Dynamic mechanical properties of suspensions of micellar casein particles

Small micellar casein particles, so-called submicelles, were obtained by removing colloidal calcium phosphate from native casein by adding sodium polyphosphate. Aqueous submicelle suspensions were characterized using light scattering and rheology as a function of concentration and temperature. The casein submicelles behave like soft spheres that jam at a critical concentration (C(c)) of about 100 g L(-1). The viscosity does not diverge at C(c), but increases sharply, similarly to that of multiarm star polymers. C(c) increases weakly with increasing temperature, which leads to a strong decrease of the viscosity close to and above C(c). Concentrated submicelle suspensions show strong shear-thinning above a critical shear rate and the shear stress becomes independent of the shear rate. The critical shear rates at different temperatures and concentrations are inversely proportional to the zero-shear viscosity. At much higher shear rates, the shear stress fluctuates strongly in time indicating inhomogeneous flow. The frequency dependence of casein submicelle suspensions is characterized by elastic behavior at high frequencies (concentrations) and viscous behavior at low frequencies (concentrations).     

On the effect of an increase in the effective viscosity of nanodispersed Aerosil in Vaseline oil in the range of extra low shear rates

Total flow curves of the suspensions of modified (methylated) nanodispersed Aerosil (mean particle size is 40 nm) in Vaseline oil with concentrations of 2–7 wt % are recorded under quasi-equilibrium conditions. The behavior of these structured nanodisperse systems in the range of extra low shear rates (10^?6-10^?3 s^?1) is studied in detail. In this range of shear rates, the effect of an increase in the rise of effective viscosity with increasing shear stress is revealed for concentrated Aerosil suspensions for the first time.     

Stress jumps on weakly flocculated dispersions: steady state and transient results

Weakly flocculated, thixotropic suspensions have been investigated by means of fast stress jump experiments. With a suitable procedure, reliable stress relaxation data could be collected starting 20 ms after cessation of flow. This technique has been used to determine the elastic and hydrodynamic contributions to the shear stress. Steady state as well as transient flows have been studied for suspensions containing either fumed silica or carbon black particles in a Newtonian medium. In both systems, the elastic stress totally dominates the response at low shear rates and consequently also the apparent yield stress. This stress contribution becomes negligibly small at high shear rates. The hydrodynamic contribution to the viscosity has finite limits at both the low and high shear rate ends. The data are relevant for testing rheological models. As an illustration, it is shown that the data agree qualitatively with the model proposed by Potanin et al. (J. Chem. Phys. 102 (14) (1995) 5845-5853).     

Ionic strength effects on the microstructure and shear rheology of cellulose nanocrystal suspensions

The effect of ionic strength on the rheology and microstructure of Cellulose nanocrystals (CNC) aqueous suspensions are studied over a broad range of CNC (3–15 wt%) and NaCl concentrations (0–15 mM), using polarized optical microscopy combined with rheometry. The CNC suspensions are isotropic at low concentration and form chiral nematic liquid crystalline structure above a first critical concentration and gel above a second critical one. It has been shown that for isotropic CNC suspensions, increasing the ionic strength of the system up to 5 mM NaCl concentration weakens the electro-viscous effects and thus reduces the viscosity of these suspensions. For biphasic samples, which contain chiral nematic liquid crystal domains, increasing the ionic strength up to 5 mM NaCl concentration decreases the size of the chiral nematic domains, and leads the viscosity of the samples at low shear rates to increase. On the other hand, at high shear rates, where all the ordered domains are broken, the viscosity decreases with NaCl addition. For gels, the addition of NaCl up to 5 mM weakens the gel structure and decreases the viscosity. Further addition of NaCl (10 and 15 mM NaCl concentrations) results in extensive aggregation and de-stabilizes the CNC suspensions.     

Critical flux of hard sphere suspensions in crossflow filtration: Hydrodynamic force bias Monte Carlo simulations

A Monte Carlo method is developed for crossflow membrane filtration to determine the critical flux of hard sphere suspensions. Brownian and shear-induced diffusion are incorporated into an effective hydrodynamic force exerted on the hard spheres in a concentrated shear flow. Effects of shear rate and particle size on the critical flux are investigated using hydrodynamic force bias Monte Carlo simulations, providing a baseline of the critical flux.     

Hydrogen bonding in ethanol under shear

We study the dependence of viscosity of ethanol on shear rate using constant volume and constant pressure nonequilibrium molecular dynamics simulations, with the emphasis of the interrelationship between breaking, stability, and alignment of hydrogen bonds and shear thinning at high shear rates. We find that although the majority of hydrogen bond breakings occur at low shear rates, we do not observe shear thinning until there is some shear-induced alignment of the hydrogen bonds with the direction of shear.     

Dynamic yielding, shear thinning, and stress rheology of polymer-particle suspensions and gels

The nonlinear rheological version of our barrier hopping theory for particle-polymer suspensions and gels has been employed to study the effect of steady shear and constant stress on the alpha relaxation time, yielding process, viscosity, and non-Newtonian flow curves. The role of particle volume fraction, polymer-particle size asymmetry ratio, and polymer concentration have been systematically explored. The dynamic yield stress decreases in a polymer-concentration- and volume-fraction-dependent manner that can be described as apparent power laws with effective exponents that monotonically increase with observation time. Stress- or shear-induced thinning of the viscosity becomes more abrupt with increasing magnitude of the quiescent viscosity. Flow curves show an intermediate shear rate dependence of an effective power-law form, becoming more solidlike with increasing depletion attraction. The influence of polymer concentration, particle volume fraction, and polymer-particle size asymmetry ratio on all properties is controlled to a first approximation by how far the system is from the gelation boundary of ideal mode-coupling theory (MCT). This emphasizes the importance of the MCT nonergodicity transition despite its ultimate destruction by activated barrier hopping processes. Comparison of the theoretical results with limited experimental studies is encouraging.     

