Orientation In Shear Flow Research Articles

Application of constitutive equation considering fiber orientation in shear flow of fiber suspension

Application of constitutive equation considering fiber orientation in shear flow of fiber suspension

Slip behavior of liquid flow in rough nanochannels

The molecular dynamics simulation is applied to investigate the liquid flow in rough nanochannels with a focus on interfacial velocity slip via three-dimensional Couette flow system. The typical liquid spatial distribution, velocity profile and slip length for liquid flow in rough nanochannels are evaluated and compared with smooth nanochannel. The effects of liquid–solid interaction, surface roughness and shear flow orientation on slip behavior of liquid flow in rough nanochannels are all investigated and discussed. The results indicate that, regardless of whether the liquid flow in transverse or longitudinal flow configuration, the rough surface induces extra energy losses and contributes to the reduction of interfacial velocity in nanochannel when compared with smooth surface. A larger roughness size introduces a more irregular near-wall flow, which results in a smaller interfacial velocity slip. In addition, irrespective of surface condition, increases in liquid–solid interaction strength lead to small interfacial velocity slip and expand the extent of velocity nonlinearity in wall-neighboring region. In particular, the slip behavior of liquid flow in rough nanochannels is also influenced by the shear flow orientation. Interestingly, we find that interfacial velocity slip at the rough solid surface in transverse flow configuration is smaller than that in longitudinal flow configuration.

Novel Behavior in Shear Flow Orientation of Side-Chain Polymethacrylate Nematic Liquid Crystals

Shear flow orientation of nematic liquid crystals (LCs) has been investigated for two side-chain polymethacrylates having the same methoxyazobenzene mesogens but different spacers of either four (PM4MA) or six (PM6MA) methylene groups. These two polymer nematic LCs exhibited orientation behaviors distinct from each other. PM4MA arranged the nematic director in the velocity direction (the b-orientation), whereas PM6MA pointed the director to the neutral (the a-orientation) or velocity gradient (the c-orientation) directions depending on both shear rate and molar mass. Decreasing shear rate or increasing molar mass gave rise to an orientation transition from a to c. Such anomalous nematic orientation behavior is related to the presence of fluctuating smectic clusters, and explainable if fluctuations, which couple to shear flow to induce the a-orientation, have a lifetime which decreases with increasing backbone chain length so as to be sufficiently short to be averaged out under shear flow.

Influence of viscoelasticity on drop deformation and orientation in shear flow. Part 2: Dynamics

An experimental study of drop dynamics under shear is conducted for five fluid pairs: a reference Newtonian system, two systems with a viscoelastic drop in a Newtonian matrix, one with a Newtonian drop in a viscoelastic matrix, all at drop to matrix viscosity ratio λ = 1.5 , and a separate case at λ = 0.75 . The viscoelastic liquids are either a Boger fluid or a shear-thinning viscoelastic fluid satisfying an Ellis model. Deborah numbers in the range 1–2 and a range of capillary numbers from low to above breakup conditions are addressed. The results focus on three aspects: relaxation after cessation of shear, a new viscoelastic drop breakup scenario, and the effect of shear flow history on drop breakup. Numerical simulations with the 3D volume-of-fluid PROST method complement the experimental results.

Influence of viscoelasticity on drop deformation and orientation in shear flow: Part 1. Stationary states

The influence of matrix and droplet viscoelasticity on the steady deformation and orientation of a single droplet subjected to simple shear is investigated microscopically. Experimental data are obtained in the velocity–vorticity and velocity–velocity gradient plane. A constant viscosity Boger fluid is used, as well as a shear-thinning viscoelastic fluid. These materials are described by means of an Oldroyd-B, Giesekus, Ellis, or multi-mode Giesekus constitutive equation. The drop-to-matrix viscosity ratio is 1.5. The numerical simulations in 3D are performed with a volume-of-fluid algorithm and focus on capillary numbers 0.15 and 0.35. In the case of a viscoelastic matrix, viscoelastic stress fields, computed at varying Deborah numbers, show maxima slightly above the drop tip at the back and below the tip at the front. At both capillary numbers, the simulations with the Oldroyd-B constitutive equation predict the experimentally observed phenomena that matrix viscoelasticity significantly suppresses droplet deformation and promotes droplet orientation. These two effects saturate experimentally at high Deborah numbers. Experimentally, the high Deborah numbers are achieved by decreasing the droplet radius with other parameters unchanged. At the higher capillary and Deborah numbers, the use of the Giesekus model with a small amount of shear-thinning dampens the stationary state deformation slightly and increases the angle of orientation. Droplet viscoelasticity on the other hand hardly affects the steady droplet deformation and orientation, both experimentally and numerically, even at moderate to high capillary and Deborah numbers.

