Shear History Research Articles

Preparation and characterisation of cellulose nanofibres

Two different procedures were compared for the preparation of cellulose nanofibres from flax and microcrystalline cellulose (MCC). The first involved a combination of high energy ball milling, acid hydrolysis and ultrasound, whilst the second employed a high pressure homogenisation technique, with and without various pre-treatments of the fibrous feedstock. The geometry and microstructure of the cellulose nanofibres were observed by SEM and TEM and their particle size measured using image analysis and dynamic light scattering. Aspect ratios of nanofibres made by microfluidisation were orders of magnitude greater than those achieved by acid hydrolysis. FTIR, XRD and TGA were used to characterise changes to chemical functionality, cellulose crystallinity and thermal stability resulting from the approaches used for preparing the cellulose nanofibres. Hydrolysis using sulphuric acid gave rise to esterification of the cellulose nanofibres, a decrease in crystallinity with MCC, but an increase with flax, together with an overall reduction in thermal stability. Increased shear history of flax subjected to multiple passes through the microfluidiser, raised both cellulose nanofibril crystallinity and thermal stability, the latter being strongly influenced by acid, alkaline and, most markedly, silane pretreatment.

Thixotropy, yielding and ultrasonic Doppler velocimetry in pulp fibre suspensions

This paper reports thixotropy in concentrated pulp fibre suspensions and studies their transient flow behaviour using conventional rheometry coupled with a velocimetry technique. Specifically, an ultrasonic Doppler velocimeter is used in conjunction with a rate-controlled rheometer to deduce the local velocity profiles of pulp fibre suspensions. Pulp suspensions are found to exhibit a plateau in their flow curves where a slight increase in the shear stress generates a jump in the corresponding shear rate, implying the occurrence of shear banding. The velocity profiles were found to be discontinuous in the vicinity of the yielding radius where the Herschel–Bulkley model failed to predict the flow. Shear history and the time of rest prior to the measurement were found to play a significant role on the rheology and the local velocity profiles of pulp suspensions.

Rheology of carbon black suspensions. V. Heat-induced gelation and its reversibility

Linear viscoelastic properties of carbon black (CB) suspensions in a mixture of a rosin-modified phenol resin-type varnish (Varnish-1)/an alkyd resin-type varnish (Varnish-2), which exhibited a sol–gel transition on an increase in CB concentration, were investigated from 30°C to 80°C. The viscoelastic properties were reversible from 30°C to 60°C. In contrast, at temperatures above 60°C, the storage (G′) and loss (G″) moduli were irreversible and increased significantly with increasing temperature. This increase in the moduli is due to a change of the dispersion state to agglomerated state by heating. The agglomerated state was held, when the suspensions were lowered at 30°C. However, the G′ and G″ recovered to the original values upon shearing. This heat-induced gelation should be a universal feature for suspensions of weakly attractive particles. The temperature and shearing histories of the suspensions were discussed in relation to adsorption of polymeric component in the varnish on the CB particles.

Effects of humic acid on physical and hydrodynamic properties of kaolin flocs by particle image velocimetry

The physical and hydrodynamic properties of kaolin flocs including floc size, strength, regrowth, fractal structure and settling velocity were investigated by in situ particle image velocimetry technique at different humic acid concentration. Jar-test experimental results showed that the adsorbed humic acid had a significant influence on the coagulation process for alum and ferric chloride. Kaolin flocs formed with the ferric chloride were larger and stronger than those for alum at same humic acid concentration. Floc strength and regrowth were estimated by strength factor and recovery factor at different humic acid concentration. It was found that the increased humic acid concentration had a slight influence on the strength of kaolin flocs and resulted in much worse floc regrowth. In addition, the floc regrowth after breakage depended on the shear history and coagulants under investigation. The changes in fractal structure recorded continuously by in situ particle image velocimetry technique during the growth-breakage-regrowth processes provided a supporting information that the kaolin flocs exhibited a multilevel structure. It was proved that the increased humic acid concentration resulted in decrease in mass fractal dimension of kaolin flocs and consequently worse sedimentation performance through free-settling and microbalance techniques.

