Cessation Of Shear Research Articles

A Nonlinear Elastic Model for Shear Thickening of Suspensions Flocculated by Reversible Bridging

The flow of suspensions flocculated by reversible bridging of polymer with weak affinity for the particle surface is Newtonian in the limit of zero shear rate, shear-thickening at moderate shear rates, and plastic with a constant stress independent of shear rate in high shear fields. The Newtonian flow implies that the polymer bridges are constantly forming and breaking in a quiescent state. The stress after cessation of steady shear exponentially relaxes at long times. The relaxation time is not affected by the shear rate. The strain-dependent curve of storage modulus at a constant frequency shows a rapid increase when the strain is increased above a critical level. The critical strain is independent of frequency. Therefore, the shear thickening may be primarily attributed to the elastic effect of extended bridges. Since the polymer coils are forced to desorb at some degree of extension, the suspensions become nearly plastic at high shear rates. The intrinsic mechanism of shear-thickening flow for suspen...

Volume loss, fluid flow, and coaxial versus noncoaxial deformation in retrograde, amphibolite facies shear zones, northern Malawi, east-central Africa

Two retrograde, amphibolite facies shear zones were studied to explore the relationship between retrograde mineral reactions, volume strain, fluid flow, mylonitization, and coaxial versus noncoaxial deformation. The two shear zones are the contractional Mafwewu Hills shear zone and the transcurrently displacing Mkamasa River shear zone of northern Malawi. In general, shear-zone formation is characterized by the breakdown of feldspar and biotite and the formation of sillimanite, quartz, and water. Silica, alkali, and alkali earth elements were mobile. Mass-balance calculations, based on major- and trace-element geochemistry, indicate as much as 50%–60% volume loss in mylonite. Fluid to rock ratios estimated from the calculated depletions of silica are as much as 200–400, indicating that the initiation and activity of the shear zones were accompanied by large amounts of fluids that infiltrated the shear zones. These fluids caused an almost complete loss of alkalies within the shear zone, leading to the development of almost “dry” and sheet-silicate–free mylonite. It is proposed that the associated destabilization of micas destroyed the pathways for fluid circulation and ultimately caused the cessation of shearing. In the Mafwewu Hills shear zone, shear-zone–related stretching lineations are oriented parallel to tectonic transport. In the Mkamasa River shear zone, the stretching lineations curve from a subhorizontal attitude in the wall rock into a subvertical attitude in mylonite characterized by large volume loss. Shear bands in the Mkamasa River shear zone indicate a sinistral sense of shear. A synoptic analysis of mesoscale S and Z asymmetric folds indicates that the orientation of the stretching lineations in mylonite characterized by large volume loss is subperpendicular to the regional tectonic transport direction. Aspect ratios of feldspar and quartz in XZ sections (X ≥ Y ≥ Z, principal axes of finite-strain ellipsoid) show oblate grain shapes and show a positive correlation with volume strain. Mean kinematic vorticity numbers are 0.26–0.44 in the wall rock and 0.57–0.79 in mylonite. In mylonite, small kinematic vorticity numbers correlate with large volume loss. The transport-perpendicular orientation of the stretching lineations in strongly volume-deficient mylonite is thought to be the result of a pronounced component of shortening normal to the shear zone; the resulting higher degree of coaxial deformation controlled the final orientation of the stretching lineations.