Characteristic Shear-Flow Orientation in LC Block Copolymer Resulting from Compromise between Orientations of Microcylinder and LC Mesogen

The shear flow orientation in the diblock copolymers composed of polystyrene (PS) and side-chain liquid crystal polymer (LCP) was investigated by the wide and small-angle X-ray diffraction methods. Two diblock copolymers prepared by atom transfer radical polymerization have the weight fractions of the PS segment of 0.13 and 0.21, and form a well-defined microsegregation structure with PS cylinders hexagonally packed in a matrix of LCP which forms smectic A, nematic, and isotropic phases in order of increasing temperature. Two types of shear flow orientation have been observed, depending on the type of the phase in the LCP matrix. The shear flow at nematic temperatures orients well the PS microcylinders as well as the LC mesogens in the velocity direction. This parallel-b orientation is expectable from the general orientation of the microcylinders and the LC mesogens in shear flow. When the LCP matrix transforms to the smectic LC, the mesogens are aligned parallel to the velocity gradient direction entaili...

Shear Flow Orientation of Cylindrical Microdomain in Liquid Crystalline Diblock Copolymer and its Potentiality as Anchoring Substrate for Nematic Mesogens

We prepared a diblock copolymer composed of polystyrene (PS) and side-chain liquid crystal polymer (LCP) by atom transfer radical polymerization. The weight fraction of the PS segment is 0.22, resulting in a microsegregated structure with PS cylinders hexagonally packed in a matrix of LCP, which forms smectic A, nematic and isotropic phases in order of increasing temperature. Shear flow at nematic temperatures orients well the PS microcylinders, as well as the nematic mesogens of the LCP in the velocity direction. A high degree of orientation of 0.95 is attained for microcylinders while the orientational order parameter of the nematic LC is around 0.5. Interestingly, the nematic LC mesogens recover their initial orientation upon cooling after they have lost the orientational order in the isotropic melt. This spontaneous orientation shows that PS cylinders whose orientation is strongly sustained with a temperature variation acts as an anchoring substrate for the nematic mesogens.

Several Interesting Fields Exploited through Understanding of Polymeric Effects on Liquid Crystals of Main-Chain Polyesters

We review our experimental results on the structural characteristics of polymeric liquid crystals collected through our studies on the main-chain liquid crystalline polyesters with mesogenic biphenyl groups connected by flexible alkyl spacer. The polyesters studied, designated as BB-n where n is the number of carbon atoms in the alkyl spacer unit, form smectic mesophases. Of interest is that the type of smectic phase alters depending on the odd-even parity of n. An ordinal smectic A phase is formed by BB-n with even n, whereas BB-n with odd n forms a new type of smectic phase, so called smectic CA phase, in which the long axis of the mesogen is tilted against the layer normal and the tilting direction is opposite between neighboring layers. Finding of smectic CA phase with specific packing symmetry leads to a distinct discovery of ferroelectric and antiferroelectric phases in achiral molecular systems. In these smectic phases, the polymer chain assumes more extended conformation. Such an extended conformation, however, does not continue over the whole length of the polymer and hairpin folding arises to gain entropy. In the smectic phases, the hairpin foldings are located at a limited space, forming chain folded lamellae. Unusual shear flow orientation and anomalous crystallization behavior are explained based on the chain folded lamellar structure. Solid state morphology of liquid crystalline polymer is also presented. Since the degree of liquid crystallinity is almost 100%, the solid phase cooled through the formation of liquid crystals is composed of crystal and liquid crystal glass. Liquid crystal glass is studied and properties are discussed in a comparison with those of conventional isotropic glass.

