Showing papers on "Rheometer published in 2016"

On different ways of measuring “the” yield stress

Abstract: Yield stress materials are ubiquitous, yet the best way to obtain the value of the yield stress for any given material has been the subject of considerable debate. Here we compare different methods of measuring the yield stress with conventional rheometers that have been used in the literature on a variety of materials. The main conclusion is that, at least for well-behaved (non-thixotropic) materials, the differences between the various methods are significant; on the other hand, the scaling of the measured yield stress with the volume fraction of dispersed phase shows the same dependence independently of the way in which the yield stress is obtained experimentally. The measured yield strain is similarly found to depend on the method employed. The yield stress values obtained for a simple (non-thixotropic) yield stress fluid are only similar for Herschel–Bulkley fits and stress-strain curves obtained from oscillatory measurements. Stress-strain curves with a continuous imposed stress or strain rate differ significantly, as do oscillatory measurements of the crossover between G′ and G″ or the point where G′ starts to differ significantly from its linear response value. The intersection of the G′ and G″ curves as a function of strain consistently give the highest value of the yield stress and yield strain. In addition, many of these criteria necessitate some arbitrary definition of a crossover point. Similar conclusions apply for a class of thixotropic yield stress materials, with the stress-strain curve from the oscillatory data giving the dynamic yield stress and the Herschel–Bulkley fit either the static or dynamic yield stress, depending on how the measurement is carried out.

Local viscoelasticity of living cells measured by rotational magnetic spectroscopy

Abstract: When submitted to a magnetic field, micron-size wires with superparamagnetic properties behave as embedded rheometers and represent interesting sensors for microrheology. Here we use rotational magnetic spectroscopy to measure the shear viscosity of the cytoplasm of living cells. We address the question of whether the cytoplasm is a viscoelastic liquid or an elastic gel. The main result of the study is the observation of a rotational instability between a synchronous and an asynchronous regime of rotation, found for murine fibroblasts and human cancer cells. For wires of susceptibility 3.6, the transition occurs in the range 0.01-1 rad s(-1). The determination of the shear viscosity (10-100 Pa s) and elastic modulus (5-20 Pa) confirms the viscoelastic character of the cytoplasm. In contrast to earlier studies, it is concluded that the interior of living cells can be described as a viscoelastic liquid, and not as an elastic gel.

Investigation of thermal conductivity and viscosity of Al2O3/PAG nanolubricant for application in automotive air conditioning system

Abstract: In this paper, thermal conductivity and viscosity of the Al2O3/polyalkylene glycol (PAG) 46 nanolubricants for 0.05 to 1.0% volume concentrations at temperatures of 303.15 to 353.15 K have been investigated. Al2O3 nanoparticles were dispersed in the PAG lubricant by a two step preparation. The measurement of thermal conductivity and viscosity was performed using KD2 Pro Thermal Properties Analyzer and LVDV-III Rheometer, respectively. The results showed that the thermal conductivity of the nanolubricants increased by concentration, but decreased by temperature. Besides, the viscosity of the nanolubricants sharply increased for concentrations higher than 0.3%. However, this parameter diminished by temperature. The highest thermal conductivity and viscosity ratio were observed to be 1.04 and 7.58 times greater than the PAG lubricant for 1.0% and 0.4% concentrations, respectively. As a conclusion, it was recommended to use the Al2O3/PAG nanolubricants with concentration of less than 0.3% for application in automotive air conditioning system.