Thixotropic rheology of concentrated alumina colloidal gels for solid freeform fabrication

An engineering model for the thixotropic rheology of alumina (Al2O3) concentrated colloidal gels is proposed and compared to rheological experimental results. Concentrated colloidal gels used in extrusion-based solid freeform fabrication are often modeled as simple shear-thinning fluids with finite yield stress such that their rheological properties depend only on instantaneous shear rate and not on shear history. Although this is a satisfactory view for steady state flow, it lacks the detail to consider phenomena associated with transient processes such as die entry and exit in extrusion processes or the shape evolution of an extruded filament as it comes to rest in a low shear environment. The rheological model proposed here considers that both elastic and viscous properties of an aqueous Al2O3 gel vary with microstructure changes of colloidal aggregates and that the aggregate structure is a function of both shear rate and time. A first-order structural kinetics assumption is used to quantify the microstructure evolution of the gel network during shear flow, where shear-induced attrition and diffusion limited aggregation are the rate limiting mechanisms. A constitutive flow equation is developed by incorporating the structural kinetics to describe unsteady shear-thinning behavior. The data collected through shear rate step-change measurements are used to determine 12 model parameters in the proposed rheological model. The model is compared and validated through hysteresis-loop experiments with satisfactory agreement. This model is quasiempirical in the sense that details of interparticle pair-potentials are not used in the formulation. Nevertheless, the model could have practical uses in simulation of the macroscopic flow behavior of colloidal gels during extrusion processes. Specifically, the model offers predictions of elastic and viscous property evolution of colloidal gels as a function of shear history.

Influence of bentonite clay on the rheological behaviour of fresh mortars

Fine mineral additives are often used in the formulation of ready-mix mortars as thickeners and thixotropic agents. Yet, these attributed fresh state properties are not clearly defined from the rheological point of view. In the present study, we consider the influence of bentonite (montmorillonite-based clay mineral) on the rheological behaviour of mortars, including in particular creep and thixotropy. The mortar pastes are subjected to different shear-rates and then allowed to creep under fixed shear stresses until reaching steady state, which corresponds to either rest if the applied stress is smaller than the yield stress or permanent flow otherwise. The evolution of the creep strain is investigated depending on shear history for different contents of bentonite. The microstructure rebuilding kinetics after shear (thixotropy) is considered by analysing the temporal evolution of the creep strain for different applied shear stresses (lower than the yield stress). As expected, bentonite is found to enhance the mortar creep (or sag) resistance. This enhancement consists of both an increase of the yield stress recovered after shear, and a diminution of the characteristic time for yield stress recovery (related to microstructure rebuilding).

A new fluid model for particles settling in a viscoplastic fluid

The study of the settling behaviour of particles in viscoplastic fluids is closely related to the study of rheology. In this paper, a thorough examination of the flow behaviour of viscoplastic fluids, in the form of aqueous polyacrylamide solutions, has been presented. The results of this study suggest that the experimental fluids exhibit time-dependent flow characteristics, where the apparent viscosity of the solutions depends highly on their shear history. This time dependency has been attributed towards the processes of destruction and rejuvenation in the ‘structural network’ of the fluids (due to the presence of hydrogen bonding between polyacrylamide and water molecules), as they are subjected to changing rates of shear. A new fluid model was thus developed to capture this flow behaviour. This model, termed as ‘semi-viscoplastic’, features temporary yield stress characteristics that tend to dissipate once the structural network of the fluid is destroyed due to the application of shear. The time dependency of the fluid viscous parameters becomes apparent in the settling sphere experiment, where it has been demonstrated that a sphere that is following the flow path of another sphere tends to attain a fall velocity that is significantly higher than the preceding sphere. Based on this finding, a new generalised correlation has been developed, through which predictions of the fall velocity of spherical particles settling through viscoplastic fluids, of various shear history, can be made.

Wall shear over high degree stenoses pertinent to atherothrombosis

Atherothrombosis can induce acute myocardial infarction and stroke by progressive stenosis of a blood vessel lumen to full occlusion. Since thrombus formation and embolization may be shear-dependent, we quantify the magnitude of shear rates in idealized severely stenotic coronary arteries (≥75% by diameter) using computational fluid dynamics to characterize the shear environment that may exist during atherothrombosis. Maximum shear rates in severe short stenoses were found to exceed 250,000 s −1 (9500 dynes/cm 2) and can reach a peak value of 425,000 s −1 for a 98% stenosis. These high shear rates exceed typical shear used for in vitro blood flow experiments by an order of magnitude, indicating the need to examine thrombosis at very high shear rates. Pulsatility and stenosis eccentricity were found to have minor effects on the maximum wall shear rates in severe stenoses. In contrast, increases in the stenosis length reduced the maximum shear to 107,000 s −1 (98% stenosis), while surface roughness could increase focal wall shear rates to a value reaching 610,000 s −1 (90% stenosis). The “shear histories” of circulating platelets in these stenoses are far below reported activation thresholds. Platelets may be required to form bonds in 5 μs and resist shear forces reaching 8000 pN per platelet. Arterial thrombosis occurs in the face of pathological high shear stress, creating rapid and strong bonds without prior activation of circulating platelets.