The structure and dynamics of overturns in stably stratified homogeneous turbulence

Direct numerical simulations of stably stratified homogeneous turbulence, with and without mean shear, are used to investigate the three-dimensional structure, evolution and energetic significance of density overturns. Although the flow conditions are idealized, examination of the full-field simulation data provides insight into flow energetics and mixing which may assist in the interpretation of physical measurements, typically limited to one-dimensional vertical profiles. Overturns, defined here through the density field as contiguous regions of non-zero Thorpe displacement, are initially generated by the stirring action of coherent vortex structures present in the flow and further develop through merging with adjacent overturns. During this growth phase, overturns exhibit irregular spatial structure in unsheared flow and elongated structure with distinct orientation in shear flow. Although most of the available potential energy (APE) and buoyancy flux are associated with stable (non-overturning) regions in the flow, young overturns actively contribute to the flow energetics. In particular, overturn peripheries are sites of high levels of APE, buoyancy flux and diapycnal mixing. A collapse phase may follow the growth phase in the absence of adequately strong mean shear. During this phase, buoyancy gradually assumes control of the overturns and their vertical scale steadily decreases. The energetic significance of the overturns diminishes, although high APE and diapycnal mixing continue to occur near their boundaries. In the final phase of their evolution, overturns contribute negligibly to the energetics. The remaining overturns are characterized by a viscous–buoyant balance which maintains their vertical scale. The overturns eventually vanish due to homogenization of their internal density distribution by diffusion. Activity diagrams, sampled at different points of flow evolution, show significant variation in overturn Reynolds and Froude numbers which may have implications for vertical sampling of a turbulent event.

Shear-induced deformations of polyamide microcapsules

The application of microcapsules for technical, cosmetic and pharmaceutical purposes has attracted increased interest in recent years. The design of new capsule types requires a profound knowledge of their mechanical properties. Rheological studies provide interesting information on intrinsic membrane properties and this information can be used to avoid premature release of encapsulated compounds due to the action of external mechanical forces (stirring, swallowing, spreading). In this publication we report a systematic study of polyamide microcapsules. These particles were synthesized by reacting 4-aminomethyl-1,8-diaminooctane and sebacoyl dichloride at the interface between silicone oil and water. Two different experiments were performed to get information on the mechanical properties of the capsule walls. First of all, we used an optical rheometer (rheoscope) to observe the capsule deformation and orientation in shear flow. The polymerization kinetics, relaxation properties, the regime of linear-viscoelastic behavior and the shear modulus of the flat membranes were independently measured in an interfacial rheometer. Both experiments gave complementary results. It turned out that the two-dimensional elongational modulus was about 3–4 times larger than the shear modulus. This result is in fairly good agreement with a theoretical model recently proposed by Barthes-Biesel. Due to the simple synthesis and well-defined structure, polyamide microcapsules can also serve as simple model systems to understand the complicated flow properties of red blood cells.

Phenomenological theory of textured mesophase polymers in weak flows

AbstractA macroscopic viscoelastic model for incompressible, isothermal, homogeneous lyotropic mesophases exhibiting the nematic polydomain texture is presented. Under equilibrium static conditions the model describes a three dimensional tessellation, where each region or nematic domain has a characteristic size and orientation, and where the polydomain texture has a random orientation. Close form expressions that define the characteristic texture size and the number of randomly oriented domains are given. When subjecting the model lyotropic liquid crystalline polymer displaying the polydomain texture to a steady rectilinear shear flow, the predicted characteristic texture size decreases with increasing shear rates. The power law scaling relations of texture size with shear rate are in excellent agreement with the experimental measurements. The steady shear flow orientation predictions, characterized by a positive shear dependent alignment angle and a low orientation, are in agreement with experimental data.