Microfluidic viscometers for shear rheology of complex fluids and biofluids

Abstract: The rich diversity of man-made complex fluids and naturally occurring biofluids is opening up new opportunities for investigating their flow behavior and characterizing their rheological properties. Steady shear viscosity is undoubtedly the most widely characterized material property of these fluids. Although widely adopted, macroscale rheometers are limited by sample volumes, access to high shear rates, hydrodynamic instabilities, and interfacial artifacts. Currently, microfluidic devices are capable of handling low sample volumes, providing precision control of flow and channel geometry, enabling a high degree of multiplexing and automation, and integrating flow visualization and optical techniques. These intrinsic advantages of microfluidics have made it especially suitable for the steady shear rheology of complex fluids. In this paper, we review the use of microfluidics for conducting shear viscometry of complex fluids and biofluids with a focus on viscosity curves as a function of shear rate. We discuss the physical principles underlying different microfluidic viscometers, their unique features and limits of operation. This compilation of technological options will potentially serve in promoting the benefits of microfluidic viscometry along with evincing further interest and research in this area. We intend that this review will aid researchers handling and studying complex fluids in selecting and adopting microfluidic viscometers based on their needs. We conclude with challenges and future directions in microfluidic rheometry of complex fluids and biofluids.

Thermal conductivity and rheological studies for Cu–Zn hybrid nanofluids with various basefluids

Abstract: The trend toward renewable lubricant technology is now irreversible; an eco-friendly cutting fluid will go along with machining to achieve relative sustainability. Eco-friendly nanofluids are developed in this work and compared with traditional cutting fluids used for heat transfer. In this work, in-situ novel hybrid nanoparticles of Cu–Zn (50:50) alloy were synthesized. The stability of the hybrid nanofluids was carried out using photo capturing method and DLS method. The flash point was measured using Pensky–Martens open cup apparatus as per ASTM D6450. The thermal conductivity and rheological studies were carried out using KD-2 Pro and Rheometer. The results illustrate that the hybrid nanofluids exhibited stability for 72 h and then the agglomeration of particles starts and by 168 h almost all the particles tend to settle down. Nanofluids with vegetable oil as basefluid showed better increment in flash point, thermal conductivity and showed marginal less stability. Vegetable oil behaved as Newtonian fluid and the change in viscosity with application of shear was less noticed compared with other two fluids. On the basis of integrated study of thermal conductivity and viscosity, in the point of cost and higher relative thermal conductivity to relative viscosity for effective heat transfer vegetable oil based nanofluid showed better results.

The influence of carbon nanotubes on quasi-static puncture resistance and yarn pull-out behavior of shear-thickening fluids (STFs) impregnated woven fabrics

Abstract: This study focuses on the puncture resistance performance of woven high modulus polypropylene (HMPP) fabric impregnated with shear-thickening fluids (STFs) composed of fumed silica nanoparticles suspended in polyethylene glycol (PEG) and those containing carbon nanotubes (CNTs). The shear-thickening characteristics and rheological features of suspensions were determined at steady and oscillatory shear stress using a stress controlled rheometer. The puncture resistance performance of STF-treated HMPP fabrics were evaluated by the quasi-static puncture test. To gain a better understanding of the effect of these two suspensions on puncture resistance mechanism of fabric, yarn pull-out test was conducted to examine internal shear strength and inter yarn frictional behavior of fabric. The results showed that both the STF-treated fabrics exhibited significant enhancement in puncture resistance performance as compared to neat fabric. However, the fabric impregnated with the suspension containing CNTs showed lower enhancement, due to its lower degree of shear-thickening as confirmed by rheological measurement. The possible improving mechanisms responsible for puncture resistance performance of STF-treated fabrics were also discussed.

Microfluidic extensional rheometry using stagnation point flow.

Abstract: Characterization of the extensional rheometry of fluids with complex microstructures is of great relevance to the optimization of a wide range of industrial applications and for understanding various natural processes, biological functions, and diseases. However, quantitative measurement of the extensional properties of complex fluids has proven elusive to researchers, particularly in the case of low viscosity, weakly elastic fluids. For some time, microfluidic platforms have been recognized as having the potential to fill this gap and various approaches have been proposed. This review begins with a general discussion of extensional viscosity and the requirements of an extensional rheometer, before various types of extensional rheometers (particularly those of microfluidic design) are critically discussed. A specific focus is placed on microfluidic stagnation point extensional flows generated by cross-slot type devices, for which some important developments have been reported during the last 10 years. Additional emphasis is placed on measurements made on relevant biological fluids. Finally, the operating limits of the cross-slot extensional rheometer (chiefly imposed by the onset of elastic and inertial flow instabilities) are discussed.

