Showing papers in "Rheologica Acta in 2015"

Approximation of the inverse Langevin function revisited

Abstract: The main purpose of this paper is to provide an easy-to-use approximation formula for the inverse Langevin function. The mathematical complexity of this function makes it unfeasible for an analytical manipulation and inconvenient for computer simulation. This situation has motivated a series of papers directed on its approximation. The best known solution is given by Cohen. It is used in a lot of statistically based models of rubber-like materials. The formula is derived from rounded Pade approximation [3/2]. The main idea of the presented approach in this paper relies on improvement of the precision of approximation formula for the inverse Langevin function by using multipoint Pade approximation method. We focused our study strongly on obtaining a simple and accurate approximation. It is assumed that the proposed approximation formula may be considered a useful tool for verification of the results obtained in other ways. Our results are supported by investigating several numerical examples. The paper also presents a few applications of computer software named Mathematica which can be used to calculate symbolically one point Pade approximants and numerically multipoint Pade approximants. Using this software, we showed also how to compute higher order derivatives of the inverse function in a simple and elegant way. This issue was discussed by Itskov et al.

A generalized equation for rheology of emulsions and suspensions of deformable particles subjected to simple shear at low Reynolds number

Abstract: We present analyses to provide a generalized rheological equation for suspensions and emulsions of non-Brownian particles. These multiparticle systems are subjected to a steady straining flow at low Reynolds number. We first consider the effect of a single deformable fluid particle on the ambient velocity and stress fields to constrain the rheological behavior of dilute mixtures. In the homogenization process, we introduce a first volume correction by considering a finite domain for the incompressible matrix. We then extend the solution for the rheology of concentrated system using an incremental differential method operating in a fixed and finite volume, where we account for the effective volume of particles through a crowding factor. This approach provides a self-consistent method to approximate hydrodynamic interactions between bubbles, droplets, or solid particles in concentrated systems. The resultant non-linear model predicts the relative viscosity over particle volume fractions ranging from dilute to the the random close packing in the limit of small deformation (capillary or Weissenberg numbers) for any viscosity ratio between the dispersed and continuous phases. The predictions from our model are tested against published datasets and other constitutive equations over different ranges of viscosity ratio, volume fraction, and shear rate. These comparisons show that our model, is in excellent agreement with published datasets. Moreover, comparisons with experimental data show that the model performs very well when extrapolated to high capillary numbers (C a≫1). We also predict the existence of two dimensionless numbers; a critical viscosity ratio and critical capillary numbers that characterize transitions in the macroscopic rheological behavior of emulsions. Finally, we present a regime diagram in terms of the viscosity ratio and capillary number that constrains conditions where emulsions behave like Newtonian or Non-Newtonian fluids.

Experimental investigation on viscosity of water-based Al2O3 and TiO2 nanofluids

Abstract: This article investigates the influence of temperature, concentration, and size of nanoparticles, and addition of surfactants on dynamic viscosity of water-based nanofluids containing alumina (Al2O ...

Large amplitude oscillatory shear rheology of three different shear-thickening particle dispersions

Abstract: We present a large amplitude oscillatory shear rheology (LAOS) investigation of three different shear-thickening particle dispersions - fumed silica in polyethylene oxide (FLOC), fumed silica in polypropylene glycol (HydroC), and cornstarch in water (JAM). These systems shear-thicken by three different mechanisms - shear-induced formation of particle clusters flocculated by polymer bridging, hydrocluster formation, and jamming. The viscoelastic non-linearities of the three fluids were studied as a function of strain and strain-rate space through the use of Lissajous-Bowditch curves and local nonlinear viscoelastic moduli of an oscillatory shear cycle. The nonlinear behaviors of the three fluids were compared and contrasted to understand the nonlinear shear-thickening mechanism of each. Both HydroC and JAM dispersions were found to exhibit strong strain stiffening of the elastic moduli and strain thickening of the loss moduli behavior associated with possible hydrocluster formation and particle jamming. However, the FLOC dispersion, in contrast, showed strong strain softening and strain thinning behavior at large strain amplitudes associated with yielding of the microstructure. The expected thickening of the loss modulus of FLOC in LAOS with increasing strain was not observed even though viscosity of FLOC was found to shear-thicken in steady-shear measurements. This disagreement is likely due to very large strain amplitudes required for shear-thickening to occur by shear-induced polymer bridging mechanism. The hypothesis was confirmed through stress growth experiments. Conversely, the HydroC and JAM dispersions required relatively small applied strains for shear-thickening to occur by hydrocluster and jamming mechanism. The comparison of local intra-cycle nonlinearity through Lissajous-Bowditch plots and nonlinear viscoelastic parameters indicated that the elastic nonlinearities of all three systems are primarily driven by a strong dependence on the magnitude of the applied strain-rates within an oscillatory cycle rather than the amplitude of the applied strain. A close inspection of the LAOS data reveals strong differences in the viscoelastic nonlinearities of these three different shear-thickening dispersions which can be used to create a nonlinear rheological fingerprint for each and offers valuable new insights into the nonlinear dynamics associated with each of the shear-thickening mechanisms.

