Index of content:
Volume 57, Issue 6, November 2013

Surfactant solutions containing wormlike micelles often show complex structural changes in laminar shear flow. These structural changes produce anomalous rheological phenomena in which the shear induced structure (hereafter SIS) increases the viscosity of surfactant solutions. In turbulent shear flow, dilute solutions of 5 nm diameter and micron length wormlike micelles reduce frictional drag. It has been speculated that this drag reduction is related to wormlike micelles or SIS, although to date there have been no studies that have made their direct link to drag reduction in turbulent flow. The mechanism of drag reduction and the role of the micellar structure remain unclear. Here, we identify SIS with a long (several centimeters), threadlike structure. The threads form a gellike phase under shear, which is clearly separated from the surrounding fluid. The threads breakdown at high shear stresses and disappear at the same time as the drag reduction does. The correlation between the existence of surfactant threads and turbulent drag reduction is discussed.

Precision rheometry: Surface tension effects on lowtorque measurements in rotational rheometers
View Description Hide DescriptionFor rotational rheometers, surface tension results in a torque that should not occur in an ideal, rotationally symmetric geometry. This paper identifies and explains the effect, which is due to surface tension and contact line traction forces, and not surface rheology at the liquidair interface. For water, we show that this torque can be more than two orders of magnitude larger than the viscous torque. In steady shear flow, the effect appears as a constant torque independent of rate, which would appear inaccurately as apparent shear thinning of water. In oscillatory tests, this may appear inaccurately as an elastic modulus. This surface tension torque is sensitive to wetting conditions and contact line asymmetries and cannot be deterministically corrected in experimental measurements. It therefore raises the lower bound of the instrument lowtorque limit. The surface tension torque is reduced by maximizing rotational symmetry of the contact line, minimizing evaporation and the migration of the contact line, reducing the radial location of the contact line, and lowering the surface tension. Identifying and eliminating the surface tension torque is critical for low viscosities, intrinsic viscosities, soft materials, subdominant viscoelastic components, small gaps, and any circumstance where the lowtorque limit is experimentally important.

The rheology and processing behavior of starch and gumbased dysphagia thickeners
View Description Hide DescriptionThe addition of a starch or gumbased thickener to patient fluids with dysphagia is commonly carried out, but the mechanism behind the efficacy of this treatment is not fully understood. This paper describes the rheological behavior of two commercially available thickening powders and an additional xanthan gum solution with a view to explaining the efficacy of thickened fluids in terms of their rheology. Both linear viscoelastic and steady shear data were obtained for the fluids together with filament extensional stretch, decay, and breakup data. In order to follow the behavior of the fluids in a processing situation, a mechanical “Cambridge Throat” was designed and tested. The action of the tongue was modeled using a constant torque cam that forced fluid contained within a flexible membrane through a model throat. Movie photography captured images of the fluid behavior and showed that for a constant tongue torque, the transit time within the model throat increased with increasing fluid viscosity, with implications for the time available for the successful function of the larynx, throat muscles, and epiglottis.

Divergence in the low shear viscosity for Brownian hardsphere dispersions: At random close packing or the glass transition?
View Description Hide DescriptionModels, theories, and simulations for the low shear viscosity of Brownian, hardsphere dispersions are examined and evaluated against literature data. Hydrodynamic and thermodynamic contributions are explicitly treated and the volume fraction dependence of the experimental data is validated against the high frequency or high shear viscosity. A good description of the data is realized by a new, semiempirical model based on mode coupling theory, where , supporting dynamical arrest at the ideal glass transition.

Response of elastoviscoplastic materials to large amplitude oscillatory shear flow in the parallelplate and cylindricalCouette geometries
View Description Hide DescriptionMost investigations of large amplitude oscillatory shear (LAOS) rheometry to date have presumed uniform shear. The study of structured materials would especially benefit from LAOS rheometry but require the use of the larger gaps and roughened surfaces in parallelplate and cylindricalCouette geometries. However, in these geometries, the shear profiles are not homogeneous throughout the deformation field. For elastoviscoplastic materials undergoing LAOS in these geometries, both elastic and viscoplastic deformations may occur simultaneously, complicating the data analysis. By means of model simulations, we provide a comprehensive picture of a model elastoviscoplastic material undergoing oscillatory shear deformation in the parallelplate and cylindricalCouette geometries, and we compare the oscillatory signals to those obtained from a uniformshear field. Both displacementcontrolled and torquecontrolled oscillatory flows were simulated. We show that using popular linear formulas for mapping displacement to strain and torque to stress results in strain and stress signals that deviate significantly from their uniformshear counterparts. For some limited cases, specifically displacementcontrolled parallelplate and torquecontrolled cylindricalCouette oscillatory rheometry, the use of advanced mapping methods to improve the calculation of strain and stress signals was demonstrated. As an alternative, we suggest that analyzing LAOS signals via constitutive modeling provides a unifying approach.