Elastic instabilities in planar elongational flow of monodisperse polymer solutions

Abstract: We investigate purely elastic flow instabilities in the almost ideal planar stagnation point elongational flow field generated by a microfluidic optimized-shape cross-slot extensional rheometer (OSCER). We use time-resolved flow velocimetry and full-field birefringence microscopy to study the behavior of a series of well-characterized viscoelastic polymer solutions under conditions of low fluid inertia and over a wide range of imposed deformation rates. At low deformation rates the flow is steady and symmetric and appears Newtonian-like, while at high deformation rates we observe the onset of a flow asymmetry resembling the purely elastic instabilities reported in standard-shaped cross-slot devices. However, for intermediate rates, we observe a new type of elastic instability characterized by a lateral displacement and time-dependent motion of the stagnation point. At the onset of this new instability, we evaluate a well-known dimensionless criterion M that predicts the onset of elastic instabilities based on geometric and rheological scaling parameters. The criterion yields maximum values of M which compare well with critical values of M for the onset of elastic instabilities in viscometric torsional flows. We conclude that the same mechanism of tension acting along curved streamlines governs the onset of elastic instabilities in both extensional (irrotational) and torsional (rotational) viscoelastic flows.

The effect of carbide particle additives on rheology of shear thickening fluids

Abstract: In this paper, shear thickening fluids (STFs) including silicon carbide particles are presented. We fabricated a kind of STF based on nanosize fumed silica suspended in a liquid medium, polyethylene glycol, at a constant concentration of 20 wt.%. Then, different particle size silicon carbide (SiC) particles were added to the STF with various amounts. Their rheological properties under various temperatures were tested by using a rheometer. The suspension exhibits different systematic variations with respect to the varied parameters.

The Static and Dynamic Mechanical Properties of Magnetorheological Silly Putty

Abstract: A novel magnetorheological material defined as magnetorheological Silly Putty (MRSP) is prepared by dispersing soft magnetic particles into Silly Putty matrix with shear stiffening property. Static mechanical properties including creep and stress relaxation and dynamic rheological properties of MRSPs are tested by rheometer. The experimental results indicate that the external magnetic field exerts significant influence on the creep and relaxation behaviors. Moreover, the storage modulus of MRSPs increases sharply in response to the external stimuli of increasing angular frequency automatically and can be enhanced by external magnetic field. Besides, temperature plays a key role in shear stiffening and magnetorheological effect of MRSPs. Furthermore, considering the obstruction to the particle chains formation induced by Silly Putty matrix, a nonperforative particle aggregated chains model is proposed. The model curve is in consistency with experimental data, which means it can describe magnetoinduced behavior of MRSPs well.

The Field-Dependent Rheological Properties of Magnetorheological Grease Based on Carbonyl-Iron-Particles

Abstract: This paper presents dynamic viscoelastic properties of magnetorheological (MR) grease under variation of magnetic fields and magnetic particle fractions. The tests to discern the field-dependent properties are undertaken using both rotational and oscillatory shear rheometers. As a first step, the MR grease is developed by dispersing the carbonyl iron (CI) particles into grease medium with a mechanical stirrer. Experimental data are obtained by changing the magnetic field from 0 to 0.7 T at room temperature of 25 °C. It is found that a strong Payne effect limits the linear viscoelastic region of MR grease at strains above 0.1%. The results exhibit a high dynamic yield stress which is equivalent to Bingham plastic rheological model, and show relatively good MR effect at high shear rate of 2000 s-1. In addition, high dispersion of the magnetic particles and good thermal properties are proven. The results presented in this work directly indicate that MR grease is a smart material candidate that could be widely applicable to various fields including vibration control.

Elasticity of fibrous networks under uniaxial prestress.

