Showing papers on "Rheometer published in 2015"

Experimental Challenges of Shear Rheology: How to Avoid Bad Data

Abstract: A variety of measurement artifacts can be blamed for misinterpretations of shear thinning, shear thickening, and viscoelastic responses, when the material does not actually have these properties. The softness and activity of biological materials will often magnify the challenges of experimental rheological measurements. The theoretical definitions of rheological material functions are based on stress, strain, and strain-rate components in simple deformation fields. In reality, one typically measures loads and displacements at the boundaries of a sample, and the calculation of true stress and strain may be encumbered by instrument resolution, instrument inertia, sample inertia, boundary effects, and volumetric effects. Here we discuss these common challenges in measuring shear material functions in the context of soft, water-based, and even living biological complex fluids. We discuss techniques for identifying and minimizing experimental errors and for pushing the experimental limits of rotational shear rheometers. Two extreme case studies are used: an ultrasoft aqueous polymer/fiber network (hagfish defense gel) and an actively swimming suspension of microalgae (Dunaliella primolecta).

Extensional Relaxation Times of Dilute, Aqueous Polymer Solutions

Abstract: We show that visualization and analysis of capillary-driven thinning and pinch-off dynamics of the columnar neck in an asymmetric liquid bridge created by dripping-onto-substrate can be used for characterizing the extensional rheology of complex fluids. Using a particular example of dilute, aqueous PEO solutions, we show the measurement of both the extensional relaxation time and extensional viscosity of weakly elastic, polymeric complex fluids with low shear viscosity η < 20 mPa·s and relatively short relaxation time, λ < 1 ms. Characterization of elastic effects and extensional relaxation times in these dilute solutions is beyond the range measurable in the standard geometries used in commercially available shear and extensional rheometers (including CaBER, capillary breakup extensional rheometer). As the radius of the neck that connects a sessile drop to a nozzle is detected optically, and the extensional response for viscoelastic fluids is characterized by analyzing their elastocapillary self-thinning...

Temperature induced gelation transition of a fumed silica/PEG shear thickening fluid

Abstract: The effect of temperature on the rheological behaviors of a shear thickening fluid (STF) prepared by dispersing fumed silica (SiO2) particles into polyethylene glycol (PEG) under mechanical stirring and ultrasonication was investigated using a rotational rheometer. Under steady shear, the system showed an obvious shear thickening behavior due to the formation of “hydroclusters” of SiO2 particles driven by hydrodynamic lubrication forces. The value of the critical shear rate at which the shear thickening begins grows monotonically with temperature. Dynamic temperature sweeps show that elevating the temperature induces a gelation transition of the SiO2/PEG system when the concentration of SiO2 exceeds a critical value, which is found to be lower for the system consisting of higher average molecular weight PEG. The gelation process also becomes more remarkable at a higher concentration of SiO2 particles. It is found that the temperature induced gelation of SiO2/PEG sol is essentially related to the disappearance of the solvation layer on the surface of SiO2 particles as well as the change of hydrogen bonds.

Nonmonotonic flow curves of shear thickening suspensions.

Abstract: The discontinuous shear thickening (DST) of dense suspensions is a remarkable phenomenon in which the viscosity can increase by several orders of magnitude at a critical shear rate. It has the appearance of a first-order phase transition between two hypothetical ``states'' that we have recently identified as Stokes flows with lubricated or frictional contacts, respectively. Here we extend the analogy further by means of stress-controlled simulations and show the existence of a nonmonotonic steady-state flow curve analogous to a nonmonotonic equation of state. While we associate DST with an $\mathsf{S}$-shaped flow curve, at volume fractions above the shear jamming transition the frictional state loses flowability and the flow curve reduces to an arch, permitting the system to flow only at small stresses. Whereas a thermodynamic transition leads to phase separation in the coexistence region, we observe a uniform shear flow all along the thickening transition. A stability analysis suggests that uniform shear may be mechanically stable for the small Reynolds numbers and system sizes in a rheometer.