Analytical solutions for the flow of Carreau and Cross fluids in circular pipes and thin slits

Abstract: In this paper, analytical expressions correlating the volumetric flow rate to the pressure drop are derived for the flow of Carreau and Cross fluids through straight rigid circular uniform pipes and long thin uniform plane slits. The derivation is based on the application of Weissenberg-Rabinowitsch-Mooney-Schofield (WRMS) method to obtain flow solutions for generalized Newtonian fluids through pipes and our adaptation of this method to the flow through slits. The derived expressions are validated by comparing their solutions to the solutions obtained from direct numerical integration. They are also validated by comparison to the solutions obtained from the variational method which we proposed previously. In all the investigated cases, the three methods agree very well. The agreement with the variational method also lends more support to this method and to the variational principle which the method is based upon. We also compared the derived analytical solutions of Carreau and Cross fluids to the analytical solutions of power law fluids with comparable rheology and observed logical trends in the relation between the corresponding flow rates as a function of the applied pressure field.

Rheological investigation of pectin-based emulsion gels for pharmaceutical and cosmetic uses

Abstract: Emulsion gels are structured emulsions suitable for different uses for their specific behaviour, which is strongly dependent on the characteristics of the gelled dispersing phase. Therefore, it is important to adopt the specific gelling agent to tune the final emulsion rheological behaviour properly. Pectin is extremely interesting among potential hydrophilic gelling agents owing to its specific characteristics. In the present work, four different low-methoxyl pectins were adopted to prepare gels to be used as the dispersing phase in cosmetic or pharmaceutical emulsion gels. The rheological characterisation of pectin gels, prepared at room temperature to avoid the damage to potential thermolabile components, was carried out with small amplitude oscillations. The obtained gels were used, together with a common non-ionic surfactant (Tween 60), to prepare olive oil emulsion gels suitable to design new cosmetic products. A simple empirical model, proposed to relate the emulsion complex modulus to the oil fraction and properties of the dispersing phase, has shown itself to be a potentially useful tool to design formulations with desired properties.

Mechanisms of onset for moderate Mach number instabilities of viscoelastic flows around confined cylinders

Abstract: An examination of the upstream and downstream instabilities that occur for highly confined and bounded viscoelastic flows around a cylinder is presented. An elasticity number-Mach number (El-Ma) phase space is used to demonstrate that the onset of downstream instabilities at Ma = 1; these instabilities are instigated by the disturbance of the base flow by traveling elastic waves. After the onset of the downstream instability, an upstream instability eventually occurs for Ma crit ~ 10; however, this number is affected by both channel geometry and fluid properties. The upstream instability is instigated by perturbations of the flow field caused by the downstream instability and does not occur until a critical perturbation size at a given velocity is exceeded. The effects of these flow regimes on flow resistance are characterized by examining excess pressure drop; a significant increase in flow resistance is observed at the onset of the upstream instability.

A simple and accurate approximation of the inverse Langevin function

Abstract: The inverse Langevin function cannot be represented in an explicit form and requires an approximation by a series, a non-rational or a rational function as for example by a Pade approximation. In the current paper, an analytical method based on the Pade technique and the multiple point interpolation is presented for the inverse Langevin function. Thus, a new simple and accurate approximation of the inverse Langevin function is obtained. It might be advantageous, for example, for non-Gaussian statistical theory of rubber elasticity where the inverse Langevin function plays an important role.

