Shear History Research Articles

Effects of Layered Silicate on Nanoscopic Internal Structure and Morphology in Polymer Blend

Organically modified layered silicate was introduced into miscible poly(ethylene oxide) (PEO) / poly(methyl methacrylate) (PMMA) blends for the synthesis and characterization of polymer/clay nanocomposite through solvent casting method. As a part of our continuous efforts on the study of nanoscopic internal structures and changes of material properties caused by the presence of organically modified layered silicate, we systematically investigated the characteristic morphological changes and internal structures with scanning electron microscope (SEM) and transmission electron microscopy (TEM). Various factors such as clay concentration, thermal history, and shear history were employed to give rise to the changes of morphological and internal structures.

Effect of shear on concentrated hydrous ferric floc rheology

The primary interest of this work is to determine the effect of continuous shear, variable shear rate, and shear history on concentrated hydrous ferric floc rheology. The floc suspensions are strongly shear degrading with the behaviour being adequately described by a power law model. This is supported by images of the floc particles, which indicate a breakdown in floc structure, with the release of water and formation of smaller particles, after the application of shear. A shear rate sweep of the sample produces a hysteresis loop due to the time dependency of the samples. Higher shear rates are shown to destroy more structure, resulting in an overall decrease in floc viscosity. This is irreversible, and occurs due to a breakdown in floc structure to release trapped water and produce smaller floc particles. Shear also causes an instantaneous reduction in viscosity. This effect is instantaneously recoverable, and is due to a decrease in floc particle aggregation. Floc behaviour within the shear rate range 20–250 s −1 is also adequately modelled by the Sisko model.

Parametric Study on Cement Grout and Aggregate Media for Preplaced Aggregate Grouting

Abstract Experimental programs were designed carefully and appropriate testing materials were selected for the parametric study on the flow of cement grout through aggregate media during the preplaced aggregate grouting process in concrete construction. Experimental investigations include the testing and evaluation of the viscosity of cement grout, the intrinsic permeability coefficient, and the porosity of the aggregate media. The time dependency and shear history dependency of the rheological properties of the cement grout are incorporated in the viscosity tests so as to be able to simulate more accurately the grouting process. The average shear rate of the flow is estimated from the flow velocity by determining the average nominal radius of the imaginary flow channel from the permeability testing. The resulting parameters include the yield stress and plastic viscosity of the cement grout and their time dependency, intrinsic permeability and porosity of the aggregate media, and the average nominal radius of the imaginary flow channel.

Interfacial slip at the thermotropic liquid‐crystalline polymer/poly (ethylene naphthalate) interface

AbstractThe unique rheological properties of a thermotropic liquid‐crystalline polymer (TLCP) were first studied. The thermal and shear history of the TLCP was found to play a critical role in its rheological properties. Crystallites were observed in the TLCP melt even above the melting temperature detected by differential scanning calorimetry. Because interfacial slip had long been suggested as an important reason for viscosity reduction in TLCP/thermoplastic blends, for the first time, interfacial slip at the TLCP/poly(ethylene naphthalate) (PEN) interface was investigated with an energy model. The model quantified the degree of interfacial slip at the TLCP/PEN interface by an energy factor. The calculated energy factors revealed a high degree of interfacial slip at the TLCP/PEN interface. It was proposed that the high rigidity of rodlike TLCP chains and their alignment parallel to the interface prevented mutual entanglements at the TLCP/PEN interface. The lack of mutual entanglements promoted the interfacial slip. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 302–315, 2004

Transient phenomena in vesicular lava flows based on laboratory experiments with analogue materials

Realistic lava flow models require a comprehensive understanding of the rheological properties of lava under a range of stress conditions. Previous measurements have shown that at typical eruption temperatures lavas are non-Newtonian. This is commonly attributed to the formation and destruction of crystal networks. In the present study, the effects of bubbles on the time-dependent, non-Newtonian properties of vesicular melts are investigated experimentally using analogue materials. The shear-thinning behaviour of bubbly liquids is shown to be dependent on the previous shearing history. This thixotropic behaviour, which was investigated using a rotational vane-viscometer, is caused by delayed bubble deformation and recovery when subjected to changes in shear stress. The viscoelastic transition and the transient flow behaviour of analogue fluids were studied using both a rotational vane-viscometer and oscillatory shear apparatus. These experiments have shown that vesicular suspensions are viscoelastic fluids with a yield strength, power law rheology, and a non-zero shear modulus. These properties are also found in polymer fluids commonly used as analogue materials for lava such as gum rosin. We show that, when materials with this rheology are accelerated in channels, they may be fragmented, and when they flow through a narrowing conduit, pulsating flow can develop as a consequence of a transition from slip to non-slip conditions at the conduit wall. This has important implications both for effusive and explosive volcanic eruptions.