Abstract: We present theoretical and experimental studies of the elastic response of fibrous networks subjected to uniaxial strain. Uniaxial compression or extension is applied to extracellular networks of fibrin and collagen using a shear rheometer with free water in/outflow. Both uniaxial stress and the network shear modulus are measured. Prior work [van Oosten, et al., Sci. Rep., 2015, 6, 19270] has shown softening/stiffening of these networks under compression/extension, together with a nonlinear response to shear, but the origin of such behaviour remains poorly understood. Here, we study how uniaxial strain influences the nonlinear mechanics of fibrous networks. Using a computational network model with bendable and stretchable fibres, we show that the softening/stiffening behaviour can be understood for fixed lateral boundaries in 2D and 3D networks with comparable average connectivities to the experimental extracellular networks. Moreover, we show that the onset of stiffening depends strongly on the imposed uniaxial strain. Our study highlights the importance of both uniaxial strain and boundary conditions in determining the mechanical response of hydrogels.

Relationship of Extensional Viscosity and Liquid Crystalline Transition to Length Distribution in Carbon Nanotube Solutions

Abstract: We demonstrate that the length of carbon nanotubes (CNTs) can be determined simply and accurately from extensional viscosity measurements of semidilute CNT solutions. The method is based on measuring the extensional viscosity of CNT solutions in chlorosulfonic acid with a customized capillary thinning rheometer and determining CNT aspect ratio from the theoretical relation between extensional viscosity and aspect ratio in semidilute solutions of rigid rods. We measure CNT diameter d by transmission electron microscopy (TEM) and arrive at CNT length L. By studying samples grown by different methods, we show that the method works well for CNT lengths ranging from 0.4 to at least 20 μm, a wider range than for previous techniques. Moreover, we measure the isotropic-to-nematic transition concentration (i.e., isotropic cloud point) φiso of CNT solutions and show that this transition follows Onsager-like scaling φiso ∼ d/L. We characterize the length distributions of CNT samples by combining the measurements of ...

Core-shell structured Fe 3 O 4 @SiO 2 nanoparticles fabricated by sol–gel method and their magnetorheology

Abstract: Using a sol–gel method, silica-coated magnetite (Fe3O4 @SiO2) core-shell nanoparticles were fabricated following a two-step process. In the first step, the Fe3O4 nanoparticles were prepared via a solvothermal method. In the second step, the Fe3O4 nanoparticles were coated with SiO2 formed through the hydrolyzation of tetraethyl orthosilicate. The structure and properties of the core-shell Fe3O4 @SiO2 nanoparticles were characterized and the results showed that Fe3O4 @SiO2 nanoparticles are a soft magnetic material. A magnetorheological (MR) suspension was prepared based on the synthesized Fe3O4 @SiO2 nanoparticles dispersed in silicone oil and measured using a rotational rheometer at various magnetic field strengths. Using a rotational rheometer, the MR properties of the Fe3O4 @SiO2 in silicone oil, including shear stress, shear viscosity, and yield stress were examined under an applied magnetic field.

Influence of Particle-Size Distribution and Temperature on Rheological Behavior of Coal Slurry

Abstract: The rheological behavior of coal-water slurry of Indian coal is studied using rheometer. The effect of particle size, solid concentration, and temperature on the rheology of the coal-water slurry has been investigated. The settlement analysis of various size ranges of coal particles has been carried out. A particle size less than 75 µm is used for analysis of rheological behavior of coal slurry and it is found that the increase in solid concentration caused the increase in apparent viscosity of the coal-water mixture. Also, it is observed that as the solid concentration increases the coal-water mixture converts to a non-Newtonian fluid. The rheological behavior of the slurry is also analyzed by blending the coal samples with mixture of coarse and fine particles, hence making a bimodal particle-size distribution. The slurry having the bimodal particle-size distribution is prepared by blending the fine particles of 53–75 µm with the coarse particles of 106–150 µm or 150–250 µm with various proportions. The ...