Critical quantities on the yielding process of waxy crude oils

Abstract: The yielding behavior of waxy crude oils below the gelation temperature is a fundamental aspect for the production in oil basins located in deep water. Under this state, the material exhibits a diversity of complex non-Newtonian features turning the determination of its rheology a challenging task. We performed different tests monitoring the critical stress and strain where a major rupture occurs. We could infer from these experiments a minimum critical stress value that can be associated to the static yield stress of the material. In addition, the material exhibited a remarkably constant critical strain value, turning this last parameter into a more representative fingerprint of the material.

Influence of the initial cooling temperature on the gelation and yield stress of waxy crude oils

Abstract: One of the major problems in waxy crude oil production and transportation is oil gelation that takes place within pipelines as a result of wax crystallization at low temperatures. In such cases, the pressure needed to restart the oil flow in subsea pipelines can be much larger than the usual steady-state pressure, as the temperature in such environment can be as low as 4 °C. The literature has shown that not only the temperature itself but also the fluid shear and thermal histories have significant influence on the yield stress of waxy crude oils. This paper investigates the effect of the initial cooling temperature on the waxy crude oil viscosity, gelation temperature, and yield stress. In order to accomplish that, rheological tests were carried out under static and dynamic cooling conditions. The results show that there is a critical range for the initial cooling temperature that provides maximum values for viscosity, gelation temperature, and yield stress. In other words, the highest values of those properties are observed when the cooling started within this temperature range. The effect of a thermal pretreatment usually used to remove light ends was also investigated. In spite of not changing the initial temperature critical range, the yield stress was slightly affected by the thermal treatment. It is worth noting that the yield stress varies from approximately zero to hundreds after dynamic cooling or to thousands after static cooling within the tested range of the initial cooling temperatures.

Rheology of lime paste—a comparison with cement paste

Abstract: The rheological properties of a suspension of lime in water (lime putty) are studied with the help of creep tests in a wide range of deformations including very small values. The results are compared with those obtained with a cement paste and several similarities between the two systems are observed. It is shown that the apparent yield stress of a lime suspension is the sum of two components: one due to standard reversible colloidal interactions and one due to the formation of a brittle structure associated with the formation of links due to dissolution–precipitation mechanisms. This second component increases with the time of rest as the square root of time, and the corresponding structure irreversibly breaks as soon as some significant deformation has been imposed. We show that similar structures are formed at concentrations between 25 and 34 % (solid volume fraction) and evolve in a similar way when the time is scaled by a factor decreasing with the solid fraction.

An error-minimizing approach to inverse Langevin approximations

Abstract: The inverse Langevin function is an integral component to network models of rubber elasticity with networks assembled using non-Gaussian descriptions of chain statistics. The non-invertibility of the inverse Langevin often requires the implementation of approximations. A variety of approximant forms have been proposed, including series, rational, and trigonometric divided domain functions. In this work, we develop an error-minimizing framework for determining inverse Langevin approximants. This method can be generalized to approximants of arbitrary form, and the approximants produced through the proposed framework represent the error-minimized forms of the particular base function. We applied the error-minimizing approach to Pade approximants, reducing the average and maximum relative errors admitted by the forms of the approximants. The error-minimization technique was extended to improve standard Pade approximants by way of understanding the error admitted by the specific approximant and using error-correcting functions to minimize the residual relative error. Tailored approximants can also be constructed by appreciating the evaluation domain of the application implementing the inverse Langevin function. Using a non-Gaussian, eight-chain network model of rubber elasticity, we show how specifying locations of zero error and reducing the minimization domain can shrink the associated error of the approximant and eliminate numerical discontinuities in stress calculations at small deformations.

Quiescent and shear-induced crystallization of linear and branched polylactides

Abstract: The quiescent and shear-induced isothermal crystallization behavior of linear and long-chain branched (LCB) polylactides (PLAs) was investigated at a temperature of 130 °C. LCB-PLAs were produced by the reaction with a multifunctional chain extender, Joncryl©. In quiescent crystallization, the presence of the LCB structure accelerated the nucleation process and reduced the induction time, depending on the level of branching. The impact of shear strain and shear rate on crystallization was also examined. The shear-induced crystallization of the linear and LCB-PLAs was affected by both the total shear strain and shear rate. The crystallization kinetics of the LCB-PLAs was more affected by shear than that of the linear PLA. The crystalline morphology of the linear and LCB-PLAs under quiescent and step shear rate conditions was examined using a Linkam optical shearing system. An increase in the spherulite density was observed in the strained melt of both linear (33 %) and LCB-PLAs (15 %), in comparison with those of unstrained counterparts. Optical micrographs confirmed that the crystal nucleation was affected by the shear flow. Long-chain branching significantly promoted the nucleation density (6.7 times), although it diminished the crystal growth rate from 4.4 to 2.0 μm/min.