Shear effect on the phase behavior and morphology in pmma/san-29.5 blend

The effect of simple shear flow on the miscibility and morphology of blends of poly(methyl methacrylate) (PMMA) and a styrene-acrylonitrile random copoly-mer with 29.5 wt% acrylonitrile (SAN-29.5) has been investigated using shear apparatus and transmission electron microscopy (TEM). The obtained data showed that only shear-induced mixing was observed for all of the composition ratios. The increase of the cloud point (or homogenization temperature) ΔT(γdot; = T(γdot;) - T(0) was investigated as a function of shear rate γdot;; in addition, the normalized shift in the cloud point ΔT(γdot;)/T(0) versus γdot; was also studied and compared with that of simple liquid mixtures and polymer solutions. The results showed that the polymer blends were more sensitive to the shear rate than both simple-liquid mixtures and polymer solutions. The morphology of the PMMN SAN(= 75/25) blend (the critical composition) indicated that shear-induced phase mixing occurred at a critical shear rate value, below which the two phases were highly oriented and elongated in the flow direction. Three regimes, depending on the applied shear rate values, were detected that were in good agreement with the literature data for polymer solutions. The effect of relaxation times after shear cessation showed a decrease in the orientation of the elongated particles, but it did not completely vanish even for 10 min after the shear cessation.

A Molecular Theory for Fast Flows of Entangled Polymers

The Doi−Edwards (DE) theory for the rheological properties of entangled polymer melts and solutions successfully predicts the response to large step-shear strains but fails to predict other nonlinear shear properties, such as the steady-state viscosity or the relaxation of stress after cessation of steady shearing. Many of these failures remain even in the extension of the theory by Marrucci and Grizzuti (Gazz. Chim. Ital. 1988, 118, 179)1 to allow deformation-induced “tube stretch”. Here, we find that a much more successful theory can be obtained by also accounting for “convective constraint release”, i.e., the loss of entanglement constraints caused by the retraction of surrounding chains in their tubes. (Marrucci, G. J. Non-Newtonian Fluid Mech. 1996, 62, 279 and Ianniruberto, G.; Marrucci, G. J. Non-Newtonian Fluid Mech. 1996, 65, 241).2,3 In the molecular model developed here, convective constraint release can both shorten the reptation tube and allow reorientation of interior tube segments. The revised model predicts many of the features of steady and transient shearing flows. These include a region of nearly constant steady-state shear stress at shear rates between the inverse zero-shear reptation time and the inverse Rouse time, similar to that seen in the experiments of Bercea et al. (Macromolecules 1993, 26, 7095)4 and also predicted by Marrucci and Ianniruberto (Macromol. Symp. 1997, 117, 233).5 The predictions of transient stresses after startup and cessation of shear are also in good agreement with experiments, as are predictions of nonmonotonicity in the extinction angle after stepup or stepdown in shear rate.

Shear-induced micellar crystal structures in an aqueous triblock copolymer solution

Shear-induced structures have been observed in aqueous solutions of the triblock copolymer ${\mathrm{PEO}}_{25}{\mathrm{PPO}}_{40}{\mathrm{PEO}}_{25}$ in the micellar crystalline region of the temperature-concentration phase diagram. A structure induced by steady shear, having two-dimensional (2D) hexagonal symmetry over macroscopic dimensions, transforms reversibly to a crystalline structure with different symmetry upon cessation of shear. Other structures are induced with oscillatory shear, including a 3D hexagonal phase with Bragg peaks that not only persist but sharpen with the cessation of shear. Over the range of temperature, concentration, and shear conditions investigated, no evidence for cubic or cubatic crystalline phases has been observed.

Chain orientation in sheared molten poly(ethylene oxide) fractions by measurement of infrared dichroism

AbstractRheo‐infrared spectroscopy was used to study the development of orientation of molten narrow molar mass fractions of poly(ethylene oxide) [molar masses between 18,000 and 120,000 g/mol] during non‐Newtonian shear flow at shear rates between 2 and 270 s−1 and temperatures between 75 and 100°C. The steady state degree of orientation [expressed as the Hermans orientation function (fss)] reached a saturation level with increasing shear rate; fss increased with increasing molar mass (M) according to fss = C1 − C2/M (C1 and C2 are coefficients; the latter depended on shear rate and temperature). The coefficient C1 (fss) for a polymer with infinite molar mass took a universal value close to 0.05 for the temperatures and shear rates used. Under large shear stresses, the relationship between stress and orientation deviated markedly from linearity. The time to establish a steady state level of orientation was proportional to M1/2. The recovery of the isotropic state after the cessation of shear could initially be described by a simple exponential relaxation law: f ∝ e, where τρ is the relaxation time. The latter showed a weak molar mass dependence according to τr ∝ M0.6 and an Arrhenius temperature dependence with an activation energy of ∼60 kJ/mol. The relaxation of the shear stress after the cessation of shear was more rapid than the recovery of the isotropic state.