Phenomenological improvements to predictive models of fiber orientation in concentrated suspensions
View Description Hide DescriptionThe standard FolgarTucker (FT) orientation equation is a useful method for theoretically determining isotropic fiber orientation in concentrated suspensions. However, when quantitatively compared with related experimental observations, this equation demonstrates an overprediction inaccuracy. Recently, the PhelpsTucker anisotropic rotary diffusion (ARD) model has shown an ability to handle primary anisotropic fiber orientation. Nevertheless, the ARD tensor depending upon Hand's tensor is difficult to apply in general, because numerous parameters themselves are so sensitive as to affect the stability of any numerical results. To address these critical problems in predicting fiber orientation, this study proposes an improved ARD tensor combined with a new retardant principal rate (iARDRPR) model. The RPR model is a coaxial correction of the orientation tensor for the FT equation. In addition, the iARD tensor, consisting of an identity tensor and a dimensionless fiberrotaryresistance tensor, is more concise with two available parameters. As a validation, the iARDRPR model nicely fits the orientation tensor components measured in transient simple shear flows. Of particular importance is the good agreement between the predictive fiber orientation distribution and the practical coreshell structure for the centergated disk of injection molding of fiberreinforced thermoplastics.

Flowenhanced nucleation of poly(1butene): Model application to shortterm and continuous shear and extensional flow
View Description Hide DescriptionA modeling framework for flowenhanced nucleation of polymers is applied to a broad set of data from literature. Creation of flowinduced pointlike nuclei is coupled to chain stretch of the highmolecular weight tail of the material, calculated with a rheological constitutive model. As the flowinduced nuclei grow, the crystalline volume fraction increases and with it the viscosity of the material. This is accounted for by describing the material as a suspension of spheres in a viscoelastic matrix. Calculations are compared with a broad set of experimental data from literature on three grades of poly(1butene). First, a parameter set is determined by fitting model results to flowinduced nucleation densities from shortterm shear experiments. Next, this parameter set is used to validate the framework in continuous flow experiments in which viscosity is monitored during a constant flow rate. In this way, we demonstrate the approach is applicable to not only shortterm shear but also continuous flow. It was observed in experiments that for continuous extensional flow, the viscosity shows an upturn at a constant strain, the value of which is independent of strain rate. We hypothesize that this upturn is related to long chains entering the chain stretch regime, as a result of the extension rate exceeding the inverse of the Rouse time of the longest chains.

Negative electrorheological fluids
View Description Hide DescriptionElectrorheological (ER) fluids are electroresponsive materials that have been extensively investigated for several decades. Despite negative ER fluids being one of the most promising new class of candidate materials for various potential applications, only positive ER fluids have been widely used thus far. Herein, we briefly review the negative ER fluids to encourage their application in commercial device manufacture. This short review discusses an overview of mechanisms, materials, and applications of negative ER fluids.

The effect of polymerinduced attraction on dynamical arrests of polymer composites with bimodal particle size distributions
View Description Hide DescriptionWe explore the flow properties of nanocomposite melts where the particles have bimodal size distributions and experience a weak attraction produced by suspending silica particles in polyethylene glycol melts with a molecular weight of 2000 (PEG2000). The polymer is unentangled and adsorbs to the particle surface. The volume fraction ratio of large particles to total particle volume fraction, R, is systematically varied to study the effects of this polymerinduced attraction on suspension rheology. The maximum volume fraction, ϕm , of the particles varies in a nonmonotonic manner of R as demonstrated in studies of the same mixtures when suspended in polyethylene glycol with a molecular weight of 400 (PEG400), where the particles experience excluded volume interactions. The dynamical arrest volume fraction ϕx , of nanocomposite melts in PEG2000 monotonically increases with R. In frequency sweep experiments, the plateau elastic modulus is dominated by attractive interactions and increases with the total particle volume fraction, ϕc , proportionally with 1/(h ^{2}⟨D⟩), where ⟨D⟩ is the volume average particle diameter and h is the average particleparticle surface separation. As R is varied, this universal yielding behavior occurs at constant surface separation, h, suggesting that the flow properties of the mixtures can be understood as being equivalent to flow properties of homogeneous particle suspensions experiencing shortrange attractions with an extent independent of particle size.