The rheology of aqueous solutions of ethyl hydroxy-ethyl cellulose (EHEC) and its hydrophobically modified analogue (hmEHEC): extensional flow response in capillary break-up, jetting (ROJER) and in a cross-slot extensional rheometer

Abstract: Cellulose derivatives containing associating hydrophobic groups along their hydrophilic backbone are used as rheology modifiers in the formulation of water-based spray paints, medicinal sprays, cosmetics and printable inks. Jetting and spraying applications of these materials involve progressive thinning and break-up of a fluid column or sheet into drops. Strong extensional kinematics develop in the thinning fluid neck. In viscous Newtonian fluids, inertial and viscous stresses oppose the surface tension-driven instability. In aqueous solutions of polymers such as Ethyl Hydroxy-Ethyl Cellulose (EHEC), chain elongation provides additional elastic stresses that can delay the capillary-driven pinch-off, influencing the sprayability or jettability of the complex fluid. In this study, we quantify the transient response of thinning filaments of cellulose ether solutions to extensional flows in a Capillary Break-up Extensional Rheometer (CaBER) and in a forced jet undergoing break-up using Rayleigh Ohnesorge Jetting Extensional Rheometry (ROJER). We also characterize the steady state molecular deformations using measurements of the flow-induced birefringence and excess pressure drop in an extensional stagnation point flow using a Cross-Slot Extensional Rheometer (CSER). We show that under the high extension rates encountered in jetting and spraying, the semi-dilute solutions of hydrophobically modified ethyl hydroxy-ethyl cellulose (hmEHEC) exhibit extensional thinning, while the unmodified bare chains of EHEC display an increase in extensional viscosity, up to a plateau value. For both EHEC and hmEHEC dispersions, the low extensibility of the cellulose derivatives limits the Trouton ratio observed at the highest extension rates attained (close to 105 s−1) to around 10–20. The reduction in extensional viscosity with increasing extension rate for the hydrophobically modified cellulose ether is primarily caused by the disruption of a transient elastic network that is initially formed by intermolecular association of hydrophobic stickers. This extensional thinning behavior, in conjunction with the low extensibility of the hydrophobically modified cellulose ether additives, makes these rheology modifiers ideal for controlling the extensional rheology in formulations that require jetting or spraying, with minimal residual stringiness or stranding.

Heterogeneous flow kinematics of cellulose nanofibril suspensions under shear.

Abstract: The rheology of NFC suspensions that exhibited different microstructures and colloidal stability, namely TEMPO and enzymatic NFC suspensions, was investigated at the macro and mesoscales using a transparent Couette rheometer combined with optical observations and ultrasonic speckle velocimetry (USV). Both NFC suspensions showed a complex rheology, which was typical of yield stress, non-linear and thixotropic fluids. Hysteresis loops and erratic evolutions of the macroscale shear stress were also observed, thereby suggesting important mesostructural changes and/or inhomogeneous flow conditions. The in situ optical observations revealed drastic mesostructural changes for the enzymatic NFC suspensions, whereas the TEMPO NFC suspensions did not exhibit mesoscale heterogeneities. However, for both suspensions, USV measurements showed that the flow was heterogeneous and exhibited complex situations with the coexistence of multiple flow bands, wall slippage and possibly multidimensional effects. Using USV measurements, we also showed that the fluidization of these suspensions could presumably be attributed to a progressive and spatially heterogeneous transition from a solid-like to a liquid-like behavior. As the shear rate was increased, the multiple coexisting shear bands progressively enlarged and nearly completely spanned over the rheometer gap, whereas the plug-like flow bands were eroded.