Dynamic phase diagram and onion formation in the system C10E3/D2O

The influence of the shear on the lamellar phase, L, of the system C10E3/D2O was studied along an isoplethal path (40 wt.% C10E3) in the temperature range 25-42degreesC. A dynamic phase diagram was determined by steady-state rheometry, where by shear action the lamellar phase was transformed into multilamellar vesicles (MLVs) (onions). The location of in the dynamic phase diagram, depends only on the temperature and the applied shear rate, and not on the shear history. The classical lamellar phase structure is stable at rest and at low shear rates. When exposed to higher shear rates, the lamellar structure is transformed into onions. The transition from lamellae to onions is shifted to higher shear rates with increasing temperature. In a range of shear rates in between the stable lamellae and stable onion structure, the transition is incomplete. The transformation of lamellae into onions appears to be governed by the imposed strain, in agreement with earlier studies. The effect of temperature can be understood from the general property of nonionic surfactants where the monolayer spontaneous curvature decreases with increasing temperature. (C) 2003 Published by Elsevier B.V. (Less)

Shear-induced structure formation in solutions of drag reducing polymers

The phenomenon of shear-induced structure formation has been investigated for several drag reducing polymers. Aqueous solutions of high molecular weight partially hydrolyzed polyacrylamide, polyacrylamide, and poly(ethylene oxide) have been examined in the concentration range of 100–4000 ppm by weight. Experimental protocols included rheological and rheo-optical characterization under simple shear flow, and molecular weight analysis by chromatography following various shear histories. All solutions exhibited time-dependent stresses and turbidity patterns under high shear, indicative of structure formation. Two regimes of behavior were found with the partially hydrolyzed polyacrylamide. At concentrations below 1500 ppm, aggregate formation occurred above a critical shear rate on time scales of minutes that relaxed back over similar time scales to an apparent homogenous state on cessation of flow. However, these sheared solutions developed irreversible, large-scale turbidity and eventual precipitation on quiescent storage over several days. At higher concentrations, phase-separated droplets formed under shear that also showed ripening of the turbidity and eventual precipitation upon quiescent storage for days. The kinetics of the semi-reversible aggregate formation and relaxation processes in the lower concentration regime showed a regular variation with shear rate and concentration. The strength of the structure formation and precipitation processes was lessened in the presence of salts. Similar behavior patterns were exhibited by the other systems, however, the strength of the structure formation was also somewhat lessened compared to the partially hydrolyzed polyacrylamide. Correlations with solutions being used in ongoing turbulent drag reduction studies suggest that such structure formation may play an important role in the drag reduction process.

Correlation of Material and Processing Time Scales with Structure Development in Isotactic Polypropylene Crystallization

The influence of average molar mass and shear on the crystallization of isotactic polypropylene (iPP) has been studied. The Weissenberg number, a dimensionless parameter consisting of the product of the shear rate and a characteristic relaxation time (γ0λr), provides a criterion to locate the morphological transition from spherulitic to extended growth. The Weissenberg criterion applies to polymers of high and of low molecular weight. This is attributed to the importance of the initial nucleation process. Comparison of materials of different molar masses and various shear histories is allowed by the introduction of the normalized crystallization time τ, which is the ratio of the experimental time and a characteristic crystallization time λVvmax that needs to be defined from the several possible choices. Rheological and rheooptical experiments furnish the material characteristic relaxation and crystallization time scales, respectively. The time λVvmax, defined with light scattering as the moment at maximu...