A new look at extensional rheology of low-density polyethylene

Abstract: The nonlinear rheology of three selected commercial low-density polyethylenes (LDPE) is measured in uniaxial extensional flow. The measurements are performed using three different devices including an extensional viscosity fixture (EVF), a homemade filament stretching rheometer (DTU-FSR) and a commercial filament stretching rheometer (VADER-1000). We show that the measurements from the EVF are limited by a maximum Hencky strain of 4, while the two filament stretching rheometers are able to probe the nonlinear behavior at larger Hencky strain values where the steady state is reached. With the capability of the filament stretching rheometers, we show that LDPEs with quite different linear viscoelastic properties can have very similar steady extensional viscosity. This points to the potential for independently controlling shear and extensional rheology in certain rate ranges.

Influences of Normal Loading Rate and Shear Velocity on the Shear Behavior of Artificial Rock Joints

Abstract: Recently, the Chinese government has planned to build a large underground rock reservoir for compressed air energy storage plant in Inner Mongolia for electric power generation. The inflation and discharge of compressed air will cause a fluctuation of air pressure on the cavern surface, subsequently leading to a variation of loading on the rock joints around the cavern. Usually, the rate of inflation or discharge of compressed air is not constant (determined by the demand, from tens of kilopascal to more than one megapascal per hour). A similar effect on the rock joints is also encountered in large underground petroleum storage caverns, such as Jurong Rock Cavern under construction in Singapore. As such, the effect of loading rate on the shear behavior of rock joints should be considered. However, this is fundamentally different from the creep behavior of rock joint due to the far shorter loading and unloading time. In the past decades, most of the researches focused on the importance of roughness on the static (or quasi-static) shear behavior of rock joints under constant normal loading (CNL) condition, such as Patton (1966), Ladanyi and Archambault (1969), Barton (1973), Barton and Choubey (1977), Reeves (1985), Kulatilake et al. (1995), Zhao (1997), Grasselli and Egger (2003), Grasselli (2006), Ghazvinian et al. (2012) and Xia et al. (2014). A few studies have been conducted to investigate the effect of shear velocity on the shear behavior of rock joints (Crawford and Curran 1981, 1982; Larson 2003; Atapour and Moosavi 2014). There is no consensus of the extent of effects caused by the shear velocity. Hassani and Scoble (1985) found no dependency of shear strength on shear velocity for sandstone joints within the shear velocity range of 0.012–0.180 mm/min. Several researchers investigated the effect of normal loading history (e.g., over-consolidation) on the shear behavior of rock joints (Barton 1973; Babanouri et al. 2011).However, few studies focusedon the influenceof normal loading rate on the shear behavior of rock joints, although several researchers found that the extent of velocity effect was related to the normal stress level (Schneider 1977; Crawford andCurran 1981, 1982) and creep behavior of the usedmaterial (Wang and Scholz 1994). To further investigate the shear behavior of rock joints, a number of direct shear tests were carried out under three different normal stress levels by using three different normal loading rates under the CNL condition.

Role of capillarity and microstructure on interfacial viscoelasticity of particle laden interfaces

Abstract: The correlation between interfacial interparticle forces, microstructure, and rheology of particle laden interfaces is characterized for a system of polystyrene particles at an air/water interface by using both aqueous salt concentration and particle surface concentration as control parameters. Characterizing linear, interfacial shear viscoelastic moduli and microstructure simultaneously using a custom imaging system attached to a double wall ring interfacial rheometer allows the relationship between these properties to be evaluated. Interfacial viscoelastic moduli magnitude is found to be determined locally by the degree of restricted particle motion, which is dictated to both by interparticle attraction due to capillarity and severity of caging caused by local microstructure. However, macroscale rheological behaviors, such as elasticity and yield, are tied to the mesostructural organization of local microstructure. Large domains of aligned hexagonal packed particles create elastic interfaces; when these...