On a new method to determine the yield stress in lubricating grease

Abstract: An experimental study using both a controlled stress and a controlled strain rheometer has been undertaken to characterize lubricating grease in shear, creep, stress relaxation, and oscillatory flow, with a main focus on determining the yield stress. The yield stress was examined using a cone–plate and parallel-plate system with smooth and rough surfaces. Clear discrepancies were observed in the yield stress values obtained using different techniques where oscillatory strain sweep measurements seem to be the best choice. This technique is less sensitive to wall slip, shows good reproducibility, and is relatively easy to perform. The method also shows that the yield stress is a function of the imposed frequency and therefore of the time domain. At lower values of shear—that is, in the linear viscoelastic regime—there is no structural breakdown and the rheology of the grease can be described by the Maxwell model where the stress and the strain are almost proportional to each other. Based on this observation, a novel method to determine the yield stress is proposed: “The yield stress can be determined from the point where this linearity no longer applies.” This method is compared to those that are commonly used. The yield stress was found to depend exponentially on temperature and linearly on frequency.

S-shaped flow curves of shear thickening suspensions: Direct observation of frictional rheology

Abstract: We study the rheological behavior of concentrated granular suspensions of simple spherical particles. Under controlled stress, the system exhibits an S-shaped flow curve (stress vs shear rate) with a negative slope in between the low-viscosity Newtonian regime and the shear thickened regime. Under controlled shear rate, a discontinuous transition between the two states is observed. Stress visualization experiments with a fluorescent probe suggest that friction is at the origin of shear thickening. Stress visualization shows that the stress in the system remains homogeneous (no shear banding) if a stress is imposed that is intermediate between the high- and low-stress branches. The S-shaped shear thickening is then due to the discontinuous formation of a frictional force network between particles upon increasing the stress.

Yield stress measurements of cyclopentane hydrate slurry

Abstract: A hydrate slurry is prepared under shear using liquid cyclopentane (CP) as the hydrate former at atmospheric conditions from a density-matched water-in-oil emulsion, by quenching it to a lower temperature at a fixed shear rate. The typical average drop size is 10 μm. A transition, from initial deformable water drops dispersed in a continuous oil to a solid particle suspension, takes place during which the viscosity increases by several orders of magnitude and jamming may occur. Hydrate slurries so obtained are used for further rheological measurements. In particular, yield stresses are examined for varying water volume fraction using a stress-controlled rheometer. Yield stresses ranged from about 5–4600 Pa between 16% and 30% water volume fraction, with 16% being the minimum water fraction for which there is any measurable yield stress. The yield stress and the final slurry viscosity both scale with water volume fraction as τ y and η f ∼ ( ϕ - ϕ c ) 2.5 , where ϕ c ≐ 0.15 as no yield stress is observed for ϕ 15 %. Yield stress measurements of CP hydrate slurries allowed to rest after formation under no shearing for a duration are also carried out. Hydrate slurries were found to exhibit shear thinning and significant thixotropic behavior. Direct visualization of hydrate formation on water drops in water-in-oil emulsions is presented.

A comparison of rheology and FTIR in the study of polypropylene and polystyrene photodegradation

Abstract: Rheology and FTIR spectroscopy are compared as methods to study the degree of photodegradation in polypropylene (PP) and polystyrene (PS) sheets. The materials are hot pressed, artificially photo-aged with fluorescent lights for 4–2048 h and then measured with a rotational rheometer and FTIR. Both materials show a tendency for chain scission which can be seen as a reduction in viscosity. Changes in PP can be observed with both methods after 256 h of irradiation. Changes in PS become significant in rheology after 64 h but in FTIR only after 1024 h of irradiation. Due to the different chemical nature of the materials, the degradation of PS is rather linear with exposure, whereas the degradation of PP is more exponential. Using the zero shear viscosities obtained through extrapolations of the Cole–Cole and Carreau–Yasuda models, relative molecular weights are estimated with the aid of the power–law relationship between these two. These results are compared with the carbonyl indices determined from the FTIR spectra. Rheology is found to be a viable alternative for FTIR in certain situations. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42246.

Flow enhancement due to elastic turbulence in channel flows of shear thinning fluids.