Investigation of the rheological behavior of industrial tubular and autoclave LDPEs under SAOS, LAOS, transient shear, and elongational flows compared with predictions from the MSF theory
View Description Hide DescriptionThe molecular structure of two different types of industrial lowdensity polyethylenes (LDPE), i.e., tubular and autoclave, was investigated by gel permeation chromatography and different rheological methods and then compared with predictions from theoretical modeling. Linear rheological data generated from small amplitude oscillatory shear (SAOS) are presented using the framework of a Van GurpPalmen plot while nonlinear rheological data obtained from either transient shear, transient extension, or medium amplitude oscillatory shear (MAOS) are compared with simulations using a generalized form of the molecular stress function theory with only three nonlinear material parameters: a 2, β, and f max which represent the dissipation in simple shear flow, the ratio of molar mass of the branched polymer to the molar mass of the backbone, and the maximum stretching of the polymer chain before chains slip past one another without further stretch, respectively. The effect of these parameters on the predictions is investigated for the aforementioned nonlinear deformations. As a result of these comparisons, a 2 was found to have a constant value within experimental error (a 2 = 0.07 ± 0.03) when simulating the shear stress growth coefficient. Next, β was defined by fitting the extensional data with this model resulting in a constant value of 1.8 for tubular LDPEs and a value between 1.6 and 2.1 for autoclave LDPEs depending on the molecular structure. The universality of these β values was found by simulating the intrinsic nonlinearity (defined by Fourier transform rheology method) in the MAOS region. Finally, f max was evaluated by fitting the model to tensile stress growth coefficient data. It was found that f max is independent of the extensional rate for tubular LDPEs but exhibits a power law behavior with the extensional rate for autoclave LDPEs. Our investigations demonstrated that nonlinear deformations instead of SAOS deformations are the preferred method for characterizing the molecular structure of these polymers due to its sensitivity to the complex structure of LCB present in these materials along with their broad molecular weight distribution.

Rheology of bubble suspensions using dissipative particle dynamics. Part I: A hardcore DPD particle model for gas bubbles
View Description Hide DescriptionIn this paper, the rheology of dilute bubble suspensions is studied using dissipative particle dynamics (DPD). Each gas bubble is modeled by a hardcore DPD particle. The approach addresses the issue of zeroviscosity arising from modeling of a gas bubble by a set of DPD particles. Moreover, it helps to reduce significantly the computational demand due to a much less number of DPD particles required in the simulation. A dissipative layer is created outside the effective region of the hardcore DPD particle to manage the hydrodynamic force acting on it, and different phases can be defined accordingly. The model is further examined in the simulation of dilute bubble suspensions, and measurements on the Newtonian viscosity for the volume fraction less than 15% are consistent with experimental results and results from theoretical models in continuum mechanics at low Ca limit.

Rheology and structure of poly(vinyl alcohol)poly(ethylene glycol) blends during aging
View Description Hide DescriptionWe have used rheometry and dynamic light scattering to study the evolution of the properties of concentrated aqueous solutions of poly(vinyl alcohol) (PVA) and blends of PVA and poly(ethylene glycol) (PEG) as they age. We find that freshly prepared materials are primarily viscous, though they shear thin at lowshear rates and show signs of shearinduced structure at higher shear rates. The viscosity of pure PVA solutions increases slowly as they age. The aging of the PVA/PEG blends is much faster, however, and for sufficiently high concentrations of PEG, the blends gel over the 1month time scale of our observations. Dynamic light scattering experiments show the presence of two relaxation processes in both the pure PVA solution and the blends, with relaxation times that depend on the polymer concentration and the aging time. Both the rheological and dynamic light scattering measurements show the existence of microphase separation in the blends and suggest that the presence of PEG forces the PVA solution to phase separate and eventually to gel. However, we find that the gel transition is not sharp, with different signatures of gelation occurring at different times, depending on the length scale at which the gel is probed.

Colloidal microstructure effects on particle sedimentation in yield stress fluids
View Description Hide DescriptionYield stress fluids are widely used in industry, deeply studied as an example of soft matter, and easy to conceptually describe: A solidlike material that can be yielded and made to flow by applying a minimum stress but will resolidify once the applied stress is removed. Similarly, a particle will be stably suspended against sedimentation by a yield stress fluid if the stress it exerts on the fluid does not exceed the yield stress. In this article, we examine the current approach to predicting particle suspension in a yield stress fluid. We focus on a key cause of variability in both the fluid yield stress and propagation of particle stress: The fluid microstructure. We measure the prevention of particle sedimentation by examples of the two key microstructures used to create a yield stress suspension: A colloidal glass, Carbopol, which forms high volume fraction elastic structures by crowding, and a colloidal gel, microfibrous cellulose or MFC, which forms a sparser low volume fraction elastic network by interparticle attachments. Comparing the sedimentation behavior of a single sphere in Carbopol and in MFC indicates that fluids with the same yield stress value can differ by a factor of 6 in their stability against particle sedimentation as a result of microstructure differences. Such suspensions cannot be characterized by yield stress alone, so the different fluids' yielding, and possibly recovery, from applied stress must also be studied. The work points to methods of improved design of microstructured fluids in a range of formulated product applications and also links shared goals of the rheology and microrheology communities.

