
obtain the fracture diameter. The edge fracture phenomenon was examined to find its dependence on the physical dimensions
of the flow (i.e. parallel plate gap or cone angle), on the surface tension coefficient, on the critical shear rate and on
the critical second normal stress difference. The critical second normal stress difference was found to depend on the surface
tension coefficient and the fracture diameter, as shown by the theory of Tanner and Keentok (1983); however, the experimental
data were best fitted by the equation −N
2c = 1.095Γ/a. It was found that edge fracture in viscoelastic liquids depends on the Reynolds number, which is in good agreement with
the inertial theory of edge fracture. Edge fracture in lubricating grease and toothpaste is broadly consistent with the CEF
model of edge fracture. A finite volume method program was used to simulate the flow of a viscoelastic liquid, obeying the
modified Phan-Thien-Tanner model, to obtain the velocity and stress distribution in parallel plate flow in three dimensions.
Stress concentrations of the second normal stress difference (N
2) were found in the plane of the crack; the velocity distribution shows a secondary flow tending to aid crack formation if
N
2 is negative, and a secondary flow tending to suppress crack formation if N
2 is positive.
Received: 4 January 1999 Accepted: 19 May 1999 相似文献
13.
David M. Boudreau H. Henning Winter C. Peter Lillya Richard S. Stein 《Rheologica Acta》1999,38(6):503-513
Orientational changes in monodomains of flow-aligning liquid crystals, 4-n-pentyl-4′-cyanobiphenyl and N-(4-methoxybenzylidene)-4-butylaniline, were studied during shear and recovery in a linear shearing device fitted to an optical
microscope. Planar alignment (director in the shear plane) allows the study of twist effects and was generated by strong planar
anchoring at the walls with orientations in a range of 0–90° with the shear direction. While being held back by the anchoring
walls, shear caused the bulk director to rotate towards a steady-state alignment angle in the shear direction (Leslie angle
θL). The transient director rotation was observed with conoscopy. It was found that increasing the initial alignment towards
the vorticity direction increased the measured θL. Upon stopping the flow, the bulk director returned to its initial state. With initial alignment orientation changing from
parallel to perpendicular to the flow direction, the rate of the twist-driven recovery process increases. This rate increase
is not seen in the splay-driven recovery which is constant and consistently faster than twist-driven recovery at all orientations
studied.
Received: 10 December 1998/Accepted: 7 June 1999 相似文献
14.
We investigate the steady-state rheological behaviour of the lamellar phase of a lyotropic system (CpCl, hexanol, brine)
and of a thermotropic system (8CB). Power laws characterize the behaviour of the imposed stress as a function of the measured
shear rate and similarities are observed for both systems; the same regime γ˙∼σ
m
with m≈1.7 is obtained at low shear stresses corresponding to a texture of oily streaks oriented in the direction of the flow, as
shown by microscopic observations. The “onion state” only exists in the case of dilute samples of the lyotropic lamellar phase;
the stress then varies as γ˙∼σ
m
with m≈4.8, as already observed by other groups with different systems. Rheological measurements at different temperatures allow
determination of different activation energies relating to the still badly understood processes involved in the different
rheological regimes. We propose a model which reproduces the experimental power laws and which is based on an analogy with
the theory of high-temperature creep in metals and alloys.
Received: 19 October 1999/Accepted: 1 November 1999 相似文献
15.
Hiroshi Watanabe Tomohiro Sato Motoyuki Hirose Kunihiro Osaki Ming-Long Yao 《Rheologica Acta》1999,38(2):100-107
Rheo-dielectric behavior was examined for 4−4′−n-octyl-cyanobiphenyl (8CB) having large dipoles parallel to its principal axis (in the direction of the C≡N bond). In the
quiescent state at all temperatures (T) examined, orientational fluctuation of the 8CB molecules was observed as dielectric dispersions at characteristic frequencies
ωc>106 s−1. In the isotropic state at high T, no detectable changes of the complex dielectric constant ɛ*(ω) were found under slow flow at shear rates ˙γ≫ωc. In the nematic state at intermediate T, the terminal relaxation intensity of ɛ*(ω) was decreased under such slow flow. In the smectic state at lower T, the flow effect became much less significant. These results were related to the flow-induced changes of the liquid crystalline
textures in the nematic and smectic states, and the differences of the rheo-dielectric behavior in these states are discussed
in relation to a difference of the symmetry of molecular arrangements in the nematic and smectic textures.
Received: 1 October 1998 Accepted: 13 January 1999 相似文献
16.
A wavy texture occurs in the flows of liquid crystalline polymers through a slit cell. In the present paper the development of the wavy texture is examined in pressure-driven startup flows for four types of slit cells, using a liquid crystalline solution of 50 wt% hydroxypropylcellulose (HPC). There exists a comparatively long induction period until the wavy texture appears after the startup of the flow, and the induction time decreases with increasing apparent shear rate. However, it is found that the apparent shear strain at which the wavy texture emerges is independent of the apparent shear rate though the value of the apparent shear strain slightly varies with the type of flow cell. Furthermore, the light scattering experiments are carried out to examine the structure of wavy texture. After the startup of the flow, a homogeneous pattern of the light scattering quickly shrink in size and a spike pattern perpendicular to the flow direction is emphasized. While the wavy texture is seen, the ellipsoidal pattern of light scattering oscillates with the same frequency as the passage of the wavy texture. A structure of scattering objects in the wavy texture is proposed, based on the observation of change in the light scattering pattern with time. 相似文献
17.