Abstract: We explore the flow of highly shear thinning polymer solutions in straight geometry. The strong variations of the normal forces close to the wall give rise to an elastic instability. We evidence a periodic motion close the onset of the instability, which then evolves towards a turbulentlike flow at higher flow rates. Strikingly, we point out that this instability induces genuine drag reduction due to the homogenization of the viscosity profile by the turbulent flow. Viscoelastic polymer solutions are characterized by a relaxation time λ required by the polymer molecules to adjust to changes in the flow conditions. This time is at the origin of purely elastic instabilities observed at very low Reynolds numbers with no counterpart in pure Newtonian fluids. In shear flows, purely elastic instabilities arise due to stress tensor anisotropy inducing a destabilizing net force perpendicularly to curved streamlines [1–3]. These instabilities can lead to purely elastic turbulence [4–6]. In straight channels, theoretical studies demonstrate that the viscosimetric properties of the fluid play a major role in the stability of the flow. Plane Couette flows [7] and Poiseuille flows [8] of Oldroyd-B fluids exhibit a nonlinear subcritical elastic instability although the base flow is linearly stable [9]. Recent experiments [10,11] have shown that finite amplitude perturbation creates curved stream-lines that drive the instability. In contrast, channel flows of highly shear thinning fluids [12–14] are theoretically linearly unstable. The instability is driven in this case by the strong variations of both normal stress and viscous dissipation in the shear direction. An extreme situation is obtained with shear banding fluids, where the interface bears an unbalanced normal stress [14]. This leads to an interfacial instability which has been observed experimen-tally [15]. A similar mechanism is theoretically expected without shear banding but requires a strong gradient of shear rate [13]. At this stage, channel flow stability of such liquids has not been studied experimentally, although shear thinning is a very common feature of elastic fluids. In this Letter, we focus on highly shear thinning elastic polymer solutions with no shear banding flowing in straight channels. At low flow rates, no velocity fluctuations are observed. At higher flow rates, the power spectrum density of the velocity fluctuations displays a distinct peak indicat-ing the onset of instability. The position of the peak is in agreement with theoretical predictions for highly shear thinning fluids [13]. At even higher flow rates, the fluctuations occur at all scales and the spectrum becomes broadband with a power law decay. This instability induces genuine drag reduction: viscous losses are smaller than expected from the fluid rheology. This is a remarkable result since one expects additional energy losses due to the enhancement of velocity fluctuations. We study the flow of high molecular weight polymer solutions (18 × 10 6 g=mol partially hydrolyzed polyacry-lamide) in water at different concentrations in the semi-dilute regime. The global flow curves of these solutions are determined using the shear-rate-imposed mode of a rheometer (TA Instruments ARG2) in a sanded cone-and-plate geometry of angle θ ¼ 2°. Figure 1 reports both the shear stress σ and the Weissenberg number Wi ¼ N 1 =2σ as

Dry granular flows: Rheological measurements of the μ(I)-rheology

Abstract: Granular materials do not always flow homogeneously like fluids when submitted to external stress, but often form rigid regions that are separated by narrow shear bands where the material yields and flows. This shear localization impacts their apparent rheology, which makes it difficult to infer a constitutive behavior from conventional rheometric measurements. Moreover, they present a dilatant behavior, which makes their study in classical fixed-volume geometries difficult. These features led numerous groups to perform extensive studies with inclined plane flows, which were of crucial importance for the development and the validation of the μ(I) -rheology. Our aim is to develop a method to characterize granular materials with rheometrical tools. Using rheometry measurements in an annular shear cell, dense granular flows of 0.5 mm spherical and monodisperse beads are studied. A focus is placed on the comparison between the present results and the μ(I) -rheology. From steady state measurements of the torque and the gap under imposed shear rate γ and normal force FN , we define an inertial number I. We show that, at low I (small γ and/or large FN ), the flow goes to a quasistatic limit, and the response in terms of dimensionless stress or internal friction coefficient—μ—and solid concentration—ϕ—profiles is independent of the inertial number. Upon increasing I (large γ and/or small FN ), dilation occurs and ϕ decreases while μ increases. The observed variations are in good agreement with previous observations of the literature [Jop et al., Nature 441, 727–730 (2006) and Hatano, Phys. Rev. E 75, 060301 (R) (2007)]. These results show that the constitutive equations μ(I) and ϕ(I) of granular materials can be measured with a rheometer.