Shear-induced orientational order in the hexagonal phase of oxyethylene/oxybutylene diblock copolymer gels

Shear-induced orientational ordering in diblock copolymer gels of E 40B 10 in 0.2 mol dm −3 aqueous K 2SO 4, where E denotes oxyethylene and B denotes oxybutylene, was studied in the hexagonally packed rod phase at 85°C for a number of diblock copolymer concentrations. Large amplitude (λ = 50%) oscillatory shear at a shear rate of ω = 10 rad s −1 was applied to the gels and the degree of orientation was quantified in terms of global orientational order parameters extracted from small-angle X-ray scattering (SAXS) patterns. Shearing the gel produced a rapid drop in the dynamic shear moduli and a slower increase in orientation; up to a second rank orientation parameter of P 2 = 0.26 for the highest concentration (38 wt%) diblock gel. Time-resolved SAXS measurements showed that the steady state level of orientation was established over a time-scale of 3 min. Lower concentration gels yielded lower orientation parameters from the same deformation process, and a 25 wt% gel remained unoriented. Upon cessation of shear the dynamic shear moduli recovered rapidly to their initial values, although the shear-induced anisotropy in the SAXS patterns relaxed very slowly to zero over a time-scale of over an hour.

A rheo‐infrared‐dichroism study of sheared molten polystyrene fractions

AbstractThe development of orientation in molten narrow molar mass fractions of polystyrene during shear flow in the non‐Newtonian shear rate range (11–24 s−1) at temperatures between 225 and 275°C has been assessed by measurement of the infrared dichroism. Deviation from linearity between shear stress and orientation was found at high shear stresses in the steady state region. A sudden change in the shear rate caused a change in the shear stress that was one to two orders of magnitude‐state level of the Hermans orientation function (fss) was dependent on temperature, shear rate and molar mass (M): fss = C1–C2/M (where C1 and C2 are constants). The time to reach steady state on the inception of shear was proportional to M1/2. The recovery of the isotropic state after the cessation of shear followed initially a simple exponential law: f ∝ e−t/τ, where t is the time and τ is the relaxation time. The relaxation time showed only a weak molar mass dependence (τ ∝ M0.8), indicating that the decay of orientation was initially governed by processes other than reptation.

An integrated Couette system for in situ shearing of polymer and surfactant solutions and gels with simultaneous small angle x-ray scattering

A Couette cell designed for in situ small angle x-ray scattering (SAXS) studies of polymeric systems under shear is described. Representative results are presented for the effect of shear on the lamellar phase of a concentrated poly(oxyethylene)–poly(oxybutylene) diblock copolymer in aqueous solution. It is shown that the application of shear resulted in a reduction in the lamellar spacing and of the defect density. The former was observed as a shift in the principal peak position in the SAXS pattern, and the latter as an increase in the associated correlation length. It was further shown that the cessation of shear led to a relaxation of both shear induced effects, over a time period of hours.

Shear-induced alignment and relaxation of orientation in smectic side-chain liquid-crystalline polymers

Infrared rheometry showed that the mesogenic groups of two smectic side-chain liquid-crystalline polymers were aligned by shear until a steady-state value was achieved after approximately 10 s. The shear experiments were carried out at temperatures 10–50°C below the isotropisation temperature. The alignment of the mesogenic groups was always perpendicular to the shear flow direction. The degree of orientation increased with increasing shear rate and the highest reported value for the Hermans orientation function was −0.35 (−0.5 corresponds to perfect perpendicular orientation; it was assumed that the mesogen-group-orientation was unixaial). The turbidity of the sheared smectic samples indicates the presence of small domains in spite of their high level of orientation. A gradual loss of mesogen group orientation occurred after cessation of shear, and full recovery of the original isotropic state was in no case achieved within 60 000 s.