Numerical modeling of stress–strain behavior of sand under cyclic loading

Analysis of soil response to cyclic loading is a complex problem due to the participation of a large number of loading conditions and physical properties. In this study, a promising variant of genetic programming (GP), namely multi expression programming (MEP) is utilized to model stress–strain behavior of sands subjected to large amplitude regular cyclic loading. Generalized MEP-based formulations are derived to simulate hysteresis strain–stress curves. To construct the hysteresis curves, functional relationships are obtained between maximum shear stress at different loading cycles and several influencing parameters. New constitutive correlations are further developed for the assessment of damping ratio and shear modulus. The proposed correlations are established based on several drained strain-controlled cyclic torsional simple shear tests performed on samples of Toyoura sand. The correlations take into account the effects of initial effective confining pressure, initial relative density, initial anisotropic stress state, strain amplitude, and shear history. A subsequent parametric study is carried out and the obtained trends are confirmed with the experimental study results. For more validity verification, the MEP-based correlations are applied to simulate the stress–strain curves of a part of the laboratory results beyond the training data domain. The proposed models are effectively capable of capturing the complex hysteresis behavior of sandy soils under cyclic loading conditions. The derived correlations are particularly valuable for providing an analysis tool accessible to practicing engineers.

The Variation of the Gel Point of Daqing Crude Oil with Pipelining History

Daqing crude oil is a typical waxy crude oil with a wax content of 26.6 wt% and a gel point of 32°C. In order to conduct the safety analysis of pipeline operation, the variation of gel point of Daqing crude oil with thermal and shear history was studied through pipelining simulation experiments. The experiments were performed under different annual throughputs and shear rates of pipeline flow. The results suggest that the change of shear action has little effect on the gel point of crude oil. As the final dynamic cooling temperature is increased, the change of gel point experiences two stages, staying constant at first and then decreasing. Through 13 groups of pipelining simulation experiments, the effects of heating temperature ranging from 45°C to 65°C on gel point of Daqing crude oil were studied. It has been indicated that the gel point increases as the heating temperature is increased under the same final dynamic cooling temperature. The relation equation was constructed to be used to predict the gel point under certain heating temperatures and final dynamic cooling temperature. The average absolute deviation of 52 predicted values of gel point is 0.58°C.

Rheological properties of gum arabic solution: From Newtonianism to thixotropy

The influence of molecular association on the rheological behavior of gum arabic solution was studied. Using time-dependence of transient shear stress a reversible molecular association could be arrested and the thixotropy of gum arabic solution was interpreted. The structural breakdown, buildup and the effect of shear history due to molecular association could be related using the Casson’s equation. Stress jump measurements distinguished the effect of molecular association on the elastic and viscous contributions of shear stress. An elastic-like contribution dominated at low shear rates whereas the viscous component became important at high shear rates. The elastic contribution was strongly affected by the molecular association of gum arabic, with increased molecular association contributing to a larger elastic contribution. The elastic and viscous contributions were also investigated in transient flow. Both of the contributions increased with rest time, indicating a buildup of molecular association. With increasing the rest time the elastic contribution increased much faster than the viscous contribution to the total stress.

Influence of filler content and processing parameter on the crystallization behaviour of PP/kaolin composites

Kaolin-filled polypropylene (PP) composites with various kaolin content, processing temperature and shear histories were compounded using a heated two roll-mill. Prior to thermal analysis, the samples were subjected to extrusion process via capillary rheometer. The influences of kaolin content, processing temperature and shear stress on crystallization of all samples, including isothermal and nonisothermal crystallization behaviour were investigated by differential scanning calorimetry (DSC). The results showed that the increasing kaolin content, processing temperature and shear stress have shifted the crystallization exothermic peak to higher temperature and reduced the overall crystallization time.