DNA orientation in shear flow

AbstractThe linear dichroism (LD) has been measured for DNA molecules 239–164,000 base pairs long oriented in shear flow over a large range of velocity gradients (30–3,000 s −1) and ionic strengths (2–250 mM). At very low gradients, the degree of DNA orientation increases quadratically with the applied shear as predicted by the Zimm theory [J. Zimm, (1956) Chemical Physics, Vol. 24, p. 269]. At higher gradients, the orientation of fragments ≥ 7 kilobase pairs (kbp) increases linearly with increasing shear, whereas the orientation of fragments ≥ 15 kbp shows a more complicated dependence. In general, the orientation decreases with increasing ionic strength throughout the studied ionic strength interval, owing to a decrease in the persistence length of the DNA. The effect is most dramatic at ionic strengths below 10 mM, and is more pronounced for longer DNA fragments. For fragments ≥ 15 kbp and velocity gradients ≥ 100 s−1, the orientation can be adequately described by the empirical relation: LDr= –(k1‐G)/(k2 + G), where k1is a linear function of the square root of the ionic strength and k2 depends on the DNA contour length. Since the DNA persistence length can be represented as a linear function of the reciprocal square root of the ionic strength [D. Porschke, (1991) Biophysical Chemistry, Vol. 40, p. 169], extrapolation of the empirical relation provides information about the stiffness of the DNA fibers. © 1993 John Wiley & Sons, Inc.

Experimental methods in polymer melt rheology

AbstractExperimental developments in different areas of polymer melt rheology and recent results are reviewed: Shear oscillations were performed with mixtures of polyisobutylenes (PIB) of relatively small molecular mass distributions and with blends of polystyrene (PS) and polymethylmethacrylate (PMMA). For the latter, the interfacial tension between the melts of PS and PMMA can be derived from the results. ‐ In constant shear rate flow, the measurement of the first normal stress difference N1 in a commercial cone‐and‐plate rotational rheometer is simple if the test temperature is kept extremely constant. By a further modification a partial integral of the pressure distribution is measured from which the second normal stress difference N2 can be determined. ‐ The molecular orientation in shear flow results in a rheological anisotropy that can be studied in a parallel‐plate shear rheometer in which the polymer melt sample can arbitrarily be sheared in two perpendicular directions. ‐ For melt elongation by rotary clamps constant, arbitrary ratios of the strain rate components can be applied. Planar elongations of the PIB samples indicate an influence of the width of the molecular mass distribution. More general elongations with a change of the strain rate ratio during the test are of especial interest as is documented by the comparison of the resulting stresses with predictions from different network theories. ‐ In a rheometer for simple elongation at 170°C, recovery after melt extension of the PS/PMMA blends reveals a value of the interfacial tension equal to the one obtained from shear oscillations.

Anisotropic Behavior of the Thermal Diffusivity of Polymer Materials. 2nd Report. Dependence of the Anisotropy on Flow Patterns of Molten Polymer between Parallel Walls.

In the previous report, we have studied an anisotropy of the thermal diffusivity of molded PMMA spiral-flow samples by means of the forced Rayleigh scattering method. This anisotropy of thermal properties was caused by the orientation of polymer molecules under shear flow field in the mold. In the present paper, the anisotropy of the thermal diffusivity due to molecular orientation in shear flow was experimentally measured for the sample in molten and flowing condition. One-dimensional flow of molten polymer was created in a thin rectangular visualized conduit with a plunger extruder, and the anisotropic behavior was investigated for molten polystylene at different temperatures and different velocity profiles. It was shown as possible to measure the thermal diffusivity of molton polymer in its flowing condition by mean of the forced Rayleigh scattering method. The ratio of a∥, the thermal diffusivity in the direction parallel to the flow, to a ⊥, that rectangular to the flow, increases with a mean velocity increase. It was shown that all of measured thermal diffusivity can be correlated using Re number as a parameter.

Rheological properties of liquid‐crystalline side‐group polymers in the isotropic, nematic, and smectic states

AbstractThe melt viscosity of seven liquid‐crystalline side‐group polymers with polyacrylate and polysiloxane main chains is measured as a function of shear rate and temperature. In the isotropic and also in the nematic phase none or only a very small dependence of the melt viscosity on shear rate is observed for the shear rates investigated. The melt viscosity in the nematic phase is higher than that in the isotropic phase. This is contrary to the results for liquid crystalline mainchain polymers and points to the fact, that there is no resulting orientation in shear flow of liquid‐crystalline side‐group polymers in the nematic phase. In the smectic phase the melt viscosity is very high and strongly shear‐rate dependent. These results are compared with dielectric relaxation measurements on polymethacrylates and a polychloroacrylate.