Experimental determination of interfacial slip between polyethylene and thermoplastic elastomers
View Description Hide DescriptionInterfacial slip was determined for linearlowdensity polyethylene (PE)/thermoplastic elastomer (TPE) immiscible systems by use of layered samples between rotational parallelplates according to the method described by Zhao and Macosko (2002) and Lee et al. (2009). Six different types of TPEs based on polyesters, polyethers, and polysiloxane were investigated. Interfacial slip between PE and TPE could be detected and quantified at low shear stresses in the range of 30–3000 Pa using samples consisting of only three layers. In agreement with the scaling law of BrochardWyart and de Gennes (1993) for weakly entangled interfaces at low shear stress, the polymer–polymer interfacial slip velocity was found to scale linearly with the applied shear stress and slip velocities ranging from 0.5 to 50 μm/s were observed. To quantify the amount of wall slip during the rotational measurements, the procedure presented by Yoshimura and Prud'homme (1988) was used. As a result the steady shear viscosities of three TPEs had to be corrected for wall slip. Due to differences in the first normal stresses of PE and TPE, secondary flow was observed during prolonged rotation at much higher shear stresses, leading to a complete rearrangement of the sample: Starting the rotation with two layers of PE/TPE between parallelplates, a coreshell structure was found at the end of the experiment.

HeleShaw rheometry
View Description Hide DescriptionIn this paper, we describe a novel approach to determine the flow behavior index of a powerlaw fluid by means of a microfluidic device. The concept of this method is based on a mathematical analysis by Aronsson and Janfalk [Eur. J. Appl. Math. 3, 343–366 (1992)] of HeleShaw flow of powerlaw liquids. We implement this approach by driving a nonNewtonian fluid through a glass microfluidic chip with a 100:1 contraction. The flow in this chip satisfies the HeleShaw flow conditions in most of the device. Two conjugate pLaplace equations describe the pressure and stream function in such flows. These equations depend on the flow behavior index, n. Therefore, by fitting the pLaplace equation to the velocity field obtained from a micro particle image velocimetry measurement of the flow, the flow behavior index of the fluid in the chip can be determined. Because in practice, fluids rarely show perfectly inelastic powerlaw behavior, conditions under which the assumption of inelastic flow is valid were derived by analyzing HeleShaw flow of an OldroydB fluid. The concept was tested using three different classes of model fluids, a Newtonian fluid, an inelastic powerlaw fluid, and a Boger fluid. In all three cases, satisfactory results were obtained, with values of n deviating at most 4% from values measured using conventional rheometry. The method presented here is expected to be potentially useful in online quality control in, for example, polymer or food processing.

Superposition of small strains on large: Some counterintuitive results for a concentrated colloidal system
View Description Hide DescriptionThe rheological response of a colloidal system concentrated to near the glass concentration has been investigated using a cone and plate geometry. The prime purpose of the work was to interrogate the nonlinear response using the socalled “tickle” experiment in which small deformations are superimposed on a single large deformation to probe “rejuvenation” and “reinitiated aging” behaviors as has been done in polymer glasses and solutions. Counterintuitively, we find that the superposition of a small positive probe onto the large deformation gives different results from the superposition of a small negative probe. Such a result is contrary to simple fading memory ideas of viscoelastic materials. We also find that the rejuvenation and reinitiated aging behavior in the positive probe case is dependent on whether one considers the probes at “first” probe or at “second” probe, in spite of following the protocol first suggested by Struik [Physical Aging in Amorphous Polymers and Other Materials (Elsevier, Amsterdam, 1978)], which avoids sequence effects in viscoelastic materials. It is suggested that colloidal systems, while exhibiting some features of fading memory behavior that is expected in molecular (polymer) glasses, in fact also undergo a mechanically induced structural change, perhaps similar to that found in highly filled materials such as rocket propellants and carbon black filled rubber.

Advection of shearinduced surfactant threads and turbulent drag reduction
View Description Hide DescriptionSurfactant solutions containing wormlike micelles often show complex structural changes in laminar shear flow. These structural changes produce anomalous rheological phenomena in which the shear induced structure (hereafter SIS) increases the viscosity of surfactant solutions. In turbulent shear flow, dilute solutions of 5 nm diameter and micron length wormlike micelles reduce frictional drag. It has been speculated that this drag reduction is related to wormlike micelles or SIS, although to date there have been no studies that have made their direct link to drag reduction in turbulent flow. The mechanism of drag reduction and the role of the micellar structure remain unclear. Here, we identify SIS with a long (several centimeters), threadlike structure. The threads form a gellike phase under shear, which is clearly separated from the surrounding fluid. The threads breakdown at high shear stresses and disappear at the same time as the drag reduction does. The correlation between the existence of surfactant threads and turbulent drag reduction is discussed.