Rheology and microstructure of non-colloidal suspensions under shear studied with Smoothed Particle Hydrodynamics

Abstract: In this work a Smoothed Particle Hydrodynamics model is presented to study rheology and microstructure of a non-colloidal suspension of spherical particles in a Newtonian solvent. The scheme presented in (Bian and Ellero, 2014) is extended to three-dimensions incorporating both normal and tangential short-range interparticle lubrication forces which are solved implicitly with a refined splitting strategy. The scheme allows to bypass prohibitively small time steps generally required for handling divergent lubrication forces and allows to simulate large particle systems. Rheology of a three-dimensional hard-spheres suspension confined in a plane Couette rheometer is investigated for concentrations up to ϕ = 0.5 . Results for the relative suspension viscosity ηr are analyzed against sample size and numerical convergence of the splitting lubrication scheme and compared with available experimental and simulations data. Very close agreement with the experiments is obtained for ηr up to ϕ = 0.35 . At larger concentrations, our results are still unable to explain the significant viscosity increase observed in experiments. Modest shear-thickening is also observed which is related to anisotropy of the particle radial distribution function (RDF) and presence of reversible hydrodynamic aggregates increasing in size with increasingly applied shear rates.

Dependencies of Shear Wave Velocity and Shear Modulus of Soil on Saturation

Abstract: Shear wave propagation in soil is a physical phenomenon and has been used widely for monitoring and seismic property assessment in geotechnical engineering. Shear wave velocity Vs and small-strain shear modulus G0 are the key parameters in defining material response to various dynamic loadings. To date, the dependencies of Vs and G0 on saturation, especially in high suction range, are still not well understood because of the limited testing methodology and experimental evidence. In this study, the authors present a new laboratory instrumentation of measuring shear wave propagation in different types of unsaturated soils. Low relative humidity and water mist injection environment are used for measuring shear wave velocity under both drying and wetting conditions. Bender element technique was used to measure the shear wave responses. Step function was used as excitation, and determination of a first arrival time was identified and consistently used for all shear wave measurements. Shear wave evoluti...

Elasticity of fibrous networks under axial prestress

Abstract: We present theoretical and experimental studies of the elastic response of fibrous networks subjected to uniaxial strain. Uniaxial compression or extension is applied to extracellular networks of fibrin and collagen using a shear rheometer with free water in/outflow. Both axial stress and the network shear modulus are measured. Prior work [van Oosten et al., Scientific Reports, 2015, 6, 19270] has shown softening/stiffening of these networks under compression/extension, together with a nonlinear response to shear, but the origin of such behaviour remains poorly understood. Here, we study how uniaxial strain influences the nonlinear mechanics of fibrous networks. Using a computational network model with bendable and stretchable fibres, we show that the softening/stiffening behaviour can be understood for fixed lateral boundaries in 2D and 3D networks with comparable average connectivities to the experimental extracellular networks. Moreover, we show that the onset of stiffening depends strongly on the imposed uniaxial strain. Our study highlights the importance of both axial strain and boundary conditions in determining the mechanical response of hydrogels.

Rheological Examination of Aging in Polymer-Modified Asphalt

Abstract: Polymers are commonly used to modify asphalt binder in order to satisfy the performance-grade binder’s rheological properties, such as shear, storage, loss, and bending stiffness. However, it is not known what types and percentages of polymers affect these properties under conditions of aging. To this end, base asphalt binders were modified with styrene-butadiene and styrene-butadiene-styrene polymers at 3, 4, and 5% and then aged in the laboratory using a rolling thin film oven for short-term aging, a pressure aging vessel for long-term aging, and a draft oven for intermediate aging. The stiffness properties of the modified binders were determined using dynamic shear rheometer (DSR), multiple-stress creep recovery (MSCR), Brookfield rotational viscometer (RV), and bending beam rheometer (BBR) tests. Comparison of aging indexes for storage and loss modulus indicated that aging increases the elastic or storage component more than the viscous or loss component of the complex shear modulus. It was sh...