Two-step yielding in surfactant suspension pastes

Abstract: In this work, we investigate two-step yielding behavior of pastes containing anionic surfactants, clay, and abrasive particles of calcite. Such pastes are commercially used as hand dishwash paste. The surfactant suspension pastes have soft solid-like consistency with finite elastic modulus. The total volume fraction of the particulate matter in the same is around 36 %; consequently, we attribute the elastic modulus of the same to the bonding between abrasive particles leading to attractive gel phase. We observe that the surfactant suspension pastes undergo physical aging during which evolution of relaxation time takes place. In dynamic strain sweep experiments, the pastes demonstrate clear signature of two-step yielding. It is observed that during the first yielding event, elastic modulus decreases while viscous modulus shows a maximum. For the second yielding event, however, elastic modulus demonstrates plateau before decreasing. While the first yielding event is observed to remain unaffected by the frequency of oscillations, the plateau values of elastic modulus, before the second yielding process, is observed to increase with frequency. We attribute the first yielding event in surfactant suspension pastes to rupture of network, while the second yielding event to breakage of the aggregates.

Large-amplitude oscillatory shear: comparing parallel-disk with cone-plate flow

Abstract: We compare the ratio of the amplitudes of the third to the first harmonic of the torque, , measured in rotational parallel-disk flow, with the ratio of the corresponding harmonics of the shear stress, |τ 3|/|τ 1|, that would be observed in sliding-plate or cone-plate flow. In other words, we seek a correction factor with which must be multiplied, to get the quantity |τ 3|/|τ 1|, where |τ 3|/|τ 1| is obtained from any simple shearing flow geometry. In this paper, we explore theoretically, the disagreement between and τ 3/τ 1 using the simplest continuum model relevant to large-amplitude oscillatory shear flow: the single relaxation time co-rotational Maxwell model. We focus on the region where the harmonic amplitudes and thus, their ratios, can be fully described with power laws. This gives the expression for , by integrating the explicit analytical solution for the shear stress. In the power law region, we find that, for low Weissenberg numbers, for the third harmonics , and for the fifth harmonics, . We verify these results experimentally. In other words, the heterogeneous flow field of the parallel-disk geometry significantly attenuates the higher harmonics, when compared with the homogeneous, sliding-plate flow. This is because only the outermost part of the sample is subject to the high shear rate amplitude. Furthermore, our expression for the torque in large-amplitude oscillatory parallel-disk flow is also useful for the simplest design of viscous torsional dampers, that is, those incorporating a viscoelastic liquid between two disks.

Nonlinear and linear viscoelastic properties of a novel type of xanthan gum with industrial applications

Abstract: The influence of concentration in the viscoelastic properties of an “advanced performance” xanthan gum was studied by means of both techniques, small amplitude oscillatory shear (SAOS) and large amplitude oscillatory shear (LAOS). G′ and G″, within linear viscoelastic range, were demonstrated to be higher than the ones for the conventional xanthan gum pointing out to a less fluid-like behaviour. The combination of the use of both rheological measurements revealed a regime transition within the concentration range studied. Thus, the deviations from the Cox-Merz rule for the concentrations above 0.20 % (m/m) indicate the occurrence of a more development structure. In addition, the analysis of the influence of concentration on the local shear-thickening parameter (T) obtained by LAOS confirmed the modification of the gum structure.

The effect of premature wall yield on creep testing of strongly flocculated suspensions

Abstract: Measuring yielding in cohesive suspensions is often hampered by slip at measurement surfaces. This paper presents creep data for strongly flocculated suspensions obtained using vane-in-cup tools with differing cup-to-vane diameter ratios. The three suspensions were titania and alumina aggregated at their isoelectric points and polymer-flocculated alumina. The aim was to find the diameter ratio where slip or premature yielding at the cup wall had no effect on the transient behaviour. The large diameter ratio results showed readily understandable material behaviour comprising linear viscoelasticity at low stresses, strain-softening close to yielding, time-dependent yield across a range of stresses and then viscous flow. Tests in small ratio geometries however showed more complex responses. Effects attributed to the cup wall included delayed softening, slip, multiple yielding and stick–slip events, and unsteady flow. The conclusion was that cups have to be relatively large to eliminate wall artefacts. A diameter ratio of three was sufficient in practice, although the minimum ratio must be material dependent.