The rheology of cementitious suspensions: A closer look at experimental parameters and property determination using common rheological models

Abstract: This paper investigates the influence of gap between parallel plates, surface texture of the bottom plate, and mixing intensity on the yield stress and plastic viscosity of cementitious suspensions extracted using the Bingham model. Special emphasis is paid toward understanding the effects of shear rate range and different rheological models on the flow parameters. It is shown that the use of a wider shear rate range (0.1–100/s), can be beneficial in obtaining a reasonable portion of the stress plateau in the shear stress–shear rate relationship, which facilitates a model-less, yet accurate extraction of yield stress. The Bingham model that considers only the linear region (i.e. ∼5–100/s) overestimates the yield stress as indicated by the stress asymptote while the Herschel–Bulkley (H–B) equation applied in the 0.1–100/s shear rate range underestimates the yield stress. Further lowering the evaluated shear rate range (i.e. 0.005–100/s) does substantially improve the H–B prediction of yield stress.

Physical properties of polyacrylamide gels probed by AFM and rheology

Abstract: Polymer gels have been shown to behave as viscoelastic materials but only a small amount of data is usually provided in the glass transition. In this paper, the dynamic moduli and of polyacrylamide hydrogels are investigated using both an AFM in contact force modulation mode and a classical rheometer. The validity is shown by the matching of the two techniques. Measurements are carried out on gels of increasing polymer concentration in a wide frequency range. A model based on fractional derivatives is successfully used, covering the whole frequency range. , the plateau modulus, as well as several other parameters are obtained at low frequencies. The model also predicts the slope a of both moduli in the glass transition, and a transition frequency is introduced to separate the gel-like behavior with the glassy state. Its variation with polymer content c gives a dependence , in good agreement with previous theories. Therefore, the AFM data provides new information on the physics of polymer gels.

Effect of Particle Size on Shear Stress of Magnetorheological Fluids

Abstract: Magnetorheological fluids (MRF), known for their variable shear stress contain magnetisable micrometer-sized particles (few micrometer to 200 micrometers) in a nonmagnetic carrier liquid To avoid settling of particles, smaller sized (3-10 micrometers) particles are preferred, while larger sized particles can be used in MR brakes, MR clutches, etc as mechanical stirring action in those mechanisms does not allow particles to settle down Ideally larger sized particles provide higher shear stress compared to smaller sized particles However there is need to explore the effect of particle sizes on the shear stress In the current paper, a comparison of different particle sizes on MR effect has been presented Particle size distributions of iron particles were measured using HORIBA Laser Scattering Particle Size Distribution Analyser The particle size distribution, mean sizes and standard deviations have been presented The nature of particle shapes has been observed using scanning electron microscopy To explore the effect of particle sizes, nine MR fluids containing small, large and mixed sized carbonyl iron particles have been synthesized Three concentrations (9%, 18% and 36% by volume) for each size of particles have been used The shear stresses of those MRF samples have been measured using ANTON PAAR MCR-102 Rheometer With increase in volume fraction of iron particles, the MR fluids synthesized using “mixed sized particles” show better shear stress compared to the MR fluids containing “smaller sized spherical shaped particles” and “larger sized flaked shaped particles” at higher shear rate

Experimental and modelling study of the effect of temperature on shear thickening fluids

Abstract: This paper is a study of the effect that temperature has on a shear thickening fluid made from ethylene glycol and fumed silica, with 20%–26% weight fractions. Three typical temperatures, ranging from 20°C to 60°C, were selected to study the shear rate dependence of viscosity. The temperature tests showed that a high temperature increased the critical shear rate and lowered the shear thickening ratio. A viscosity function was proposed to represent the three characteristic regions in typical shear thickening fluid and to predict viscosity at different temperatures.