Modelling the effect of shear history on activated sludge flocculation

The aim of this paper is to investigate the effect of shear history on activated sludge flocculation dynamics and to model the observed relationships using population balances. Activated sludge flocs are exposed to dramatic changes in the shear rate within the treatment process, as they pass through localised high and low mixing intensities within the aeration basin and are cycled through the different unit operations of the treatment process. We will show that shear history is a key factor in determining floc size, and that the floc size varies irreversibly with changes in shear rate. A population balance model of the flocculation process is also introduced and evaluated.

A generalized model for predicting non-Newtonian viscosity of waxy crudes as a function of temperature and precipitated wax ☆

Precipitated wax, shear and thermal history have pronounced effects on viscosity and rheological behavior of waxy crudes. On the basis of mechanism of waxy crude rheology, a shear-rate-dependent viscosity model has been developed by applying theory of suspension rheology. This model is characterized by its capability to predict viscosities of crude oils with various thermal and shear history and beneficiated with pour-point-depressants (PPD). Once viscosities at only two temperatures above the wax appearance temperature and apparent viscosities at one temperature in the non-Newtonian regime are known, viscosities or apparent viscosities at any temperatures above the gel point can be predicted by using the model together with the concentration of precipitated wax at that specified temperature. Verification by using 3458 viscosity data points ranging from 5 to 2900 mPa s from 33 virgin crudes and 14 PPD-beneficiated crudes with various thermal and shear history shows that the model predicts viscosities with an absolute average deviation of 7.43%. Furthermore parameters of rheological models such as the consistency coefficient K and the flow behavior index n of the power law model may be obtained by regressing predicted viscosity data and corresponding shear-rates.

Application of the Elastic Boundary Induced by Shear History on Saturated Cohesive Soil

Induced anisotropy under three-dimensional stress conditions is one of the important factors in the accurate prediction of ground deformation and should be a primary consideration not only in the laboratory, but also in the field. To achieve this objective, torsional shear tests for saturated cohesive soils with induced anisotropy were performed. Induced anisotropy was generated by the shear history under constant p' (effective mean principal stress), α (direction of major principal stress) and b (intermediate principal stress coefficient) conditions in this study. The parameters α and b were also set up during the shear history process, and the drained torsional shear tests were carried out under constant p', α and b conditions using these specimens. The behaviour of soils during this process was examined in detail. The elastic boundary induced by the shear history was represented in three dimensional space. Furthermore, the uniqueness of this elastic boundary was examined by changing the stress level and the material of the specimen. In conclusion, the uniqueness of the elastic boundary induced by the pure shear history (constant p') on saturated cohesive soils was verified via these experimental results. The application of the elastic boundary in a wide range of stress level and material was thus shown.

Characterisation of an Industrial Polymer Melt Through Either Uniaxial Extension or Exponential Shear Data: An Application of the Pom-Pom Model

Abstract We present new non-linear data in extension and two different shear histories. These data are used to compare the effectiveness of using exponential shear data and uniaxial extension data to characterise the non-linear response of an industrial LDPE melt with the pom-pom molecular model. We conclude that extension and exponential shear both allow good predictions to be made in simple shear. However, the characterisation spectrum obtained from exponential shear data fails to predict the correct degree of strain hardening at low extension rates. From this study we are able to suggest circumstances under which exponential shear provides a useful characterisation of branched polymer melts.

Influence of Shear Rate on Thixotropic Suspensions

The thixotropic type of a suspension can be specified by first shearing the suspension at high shear rate to destroy the structure of the suspension, then monitoring the change of the viscosity or stress with time under low shear (D l), i.e., to study the process of structure recovery. The shear history and D l can influence the recovery process, so they have an influence on the thixotropic type of a suspension. In this paper, the influence of D l on the thixotropy of Fe–Al–Mg‐mixed metal hydroxide (MMH for short)/Na‐montmorillonite (MT for short) suspensions is reported. The system with a weight ratio (R) of MMH/MT of 0 and with R = 0.013 both show positive thixotropy in the D 1 range of 10–1022 s−1, i.e., D l does not change the thixotropic type of the suspensions. The suspension with R = 0.051 shows complex thixotropy under lower D l values (10 and 170 s−1), but shows positive thixotropy under higher D l values (511 and 1022 s−1). The system with R = 0.091 shows negative thixotropy at lower D l values (10 and 170 s−1), but shows complex thixotropy at higher D l values (511 and 1022 s−1). With increasing D l, the viscosity of these suspensions decreases gradually, and the degree of change in η ∼ t slows down, i.e., the thixotropic strength becomes weak. The mechanism for these changes is discussed.