EFFECTS OF DIFFERENT PEKMEZ (FRUIT MOLASSES) TYPES USED AS A NATURAL SUGAR SOURCE ON THE BATTER RHEOLOGY AND PHYSICAL PROPERTIES OF CAKES

ABSTRACT Five different kinds of fruit pekmez (mulberry, apricot, elecampane, grape and watermelon) were used in cake formulations as a natural sugar source. The rheologic properties of the batters and the physical and sensory properties of the cakes were investigated. Batter viscosity was measured at 15, 18, 21 and 24C using the Brookfield‐type rotational viscometer. The power law model appeared to be suitable for describing the flow behavior of the cake batters containing pekmez as indicated by high r2 values. n and k values ranged from 0.2528 (elecampane) to 0.5930 (mulberry), 124.38 (elecampane) and 13.42 (grape) at 24°C, respectively. However, the consistency index (k) of all samples decreased as temperature increased. The cake batters containing pekmez exhibited a pseudoplastic behavior. The replacement of sugar with pekmez in the cake formulations decreased the cake volumes. Elecampane bush (andız) pekmez gave the most reddish (a) batter color. The others decreased the L value and increased the a value of the batters. The cakes with watermelon were preferred by the panelists in terms of texture, odor, pore structure, wetness and overall acceptability.PRACTICAL APPLICATIONSFruit molasses (pekmez) is a very important food in human nutrition because of its high sugar, especially monosaccharides such as glucose and fructose, mineral and organic acid content. Therefore, pekmezes enhance nutritional properties and shelf life, and provide the desired flavor and color in cakes and other baked products. So, they can be used in the formulations of food products like cakes as a sweetener instead of sucrose. However, different food ingredients affect the rheology of food materials. Pekmezes affect the rheologic, physical and sensory properties of cakes and their batter. Rheologic, physical and sensorial properties are valuable information in product development. The rheologic properties of fluid foods are complex and depend on many factors such as the composition, shear rate, duration of shearing, and previous thermal and shear histories. The present study was conducted to determine the rheologic, physical and sensory properties of cakes formulated with different types of fruit pekmez in substitution for sucrose.

Liver transplantation and portal hypertension: shear stress theory and history of management for small-for-size graft

Liver transplantation and portal hypertension: shear stress theory and history of management for small-for-size graft

Liquid-to-solid transition of concentrated suspensions under complex transient shear histories

Suspensions containing noncolloidal aluminum particles in a Newtonian carrier liquid and the effects of shear history on the rheological properties of suspensions are investigated showing a critical concentration phi(c), which is close to the "freezing" packing fraction of spherical particle suspensions. An apparent liquid-to-solid transition was clearly found for suspensions below phi(c) via transient step shear after long shear history as well as via the large amplitude oscillatory shear flow with controlled shear stress. The microstructures or the local flow characteristics of suspensions with phi<phi(c) and with phi>phi(c) are still different according to the Fourier Transform Rheological analysis. Although suspensions with phi<phi(c) show solidlike behaviors after long shear history, the flow behaviors keep unchanged as shear stress increases, and typically show solidlike, liquid-solid coexistence, liquidlike, and jamming behaviors, successively. However, liquidlike regime of concentrated suspensions with phi>phi(c) disappears after preshear.

Implications for Proterozoic plate margin evolution from geophysical analysis and crustal-scale modeling within the western Gawler Craton, Australia

Complexly deformed late Archean to Mesoproterozoic rocks of the western Gawler Craton are poorly exposed, thus reducing the ability to extract meaningful geological, structural and tectonic information. This study focuses on shear zone analysis via a constrained integration of qualitative aeromagnetic and Bouguer gravity data interpretation with quantitative two-dimensional forward modeling. This integration provides a means to interpret the crustal architecture as well as constrain a sequence of events, which aid the interpretation of a tectonic history for the western Gawler Craton. Results indicate a polyphase shearing history dominated by a crustal-scale array of predominantly west-dipping shear zones. The initiation of ∼ east–west trending SZ 1 structures was likely coincident with ca. 1750–1720 Ma crustal extension, high heat flow and deposition on thinned areas of Archean crust with the development of a dense, possibly underplated lower crustal component. SZ 1-R2 dextral transpressional movements reactivated SZ 1 during the ca. 1720–1670 Ma Kimban Orogeny, which is interpreted to record collision between the Gawler Craton and the North Australian Craton. During this event basins marginal to, and in the interior of the Archean crustal block of the western Gawler Craton were inverted and the dense lower crust was vertically offset, causing some of the long wavelength (∼ 30–50 km) Bouguer gravity anomalies presently observed. Shear zone activity is constrained until ca. 1680 Ma during which time subduction rollback established a new arc position at the southern margin of the Archean crust with magmatism persisting until ca. 1670 Ma. SZ 3 structures overprint and offset the interpreted axis of the Kimban Orogeny within the Archean crust. These structures record a major phase of crustal-scale sinistral strike-slip movement associated with ∼ north–south shortening between ca. 1630 and 1540 Ma prior to their reactivation at ca. 1450 Ma. This SZ 3-R4 reactivation is interpreted to cause southwest-directed crustal transport, which was focused as west-side-up movements internal to the Archean continent, whereas dextral strike-slip was focused at a major rheological boundary between thick Archean crust to the south and thick Paleoproterozoic basins to the north.