A hierarchical multi-mode MSF model for long-chain branched polymer melts part I: elongational flow

Abstract: A novel hierarchical multi-mode molecular stress function (HMMSF) model for long-chain branched (LCB) polymer melts is proposed, which implements the basic ideas of (i) the pom-pom model, (ii) hierarchal relaxation, (iii) dynamic dilution and (iv) interchain pressure. Here, the capability of this approach is demonstrated in modelling uniaxial extensional viscosity data of numerous broadly distributed long-chain branched polymer melts with only a single non-linear parameter, the dilution modulus.

A modified elasto-viscoplastic thixotropic model for two commercial gelled waxy crude oils

Abstract: A severe problem to flow assurance occurs when subsea flowlines become blocked with gelled waxy crudes. To design proper surface pump facilities, it is essential to know the minimum pressure required to restart the flow. Simulating and predicting this minimum pressure require the understanding of several physical phenomena, including compressibility, shrinkage, and rheological behavior. This study aims to characterize and simulate the rheological behavior of two commercial waxy crude oils. Based on its survey of the literature, we select the de Souza Mendes and Thompson (2013) model to fit the oil’s behavior and then conduct, using a rheometer, a considerable number of experiments with the selected oils. To verify the solution of our algorithm, we compared our theoretical solutions with some results of the literature. When comparing the simulation with experiments, the model was unable to predict the data perfectly; hence, we propose a modified version without changing the physical meaning of the equations, to improve its predictions. Once any of the empirical parameters were able to influence the elastic behavior in such a way that the shear stress decreased with time, the structural elastic modulus function was modified, which means that the relation of the structure parameter and the storage modulus was modified. One of the interesting results of the analysis is when relating the storage modulus and a new parameter added in the modification, a value was found to be, regardless of the aging time or the oil used, constant.

Large amplitude oscillatory shear (LAOS) in model colloidal suspensions and glasses: frequency dependence

Abstract: We investigate the effect of frequency on the non-linear large amplitude oscillatory shear (LAOS) response of concentrated colloidal suspensions of model soft and hard spheres at various concentrations, both below and above the glass transition. We show that the anharmonic response in the stress increases with frequency for liquid-like samples but decreases with frequency for solid-like samples. We argue that for samples below the glass transition, higher frequencies involving higher maximum shear rates promote shear thinning and increase anharmonicity. On the other hand, solid-like samples deform plastically at low frequencies as they are subjected to low shear rates within the period. Higher frequencies (higher average shear rates) lead to viscous flow over a larger fraction of the period thereby decreasing anharmonic behavior. We also demonstrate that LAOS experiments in strain-controlled rheometry at moderately high frequencies (ω > 5 rad/s) have to be very carefully interpreted, due to the superharmonic instrumental resonance effects.

Mechanical testing of small, thin samples in a humidity-controlled oven

Abstract: A new fixture for the mechanical characterization of thin polymer films under controlled temperature and relative humidity conditions is reported. Novel conducting polymers are often synthesized in small quantities and processed into films on the order of 10–100 microns thick. Standard tensile tests do not allow for adequate testing of these small sample sizes. Hence, a modification of the Sentmanat Extensional Rheometer (SER) to perform tensile testing on thin membranes is presented. Since the standard L-shaped pins do not secure thin polymer films at lower temperatures (i.e., below the melting point), screw down clamps were created to allow for mechanical characterization of solid polymer films. The new testing apparatus allows for mechanical characterization with as little as 2 % of the material needed for testing on a traditional tensile tester. In a parallel effort, a humidity delivery system developed for the TA Instruments ARES-G2 rheometer allows for testing at a range of temperatures (30–100 °C) and relative humidity conditions (0–95 % RH). The novel oven was benchmarked with low density polyethylene and Nafion 115Ⓡ. While the new experiment was built for characterization of ion exchange membranes for fuel cells, the oven is capable of characterizing any environmentally sensitive material using all standard rheometer geometries.