Free emboli formation in the wake of bi-leaflet mechanical heart valves and the effects of implantation techniques

The high incidence of thromboembolic complications of mechanical heart valves (MHV), primarily due to platelet activation by contact with foreign surfaces and by non-physiological flow patterns past the valve, still limits their success as permanent implants. The latter include elevated shear and turbulent stresses and shed vortices formed in the wake of the valve's leaflets during the deceleration phase, potentially entrapping activated and aggregated platelets. It is hypothesized that these flow patterns induce the formation of free emboli which are the source of cerebrovascular microemboli associated with MHV. Implicit to this hypothesis is that free emboli formation will be affected by the implantation technique employed and the valve orientation, as those will alter the flow characteristics past the valve and the interaction of the platelets with the flow. In this study, numerical simulations of turbulent pulsatile flow past a St. Jude Medical bi-leaflet MHV were conducted. Platelet shear histories were calculated along pertinent turbulent platelet trajectories, and the effect of a misaligned valve on platelet activation was quantified and compared to that of an aligned valve. It demonstrated that the combination of a tilted valve and subannularly sutured pledgets had an explicit detrimental effect on platelet activation, with the following entrapment of the platelets within the shed vortices of the wake leading to a significant increase of the thromboembolic potential of the valve. This numerical model depicted a viable course for free emboli formation, and indicated how the implantation technique may enhance the risk of cardioembolism.

Modelling the time-dependent rheological behavior of semisolid foodstuffs

Many of food products exhibited the thixotropic behavior, in which, the apparent viscosity of material decreases with time of shearing at constant shear rate. The structural kinetic model (SKM) was used to characterize the thixotropic behavior of three different kinds of food products. Foods selected for analysis represent the fluid and semisolid food materials. They include milled sesame, concentrated yogurt and mayonnaise. The SKM postulates that the change in the rheological behavior is associated with shear-induced breakdown of the internal structure of the food product. This model for the structure decay with time at constant shear rate assumes nth order kinetics for the decay of the material structure with a rate constant, k. The dependence of the degree and the extent of thixotropy of the materials on the temperature, composition and shear history of the food product was determined.

Development of a Portable Online Rheometer for the Characterization of PVC Melts

AbstractPoly(vinyl chloride) (PVC) is a standard polymer with a world‐wide sales quantity of more than 20 Mio. t. From a rheological point of view, PVC is much more difficult to classify than for example polyethylene. Standard thermoplastics can be plasticized by the input of thermal energy, whereas for a sufficient plastification of PVC shear energy is needed. Hence, the PVC melt properties measured with the help of standard lab‐scale rheometers are not taking the shear history into account. Therefore, the melt behavior within an extruder cannot be described. In addition, PVC melts show wall slipping effects. A new rheometer concept especially developed to characterize PVC melt is presented. The rheometer takes a representative melt stream between the screw tip and the inflow of the die. The pressure gradient of this melt stream is measured within the geometry of the rheometer. Due to this the shear history and the wall slipping effects can be accounted for. Experimental data and results are presented and the application possibilities such as the design of profile dies and the development of PVC recipes are discussed. The portable online rheometer in the assembled state.magnified imageThe portable online rheometer in the assembled state.

Phase separation in a sheared gelatin/maltodextrin mixture studied by small-angle light scattering.

The influence of shear on the structure of a gelatin/maltodextrin mixture was investigated using small-angle light scattering both during phase separation and after phase separation was allowed to occur quiescently. In all cases, phase separation occurred via spinodal decomposition to form a droplet morphology, and a characteristic length scale was formed in the structure that was prevalent during shear, as well as in quiescent conditions. Below the critical shear rate for droplet breakup, shear accelerated the coarsening rate of the droplets. A transient regime of rapid hydrodynamic coarsening was present when shear was initiated after phase separation and at late times in all cases once the droplets attained a certain size. At the critical shear rate for droplet breakup (1 s(-1)), the rapid repetition of breakup and coarsening was postulated to occur, which enabled a microstructure consisting of elongated droplets with a narrow size distribution to form. When the shear rate enabled droplets to extend to such an extent that a percolated structure could form (10 s(-1)), then the structure was relatively stable and changed very slowly over time. At very high shear rates (100 s(-1)), droplet breakup was suppressed and a highly fibrillar morphology formed that was stable only while the system was under shear. Cessation of shear at high rates led to fiber breakup and the formation of many small droplets. For a given shear rate, the final microstructure appeared to be independent of the time that shear was started when the structure consisted of discrete droplets or fibers. When a percolated structure could form, however, the shear history appeared to be important.