Change of Yield Stress of Daqing Crude Oil With Thermal and Shear History

Daqing crude oil is a typical waxy crude oil with a wax content of 26.6 wt.% and a gel point of 32°C. The yield stress of waxy crude oils is an important property in the process calculation and flow safety analysis for heated oil pipeline. The Daqing crude oil was studied and the yield stress change with shear history was explored through simulation experiment of pipelining. It was found that the change of throughput variation or shear rate showed little effect on yield stress. There existed a peak yield stress on the relation curve of yield stress with final dynamic cooling temperature. The prediction model of yield stress for waxy crude oil under the condition of shutdown was developed and can be used to predict the yield stress of Daqing crude oil at certain heating temperature, final dynamic cooling temperature, and measurement temperature. For the 139 groups of yield stress data of Daqing crude oil from the simulation experiment of pipelining, the result of prediction with this model showed that the average relative deviation between the yield stress measured and predicted is 30.27%, and the coefficient of correlation is 0.9623.

Rheological properties of PDMS/clay nanocomposites and their sensitivity to microstructure

The effect of microstructure on the rheology of clay/polymer nanocomposites is investigated using dispersions of organically treated clay in nearly Newtonian poly(dimethylsiloxane). Degree of dispersion and floc size are altered by using two different dispersion procedures and by changing the shear history. The scaling for dynamic moduli of attractive colloids applies, except for a possible relaxation mechanism at very low frequencies. The time to reach the crossover at a given frequency is found to be extremely sensitive to the dispersion procedure used. Hydrodynamic and elastic components of the steady state stress, on the other hand, evolve in a very similar fashion for the different systems. Although the relaxation times of the elastic stress components change drastically with flow-induced changes in structure, the dispersion process hardly has an effect at all. Intermittent start-up flows in the forward and reverse directions show that anisotropy persists long after the flow has been arrested, even at shear rates where no large reversible flocs are present. The degree of dispersion only had a limited effect on the anisotropy. Finally, the effect of shear on structure recovery has been studied. Very low shear rates are found to increase the rate of recovery, even for small strains.

Effects of shear and electrical properties on flow characteristics of pharmaceutical blends

AbstractThis article examines the effects and interactions of shear rate, shear strain on electrical and flow properties of pharmaceutical blends. An unexpectedly strong relation between the flow and passive electrical properties of powders is observed to depend on the shear history of the powder bed. Charge density, impedance, dielectrophoresis, flow index, and dilation were measured for several pharmaceutical blends after they were subjected to a controlled shear environment. It was found that the increase in the shear strain intensified the electrical properties for blends that did not contain MgSt. The opposite effect was found in blends lubricated with MgSt. Different shear conditions resulted in different correlations between flow index and dilation. Flow properties of powders were found to improve with continuous exposure to shear strain. It was also found that flow properties correlated to charge acquisition and impedance for different shear treatments. © 2009 American Institute of Chemical Engineers AIChE J, 2010

Population balance modelling of protein precipitates exhibiting turbulent flow through a circular pipe

Abstract The breakage of liquid–liquid, solid–liquid and solid–gas dispersions occurs in many industrial processes during the transport of particulate materials. In this work, breakage of whey protein precipitates passing through a capillary pipe is examined and an experimentally derived breakage frequency is applied to construct a suitable population balance model to characterize the breakage process. It has been shown that the breakage frequency of precipitate particles is highly dependent on their shear history and on the turbulent energy dissipation rate in the pipe. The population balance equation (PBE) uses a volume density based discrete method which is adapted from mass density based discretization. In addition to comparing the model with experimental data, predicted results at different velocities are presented. It was found that the population balance breakage model provides satisfactory results in terms of predicting particle size distributions for such processes.