Padé approximants for large-amplitude oscillatory shear flow

Abstract: Analytical solutions for either the shear stress or the normal stress differences in large-amplitude oscillatory shear flow, both for continuum or molecular models, often take the form of the first few terms of a power series in the shear rate amplitude. Here, we explore improving the accuracy of these truncated series by replacing them with ratios of polynomials. Specifically, we examine replacing the truncated series solution for the corotational Maxwell model with its Pade approximants for the shear stress response and for the normal stress differences. We find these Pade approximants to agree closely with the corresponding exact solution, and we learn that with the right approximants, one can nearly eliminate the inaccuracies of the truncated expansions.

Emulsion rheology for steady and oscillatory shear flows at moderate and high viscosity ratio

Abstract: A numerical simulation scheme based on a three-dimensional boundary integral method is used for the purpose of estimating the macroscopic flow from the microscale drop deformation for emulsions of high viscosity ratio under steady, oscillatory shear, and extensional flows. The effects of drop deformation and shear rate are examined for moderate and high viscosity ratio. The accuracy and convergence of the numerical simulations is illustrated by comparing drop deformation and rheology with experimental observations and an asymptotic theory based on a second-order small deformation theory also developed in this work. We also examine an emulsion undergoing a pure extensional flow and an extensional viscosity of the emulsion is computed by the theory and the numerical simulations. In addition, we also explore regimes of linear and nonlinear viscoelasticity when the emulsion is subjected to oscillatory shear for different viscosity ratio. Emulsion Fourier modes are examined for two different viscosity ratio. For a more concentrate emulsion with an unit viscosity ratio, a static shear elastic modulus is computed as a function of drop volume fraction and compared with experimental data. The numerical simulation results are validated against theory and experimental observation of drops in shear and extensional flows. A very good agreement is observed for moderate and high viscosity ratio. The regime of emulsions with relatively high viscosity ratios has not been much explored in the current theoretical and experimental literature. The boundary integral numerical simulations developed in this work has the ability to predict what we expect on the behavior of emulsion with moderate and high viscosity ratio.

Rheology of microgels in single particle confinement

Abstract: In this work, we investigate the shear rheology of Carbopol 981 microgel particle suspensions, confined between shearing plates with gap separations from 5 to 100 μm. We show that even for confining gaps smaller than that of the gel particle size, the yielding of concentrated microgel suspensions is delayed to stress levels above the bulk yield stress. Furthermore, for stresses below this new yield point, slip is described by elastohydrodynamic lubrication theory as long as the direct confinement of the single gel particles between the shearing surfaces is limited to a Hertzian deformation. For a strong, non-Hertzian particle deformation, the slip layer breaks down and leads to a frictional interaction of the single confined particle with the two shearing surfaces, depending on their surface roughness. Lubrication pressures and friction coefficients have been quantified with in situ normal force measurements on the confined particles, which have also been utilized to unambiguously determine the relevant swollen particle dimensions.

Linear stability analysis and nonlinear simulation of non-Newtonian viscous fingering instability in heterogeneous porous media

Abstract: The viscous fingering instability of miscible displacements in the presence of permeability heterogeneity is studied. Shear-thinning fluids are successful to suppress this instability where the mobility ratio is high and/or the heterogeneity of the porous medium is adverse. The displacing fluid is considered a non-Newtonian fluid, and the shear-thinning characters of it have been modeled using the Carreau–Yasuda constitutive equation. Two different heterogeneity models are used in this simulation, one in which the permeability decreases in transverse direction exponentially and the other in which the permeability decreases and increases exponentially in longitudinal direction, separately. In particular, the role of permeability heterogeneities on the differences between Newtonian and non-Newtonian fluids is investigated through linear stability analysis and nonlinear simulation. In nonlinear simulations, using a spectral method based on the Hartley transforms, nonlinear finger interactions, mixing lengths, sweep efficiencies, and transversely average concentration are examined and compared in Newtonian and non-Newtonian displacements.