The development of molecular orientation and morphological texture in thermotropic copolyesters

AbstractLiquid crystalline polymers can be processed to form high strength/modulus materials. In processing these materials, it is apparent that molecular orientation is an important factor in determining the physical strength of the processed materials. In this study a systematic investigation was carried out to determine how a thermotropic copolyester of parahydroxybenzoic acid (PHB) and polyethylene terephthalate (PET) responds to two basic types of flows: shear and extensional flow. This was accomplished by preparing sheared and extended samples under controlled conditions of temperature and flow history. Sheared disks were prepared using a disk and plate geometry of a Rheometrics Mechanical Spectrometer (RMS model 605), while extended ribbons were prepared using a slit die attached to an Instron capillary rheometer. Two copolymerer compositions of 60 mole percent and 80 mol percent PHB were investigated. The sheared disks and extended ribbons were investigated for molecular orientation and morphological textures using wide angle x‐ray scattering (WAXS) and scanning electron microscopy (SEM) analysis, respectively. It was found that extensional flow has a greater capacity for orienting such materials than shear flow. Samples annealed at their softening points for 1 minute (240°C for the 60 mole percent PHB/PET copolymer and 300°C for the 80 mole percent PHB/PET copolymer) showed no significant loss of orientation, indicating that once orientation is produced it may remain in the melt for a long period of time. Sheared samples prepared by shearing the sample while cooling showed significantly higher degrees of orientation than those not cooled while being sheared. This may indicate that a minimum stress level exists for the production of orientation in shear flow.

Red blood cell deformation in shear flow. Effects of internal and external phase viscosity and of in vivo aging

Shear deformation of young and old human red blood cells was examined over a range of shear stresses and suspending phase viscosities (eta o) using a cone-plate Rheoscope. The internal viscosities (eta i) of these cell types differ, and further changes in internal viscosity were induced by alteration of suspension osmolality and hence cell volume. For low suspending viscosities (0.0555 or 0.111 P) old cells tended to tumble in shear flow, whereas young cells achieved stable orientation and deformed. Changes in osmolality, at these external viscosities, altered the percentage of cells deforming, and for each cell type threshold osmolalities (Osm-50) were determined where 50% of cells deformed. The threshold osmolalities were higher for younger cells than for older cells, but the internal viscosities of the two cell types were similar at their respective Osm-50. Threshold osmolalities were also higher for the higher external viscosity, but the ratio of internal to external viscosities (i.e., eta i/eta o) was nearly constant for both external viscosities. Deformation of stably oriented cells increased with increasing shear stress and approached a value limited by cell surface area and volume. For isotonic media, over a wide range of external viscosities and shear stresses, deformation was greater for younger cells than for older cells. However, deformation vs. shear stress data for the two cell types became nearly coincident if young cells were osmotically shrunk to have their internal viscosity close to that for old cells. Increases in external viscosity, at constant shear stress, caused greater deformation for all cells. This effect of external viscosity was not equal for young and old cells; the ratio of old/young cell deformation increased with increasing eta o. However, if deformation was plotted as a function of the ratio lambda = eta i/eta o, at constant shear stress, young and old cell data followed similar paths. Thus the ratio lambda is a major determinant of cell deformation as well as a critical factor affecting stable orientation in shear flow.

EFFECT OF RED CELL RIGIDITY ON GAS TRANSPORT BY SHEARED FLOWING BLOOD†

By means of a blood flowing model the dependence of oxygen uptake by red blood cells (RBC) on the deformation of the RBC was investigated. The model allows measurements of oxygen transfer in flowing sheared blood. Simultaneously secondary flows can be produced, which enhances oxygen transfer rate of whole blood considerably. The model is applied to analyse the effect of an artificial rigidification of RBC on oxygen uptake by the cells. A diminution of RBC deformations, obtained by an oxidation of membrane SH-groups, is shown to result in a decrease of O2-uptake by RBC. This decrease is assumed to be due to a reduction of intracellular convection, which diminishes intracellular mixing of haemoglobin. Glutardialdehyde treatment results in an even higher decrease of oxygen uptake. In this case the complete lack of deformations of the RBC and of orientation in shear flow, which is typical for native RBC and is maintained in diamide RBC, is proposed to be responsible for this greater loss of O2-uptake. A shift...