The effects of extensional flow orientation on the rheological properties of two poly(styrene)-poly(ethylene-co-butylene)-poly (styrene) (PS-PEB-PS) triblock copolymers containing either spherical or cylindrical PS microdomains were
studied by oscillatory shear and oscillatory extensional experiments. Extensional measurements revealed that below the PS
block glass transition temperature pre-oriented triblocks display highly anisotropic mechanical properties. For both polymers,
the storage modulus E ′ is higher along the flow direction. Above the PS glass transition temperature the materials are no longer anisotropic and
the same storage moduli are obtained along the flow direction and perpendicular to it. Above the PS glass transition temperature
the rheological behaviour parallel and perpendicular to the flow direction was also probed in pre-oriented and non-oriented
samples by oscillatory shear rheometry. At high frequencies, the mechanical response of the triblocks was found to be independent
of the orientation for both copolymers while at low frequencies a strong effect of the flow orientation could be observed.
For both polymers the value of the storage modulus was found to be lower along the flow direction that perpendicular to it.
This was explained by the ability of PS blocks to relax more easily along the flow direction.
Received: 10 September 1999/Accepted: 1 October 1999 相似文献
18.
Mechanical spectroscopy is used to probe the structure of lyotropic liquid crystalline polymers during flow and after the
cessation of flow. The oscillatory flow is either parallel or perpendicular to the steady-state flow. The resulting moduli
provide information about the time- and shear-dependent microstructure, including anisotropy. Two different concentrations
of poly(benzylglutamate) (PBG) in m-cresol and a concentrated hydroxypropylcellulose (HPC) solution, also in m-cresol, are investigated. In all cases, the orthogonal superposition moduli evolve differently from the parallel ones. The
former are less sensitive to the flow-induced changes in structure than the latter ones. Together with the lack of sensitivity
of the superposition moduli to texture refinement during flow, this suggests a strong relation between director orientation
and superposition moduli. After the cessation of flow the parallel moduli decrease for the PBG solutions, whereas the opposite
is observed in the HPC solutions. A comparison with the orthogonal moduli provides a direct measure of anisotropy. At rest,
the PBG solutions tend toward a higher degree of anisotropy while the HPC solutions become more isotropic. In the latter systems,
all moduli are much larger, reflecting a larger contribution from the texture.
Received: 8 July 1999/Accepted: 1 October 1999 相似文献
19.
Christopher J. Dimitriou Laura Casanellas Thomas J. Ober Gareth H. McKinley 《Rheologica Acta》2012,51(5):395-411
We explore the behavior of a wormlike micellar solution under both steady and large amplitude oscillatory shear (LAOS) in
a cone–plate geometry through simultaneous bulk rheometry and localized velocimetric measurements. First, particle image velocimetry
is used to show that the shear-banded profiles observed in steady shear are in qualitative agreement with previous results
for flow in the cone–plate geometry. Then under LAOS, we observe the onset of shear-banded flow in the fluid as it is progressively
deformed into the non-linear regime—this onset closely coincides with the appearance of higher harmonics in the periodic stress
signal measured by the rheometer. These harmonics are quantified using the higher-order elastic and viscous Chebyshev coefficients
e
n
and v
n
, which are shown to grow as the banding behavior becomes more pronounced. The high resolution of the velocimetric imaging
system enables spatiotemporal variations in the structure of the banded flow to be observed in great detail. Specifically,
we observe that at large strain amplitudes (γ
0 ≥ 1), the fluid exhibits a three-banded velocity profile with a high shear rate band located in-between two lower shear rate
bands adjacent to each wall. This band persists over the full cycle of the oscillation, resulting in no phase lag being observed
between the appearance of the band and the driving strain amplitude. In addition to the kinematic measurements of shear banding,
the methods used to prevent wall slip and edge irregularities are discussed in detail, and these methods are shown to have
a measurable effect on the stability boundaries of the shear-banded flow. 相似文献
20.
W. D. Thacker S. Sarkar T. B. Gatski 《Theoretical and Computational Fluid Dynamics》2007,21(3):171-199
The influence of compressibility on the rapid pressure–strain rate tensor is investigated using the Green’s function for the
wave equation governing pressure fluctuations in compressible homogeneous shear flow. The solution for the Green’s function
is obtained as a combination of parabolic cylinder functions; it is oscillatory with monotonically increasing frequency and
decreasing amplitude at large times, and anisotropic in wave-vector space. The Green’s function depends explicitly on the
turbulent Mach number M
t
, given by the root mean square turbulent velocity fluctuations divided by the speed of sound, and the gradient Mach number
M
g
, which is the mean shear rate times the transverse integral scale of the turbulence divided by the speed of sound. Assuming
a form for the temporal decorrelation of velocity fluctuations brought about by the turbulence, the rapid pressure–strain
rate tensor is expressed exactly in terms of the energy (or Reynolds stress) spectrum tensor and the time integral of the
Green’s function times a decaying exponential. A model for the energy spectrum tensor linear in Reynolds stress anisotropies
and in mean shear is assumed for closure. The expression for the rapid pressure–strain correlation is evaluated using parameters
applicable to a mixing layer and a boundary layer. It is found that for the same range of M
t
there is a large reduction of the pressure–strain correlation in the mixing layer but not in the boundary layer. Implications
for compressible turbulence modeling are also explored.
相似文献