Rheology and Shear Orientation of a Nematic Liquid Crystalline Side-Group Polymer with Laterally Attached Mesogenic Units

The shear orientation of a nematic side-group liquid crystalline polymer (SG-LCP) with side-on fixed mesogenic units was investigated by means of rheo-birefringence and rheo-small angle light scattering. Rheological properties of the polymer depend on molecular weight. Macroscopically aligned samples were obtained in creep experiments, but the orientation was destroyed by oscillatory shear flow. The shear modulus of a low molar mass sample in the nematic and isotropic phase superposed to a common master curve. The low-frequency modulus of a high molar mass sample, however, decreased at the phase transition, and time−temperature superposition was not possible. Preshearing led to a further decrease of the modulus. Rheo-birefringence measurements revealed a clear influence of molar mass on both the final value and the orientation state after cessation of shear. Rheo-small angle light scattering also showed a strong influence of molecular weight. In polarizing microscopy, a banded texture was observed with a ...

Lyotropic chromonic liquid crystals: neutron scattering studies of shear-induced orientation and reorientation

Neutron scattering has been carried out on two dye lyotropic liquid crystalline phases (Scheme 1), one an azo dye (A) which aggregates in solution to form rods and a hexagonal phase and the second a cyanine dye (B) which aggregates in sheets forming a layer phase. The dyes were placed in a couette shear cell so that the influence of shear on the aggregate orientation could be determined. Measurements were carried out with the beam passing through the cell centre and the cell side, so that orientations along and at right angles to the flow direction were monitored. Both dyes show some unusual reorientation behaviour. The aggregates of the hexagonal phase are highly ordered along the direction of flow at low shear rate (2.78 s −1), with the degree of order being reduced at the highest shear rate (1002 s −1). Quite remarkably, on cessation of shear, the rods appear to be at right angles to the orientation direction observed for low shear. For the dye B mesophase, the layers are aligned preferentially parallel to the cell walls at low and high shear, although in the latter case the separation between the layers appears to be reduced by ca. 10%. On cessation of shear again we observe that the orientation of the aggregates is very different, with much of the material having layers perpendicular to the cell walls. This is the first report of chromonic mesophase reorientation under shear, and the first time that a shear-induced change in the separation between aggregates has been observed.

Elastic-Like and Viscous-Like Components of the Shear Viscosity for Nearly Hard Sphere, Brownian Suspensions

The shear properties of Brownian, rigid spheres consisting of sterically stabilized, crosslinked polymethylmethacrylate (PMMA) particles were studied in suspension. Three different volume fractions were used to carry out this study. The suspensions’ elastic-like and viscous-like components of the shear stress were measured by a recently developed technique utilizing cessation of steady shear. The elastic-like viscosity component, or the amount of stress retained at the instant of cessation divided by the shear rate, due to Brownian and possible interparticle forces, decreased in magnitude over the entire range of shear rates used (shear thinning). The decay of the elastic-like stress with time after cessation of flow was also analysed and found to fit a power law relation. The viscous-like component, or the amount of stress lost at the instant of cessation divided by the shear rate, is related to hydrodynamic interactions between and drag on particles and remained essentially constant with shear rate. These data are the first of this kind gathered for a model hard sphere system (i.e., mechanical means) and agrees well with results of Bender and Wagner (J. Colloid Interface Sci.172,171 (1995) who used an optical technique.