Viscometric characterisation of cement grout for prediction of prepacked concrete construction

The ability of cement grout to penetrate voids of aggregate media, during the injection process of prepacked concrete construction, is strongly dependent on its viscosity. This paper reports experimental work on the viscometric characteristics of cement grout for the prediction of prepacked grouting process. Various factors influencing the measured results have been identified. Shear history dependent viscosity of cement grout requires that an experimental programme should be designed: (a) to ensure repeatability; and (b) to obtain meaningful results for the concerned construction process. Due consideration has been given to establish proper mixing and measuring programmes. Results were obtained through a set of carefully selected parameters to simulate the shear history during the grouting process. The Bingham model was adopted to characterise the shear rate dependency of the apparent viscosity. Time dependency of the Bingham parameters was obtained from a series of tests of the same batch under the specified initial mixing condition and subsequent agitation. Thixotropy loop tests were also carried out to analyse the sensitivity of the internal structure of cement grout to further shearing, thus comparing the mixing effect on rheological property of cement grout.

Viscometric characterisation of cement grout for prediction of prepacked concrete construction

The ability of cement grout to penetrate voids of aggregate media, during the injection process of prepacked concrete construction, is strongly dependent on its viscosity. This paper reports experimental work on the viscometric characteristics of cement grout for the prediction of prepacked grouting process. Various factors influencing the measured results have been identified. Shear history dependent viscosity of cement grout requires that an experimental programme should be designed: (a) to ensure repeatability; and (b) to obtain meaningful results for the concerned construction process. Due consideration has been given to establish proper mixing and measuring programmes. Results were obtained through a set of carefully selected parameters to simulate the shear history during the grouting process. The Bingham model was adopted to characterise the shear rate dependency of the apparent viscosity. Time dependency of the Bingham parameters was obtained from a series of tests of the same batch under the specified initial mixing condition and subsequent agitation. Thixotropy loop tests were also carried out to analyse the sensitivity of the internal structure of cement grout to further shearing, thus comparing the mixing effect on rheological property of cement grout.

Post-failure mobility of saturated sands in undrained load-controlled ring shear tests

The undrained shear behavior of soils with progress of shear displacement is essential to the understanding of liquefied slope failures with long travel distance. In this paper, using a newly developed ring shear apparatus, a series of ring-shear tests were conducted on a silty sand to examine the undrained behavior of sand subjected to long shear displacement. Based on the test results, the undrained shear behavior of sands with a wide range of densities is discussed. A very low effective stress corresponding to liquefaction was observed at the steady state in all of the tests on loose, medium, as well as dense sand. The effects of stress state and shear history on the undrained shear behavior were examined by performing tests on a sample with different initial stress states and shearing the same specimen repeatedly (three times) at each initial stress state. The tests at different initial stress states proved that the initial stress state has an influence on static liquefaction resistance but has no effect on the steady-state shear strength. Repeated shear tests on the same specimen showed that with increasing shear times, both the peak shear strength and the steady-state shear strength for each specimen became greater. Detailed examination of the shear deformation revealed that the liquefaction phenomena in ring shear tests are localized in the shear zone, irrespective of the initial state of the sand. Grain crushing within the shear zone was examined. Finally, it was found that there was an optimal density at which the undrained brittleness index had a minimum value; meanwhile, the undrained brittleness index became greater with increasing initial normal and shear stresses, but decreased with shear times. These findings offer some basic understanding in assessing the postfailure mobility in landslides.Key words: excess pore pressure, localized liquefaction, shear resistance, ring-shear tests, grain crushing, silty sands.