Rheology of a dual crosslink self-healing gel: Theory and measurement using parallel-plate torsional rheometry

Abstract: Tough hydrogels can be synthesized by incorporating self-healing physical crosslinks in a chemically crosslinked gel network. Due to the breaking and reattachment of these physical crosslinks, these gels exhibit a rate-dependent behavior that can be different from a classical linear viscoelastic solid. In this work, we develop a theory to describe the linear mechanical response of a dual-crosslink gel in a parallel-plate torsional rheometer. Our theory is based on a newly developed finite strain constitutive model. We show that some of the parameters in the constitutive model can be determined by carrying oscillatory torsional experiments. For consistency, we also show that the torsion data in an oscillatory test can be predicted using our theory with parameters obtained from tension tests. Our theory provides a basis for interpreting and understanding the test data of these gels obtained from rheometry.

Concentration effect of TEMPO-oxidized nanofibrillated cellulose aqueous suspensions on the flow instabilities and small-angle X-ray scattering structural characterization

Abstract: The rheological behavior of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized nanofibrillated cellulose (NFC) suspensions at different concentrations were examined using Couette with smooth and serrated surfaces and vane-in-cup geometries. A slippage against the walls of rheometer rotor or/and stator (wall-slip) as well as flow localization within a specific volume of the sample (shear banding) took place. These phenomena were visualized using a technique based on introduction of titanium dioxide pigment into the specific volume of NFC suspension and monitoring its deformation. The flow instabilities appeared more pronounced with an increase of the NFC concentration. It was shown that the use of serrated geometry is necessary to compete with the wall-slip but is not sufficient to prevent it completely. Serrated Couette was found to be the most appropriate geometry (among the tested) to measure the rheological properties of NFC suspensions. Small-angle X-ray scattering was used to provide a supplementary information about the structural organization of NFC suspensions. The statistical average diameter of 4.8 nm was determined from the scattering data for the NFC swollen in water.

Viscosity and viscoelasticity measurements of low density polyethylene/poly(lactic acid) blends

Abstract: The rheological properties and the viscoelastic behaviour of blends of polyethylene with different percentages of poly(lactic acid), ranging from 0 to 100 wt%, were studied. In a first part, all blends were examined under steady conditions using a capillary rheometer (at 180, 190 and 200 °C) and dynamic conditions using a parallel plate rheometer. The results showed that all blends behaved like pseudoplastic fluids, with the power–law index values varying between those of polyethylene and polylactide (0.45–0.75 at 180 °C, 0.49–0.77 at 190 °C and 0.54–0.81 at 200 °C). It was also observed that at low shear rate, pure poly(lactic acid) and polyethylene possessed, respectively, the highest and the lowest flow activation energy (66.9 and 48.3 kJ/mol); however, at high shear rate, the greater the content of poly(lactic acid), the lower the activation energy. In addition, poly(lactic acid) exhibited lower viscosity and lower melt elasticity than either polyethylene or the blends. The dynamic rheological study demonstrated that all formulations displayed shear thinning behaviour and only virgin poly(lactic acid) exhibited a clear Newtonian plateau. Also, mainly at low frequencies, polyethylene had the higher values of storage modulus (325 Pa), loss modulus (937 Pa) and complex viscosity (9,740 Pa.s). However, blends had values lying between those of the two homopolymers without any improvement in the storage modulus, loss modulus or complex viscosity. In a second part, the viscoelastic characteristics were investigated using dynamic mechanical thermal analysis (DMTA). DMTA spectra showed an increase in the storage modulus with the increase of poly(lactic acid) content but the opposite was observed for the loss modulus. A cold crystallization of poly(lactic acid) is observed around 87–100 °C and the temperature of glass transition of poly(lactic acid) did not depend on the composition of the blend. These results indicate that LDPE and PLA are immiscible in all proportions either in the melt state or in the solid state.