The origin of banded textures induced by shear—a suggested scheme and a relevant rheological effect

In this paper we propose a mechanism for the ubiquitously observed, but essentially still unexplained, phenomenon of banding in sheared polymeric liquid crystals. In the course of it numerous analogies are brought in from other fields and a new rheological observation made on two lyotropic systems is announced and its relevance to banding discussed. The proposed explanation rests on two separate tenets. First, that buckling of uniaxially oriented birefringent fibrous entities accounts for the observed banding. Second, that the usually observed orientation relaxation after shearing (with which banding is usually associated) points to the existence of restoring forces which are extraneous to liquid crystals per se. These, we propose, are due to an elastic network which is part of the total system, but does not belong to its liquid crystal component and, as observed, is characteristic of polymer liquid crystals alone. These restoring forces can therefore provide the axial compression which produces the buckling. Amongst the numerous features quoted, special emphasis is given to the “self extension” phenomena which, if constrained, can be a strong contributing factor to (or even the cause of) the buckling, and thus band formation. The unusual rheological effect in the hydroxypropylcellulose-water system, the spontaneous stress build-up on the cessation of shear, while being a notable phenomenon in its own right, is discussed here because of its observed limitation of the banding process.

Determination of red blood cell shape recovery time constant in a couette system by the analysis of light reflectance and ektacytometry

Red blood cell (RBC) shape change under shear is generally reversible, with the time course of shape recovery a function of the elastic and viscous properties of the RBC membrane. RBC shape recovery can be investigated, using several different techniques, to provide information about the membrane material properties that are not directly accessible by frequently used methods to assess RBC deformability ( e.g., micropore filtration). In the present study, RBC shape recovery was studied in a Couette system after abrupt cessation of shear, either by analyzing the time course of laser light reflection or by serial measurements of elongation indexes from laser diffraction patterns. The time course of shape recovery monitored with both techniques can be described with an exponential equation. Calculated time constants for normal human RBC were 119 ± 17 msec and 97 ± 15 msec as measured by light reflection and ektacytometry, respectively. Treatment of RBC with glutaraldehyde resulted in dose-dependent decreases in the shape recovery time constant. Heat treatment (48 ° C, 20 min), which is known to increase mainly the shear elastic modulus of the membrane, decreased the time constant by 65%. In contrast, wheat germ agglutinin treatment increased the shape recovery time constant by 22%, presumably by increasing membrane surface viscosity. Our results indicate that the shape recovery time constant of RBC can be measured easily and accurately by computerized light reflection analysis.

Concentration effects on the rheology and texture of PBG/m‐cresol solutions

In this work, the rheology of PBG/m‐cresol systems is reevaluated with the specific goal of understanding the influence of concentration on low shear‐rate rheology. Comparisons of steady‐state and transient rheology and the behavior after cessation of shear allow for the separation of the contributions to the stress from the nematic fluid and defect texture. Stress relaxations demonstrate that the textural contribution to the stress increases nonlinearly from 30% to 55% as the concentration increases from 12 to 37 wt %. Recoil shows a similar nonlinear trend in the magnitude of the ultimate recovered strain γ∞. Stress relaxations and recoil, once normalized, have universal dynamics dependent only on γ̇ot and independent of concentration. Two different regions of concentration are identified and correlated to a distinct change in the nature and behavior of the textural contribution to the stress.

Thermally stable optically anisotropic polymer networks obtained from mesogenic LC cellulose derivatives

The birefringence of anisotropic polymer composites obtained by photopolymerisation of lyotropic liquid crystalline systems composed of (hydroxypropyl)cellulose derivatives (R-PC) and photopolymerisable organic acids that are also lyotropic solvents is studied. The orientation of the systems before photopolymerisation was induced by shearing. The mechanically oriented lyotropic solutions after cessation of shearing form a band texture visible under crossed polarisers. This band texture shows relaxation phenomena and the grain texture observed before shearing recovers with time. The decay time of the relaxation of the band texture at room temperature is much longer than the time of band texture formation after cessation of shearing. The photopolymerisation of acrylic acid (or methacrylic acid) in the oriented R-PC/acid lyotropic solutions leads to birefringent polymer composites. These composites show reversible anisotropic optical properties as examined by thermo-optical analysis (In contrast with the samples before photopolymerisation); therefore they can be classified as reversible anisotropic polymer networks. It is demonstrated that the thermal stability of the birefringence of the R-PC/poly(acid) composites is mainly determined by the molecular mobility of the poly(acid) connected with its glass transition temperature.