Self-associations and temperature dependence of aqueous solutions of zwitterionically modified N-isopropylacrylamide copolymers

Abstract: Thermosensitive copolymer solutions are prepared from various molar ratios of N-isopropylacrylamide (NIPAM) and the zwitterionic monomer N-(methacryloxypropyl)-N,N-dimethyl-N-(3-sulfopropyl) ammonium betaine (Zw) by free radical polymerization. In the current study, we examined the rheological properties of poly (NIPAM/sulfobetaine) copolymer solutions. We found that the rheological properties are conceptually linked with the copolymer chemical structure and chain topology. We also report the effects of bulk molar mass and zwitterions on flow curve ɳ ( $$ \overset{\cdot }{\upgamma} $$ ). With the introduction of the charged group in the PNIPAM-chain, its viscoelastic phase behavior and lower critical solution temperature (LCST) are affected. At varying concentrations of zwitterionic copolymer, they showed a shear thinning behavior with two relaxation regimes for entanglement relaxation and zwitterionic interaction. Interestingly, the zwitterionic interactions are related to the molar concentration of zwitterionic monomer. This is eventually associated with the topology of the copolymer chain. Our results also showed that there was a linear increase in the LCST of these solutions as a function of zwitterionic moieties.

Startup shear of a highly entangled polystyrene solution deep into the nonlinear viscoelastic regime

Abstract: We have employed optical particle tracking velocimetry to directly visualize the deformation field of a highly entangled polystyrene/diethyl phthalate (PS/DEP) solution (entanglement density Z = 61) in a parallel plate geometry. To probe the existence of shear banding, startup shear tests were carried out deep into the nonlinear viscoelastic (NLVE) regime. By roughening the surface of the rheometer plates, wall slip was effectively suppressed. In the presence of edge fracture and prior to it, steady-state shear banding was not seen as evidenced by velocity profiles that were linear for most of the shear tests. An exception was for an extremely high Weissenberg number Wi = 255, for which weak transient banding was observed. For the PS/DEP system studied, the results suggest that in the absence of wall slip, shear banding is not a steady-state phenomenon and weak shear banding is observed only in the transient state and this could possibly correlate with the edge fracture effects.

Structural breakdown and recovery of waxy crude oil emulsion gels

Abstract: The structural breakdown and recovery behaviors of waxy crude emulsion gels were investigated. First, the tests of stepwise increase in shear rate and hysteresis loop were carried out, and the structural breakdown process was further analyzed. Then, the structural recovery behaviors were investigated from the recoveries of apparent viscosity, storage modulus, and yield characteristics. It was found that the thixotropy of emulsion gels weakens with increasing water cut and the structural breakdown process gradually changes from solid-like brittle fracture to ductile failure. The broken-down structure of emulsion gels can only recover partially, and both the recovery rate and the recoverability are related to water cut, precipitated wax content, and pre-shear rate. To be specific, the storage modulus recovers faster with increasing water cut and decreasing precipitated wax crystals, or after pre-sheared at a higher rate, while the effects of water cut and precipitated wax on the recovery rate of yield stress are opposite. The recovery degree of both storage modulus and yield stress decreases obviously with increasing amount of wax crystals but is barely influenced by the water cut.

Stick-slip behavior of magnetorheological fluids in simple linear shearing mode

Abstract: The stick-slip motion of magnetorheological (MR) fluids under a simple linear shear mode has been experimentally reported. The critical shear strain for the onset of the stick-slip motion decreases logarithmically with the magnetic flux intensity applied. The rate of energy accumulation and releasing during stick-slip motion changed in a power law of the viscosity of the continuous phase in a range of 0.01~1000 mPa S. Different carrier fluids led to different stick-slip characteristics. The stick-slip motion can be fully suppressed with certain carrier fluids. When the MR fluids are in a shear-thickening state, the critical shear strain of the onset of the stick-slip motion is larger than a normal state. But, the relative spike amplitude of the stick-slip remains similar.

An exact analytical solution for creeping Dean flow of Bingham plastics through curved rectangular ducts

Abstract: In this paper, an exact analytical solution for creeping flow of Bingham plastic fluid passing through curved rectangular ducts is presented for the first time. The closed form of axial velocity distribution, flow resistance ratio, and wall shear stress are derived using bounded Fourier transformation. An extensive investigation on mutual effects of Hedstrom number, curvature ratio, and aspect ratio is conducted. The results indicate that a drag reduction is caused in the flow field by increasing the Hedstrom number. It is shown that unlike the Newtonian creeping Dean flow, the critical aspect ratio (an aspect ratio in which the flow resistance ratio is independent from curvature ratio) does not exist at large enough Hedstrom numbers. Analytical solution also indicated that as Hedstrom number is increased, the value of Poiseuille number is enhanced, and unlike the Newtonian flows, the value of Poiseuille number is not zero at edges of cross section.