Unusual Thixotropic Properties of Aqueous Dispersions of Laponite RD

The rheological properties of aqueous dispersions of the synthetic hectorite clay Laponite RD that form gel-like structures have been investigated. Special emphasis has been laid on the phenomenon of thixotropy. Structural recovery at rest after steady shear is considered a fundamental thixotropic process and has been characterized by small amplitude oscillatory shear measurements. A special sealing technique was used to prevent the evaporation of water during these experiments. After cessation of steady shear |η*| increases monotonically with time and even after 16 days no equilibrium viscosity value is reached. A single power law |η*| ∼tnholds within the time regime from 10 to 106s. The exponentn= 0.13 ± 0.02 is independent of clay concentration and mechanical pre-treatment of the material. This type of kinetics has not been reported so far for the thixotropic recovery of any clay dispersion and it does not fit the common definition of thixotropy. The reorganization of the gel structure determining this thixotropic phenomenon is interpreted as a cooperative self delaying process similar to the aging of glassy polymers or precipitation from supersaturated solid solutions.

Influences of substrate surfaces on the band formation behaviour in an oriented thermotropic liquid crystalline polyester

The influences of substrate surfaces on the formation behaviour of banded texture and its thermal stabilities have been studied for an aromatic X-shaped main-chain liquid crystalline polyester. The oriented polymer films prepared by shear on a glass substrate were found to exhibit a strong bounding effect, which was very effective in stabilizing or keeping the structural orders of oriented polymer chains, even for a long time in the mesomorphic state and also in stimulating the band formation process during cooling. Such an influence is considered to arise from strong interfacial interactions between the solid substrate and the mesomorphic polymer molecules deposited on it. Similar phenomena were also observed in the case of PET film adopted as substrate. However, they were quite different when the glass substrate was replaced by a Teflon sheet: the uniformity of polymer chain alignments on it was difficult to maintain even in a rapid cooling process after shear cessation, resulting in a formation of irregular banded texture. The melting-induced re-orientation behaviour of polymer chains was also examined for a drawn fibre.

Rheological investigation of shear-induced structure changes in multiblock copolymers

Styrene-butadiene multiblock copolymers were examined with both newly introduced and established rheological techniques and by transmission electron microscopy (TEM) to evaluate shear-induced structural changes in these polymers. Transient rheological tests (based on superposed flow principles) were developed which probed structural changes that occur in the copolymers during and at the cessation of steady shear. Data from these tests indicated that for the cylindrical morphology copolymer (SB1) there were structural changes occurring during steady shear that were recovered upon cessation of shear. The recovery process took place on time scales that could be significant in processing. The lamellar morphology material (SB2) did not exhibit this recovery behavior. Longer-term structure changes were investigated using established techniques and showed differences between the cylindrical and lamellar copolymers. When tested at 210°C, peaks in tan δ occurred at 30 rad/s for SB1 and at 0.5 rad/s for SB2 with saturation strain levels of 150 strain units (SU) for SB1 and 80 SU for SB2. TEM analysis of SB2 indicated that, although rheological changes are significant up to 80 SU, better alignment of the domain morphology can be achieved at moderately low strains of 7 SU. This indicates that the copolymers' rheological changes, which occur as a result of steady shearing, may not be due entirely to domain alignment, but may also be due to more local molecular rearrangements (e.g., chain disentanglement). © 1996 John Wiley & Sons, Inc.