Structure and rheological behavior of highly charged colloidal particles in a cylindrical pore I. Effect of pore size

In this work we performed nonequilibrium Brownian dynamics (NEBD) computer simulations of highly charged colloidal particles in diluted suspension under a parabolic flow in cylindrical pores. The influence of charged and neutral cylindrical pores on the structure and rheology of suspensions is analyzed. A shear-induced disorder-order-disorder-like transition was monitored for low shear rates and small pore diameters. We calculate the concentration profiles, axial distribution functions, and axial-angular pair correlation functions to determine the structural properties at steady state for a constant shear flow for different pore sizes and flow strengths. Similar behavior has been observed in a planar narrow channel in the case of charged interacting colloidal particles (M.A. Valdez, O. Manero, J. Colloid Interface Sci. 190 (1997) 81). The mobility of the particles in the radial direction decreases rapidly with the flow and becomes practically frozen. The flow exhibits non-Newtonian shear thinning behavior due to interparticle interactions and particle-wall interaction; the apparent viscosity is lower as the pore diameter decreases, giving rise to an apparent slip in the colloidal suspension. The calculated slip velocity was higher than that obtained in a rectangular slit under shear flow.     

Shear-induced structural transformation of pentaethylene glycol <Emphasis Type="Italic">n</Emphasis>-dodecyl ether and lithium perfluorooctane sulfonate mixed-surfactant lamellar solution

The viscoelastic behavior of the shear-induced structural transformation from the lamellar phase to multilamellar vesicles (MLVs) of a mixed-surfactant system was investigated. The transformation was divided into two processes on the basis of the strain dependence of the apparent viscosity. The first stage is a lamellar-to-intermediate structure transformation. It was found that a strain, not an applied shear rate, governed this process. The second stage is an intermediate-to-MLV phase transformation, which was not controlled by the strain. These structure developments were found in the shear-thickening viscosity regime. The MLV phase formed by applying shear flow exhibited shear-thinning viscosity behavior and reversible response to shear flow. The viscoelastic properties of the MLV phase were investigated by dynamic viscoelastic measurements. Under oscillating shear deformation, the amplitude dependence of the dynamic modulus indicated that the viscoelasticity of the MLV depended on the initial structure, such as the number of vesicle shells and the size of the MLV, which is governed by the preshear rate.     

Shear-thickening flow of nanoparticle suspensions flocculated by polymer bridging

The steady-shear viscosity, dynamic viscoelasticity, and stress relaxation behavior were measured for suspensions of silica nanoparticles dispersed in aqueous solutions of poly(ethylene oxide) (PEO). The suspensions of silica with diameters of 8-25 nm show striking shear-thickening profiles in steady shear and highly elastic responses under large strains in oscillatory shear. Since the silica particles are much smaller than the polymer coils, one molecule can extend through several particles by intrachain bridging. Each polymer coil may remain isolated as a floc unit and the silica particles hardly connect two flocs. Therefore, the flow of suspensions is Newtonian with low viscosity at low shear rates. When the polymer coils containing several nanoparticles are subjected to high shear fields, three-dimensional network is developed over the system. The shear-thickening flow may arise from the elastic forces of extended bridges. But, the polymer chain is easily detached from particle surface by thermal energy because of large curvature of particles. As a result, the network structures are reversibly broken down in a quiescent state and the suspensions behaves as viscoelastic fluids with the zero-shear viscosity.     

The influence of salt concentration on negative thixotropy in solutions of partially hydrolyzed polyacrylamide

The influence of NaCl concentration on negative thixotropy in aqueous glycerol solutions of partially hydrolyzed polyacrylamide have been investigated. It was found that negative thixotropy type I (a slow increase in viscosity with time of shearing) sets in at higher critical shear rates when the salt is present. On the other hand, critical shear rates for negative thixotropy type II (a rapid increase in viscosity followed by viscosity oscillation) did not depend on the salt addition. Using the critical shear stress as a hydrodynamic criterion for the occurrence of negative thixotropy, a possible explanation of the behavior is proposed.     

Flow-Induced Anisotropy in Mixtures of Associative Polymers and Latex Particles

The effect of associative polymers on the structure and rheological behavior of colloidal suspensions is discussed. Adding associative polymer is known to increase the viscosity of the suspensions. At high shear rates the increase is close to what could be expected on the basis of the hydrodynamic effects of the added polymer. At low shear rates the viscosity increases much more. Small-angle light scattering (SALS) during flow is used here to investigate the underlying structural mechanisms. The SALS patterns indicate that the associative polymer changes the particulate structure: characteristic butterfly patterns appear even at relatively low particle volume fractions. They are not present in the suspensions without associative polymer. The patterns indicate that fluctuations in particle concentration are more pronounced in the flow direction than in the vorticity direction and that anisotropic particulate structures with an orientation along the vorticity direction develop. The evolution of their characteristic length scale during flow has been followed over time. Changing the hydrophilic part of the polymer from polyacrylamide to polyacrylic acid induces stronger associative interactions. In the suspensions this results in a reduction of the relative viscosity rather than an increase. The difference in degree of associativity between the polymers also has an effect on the SALS patterns in the suspensions both at rest and during flow. The rheology as well as the SALS suggest the presence of a strong polymer network in the second system. The competition between adsorption of the associative polymer on the particles with the intermolecular associations between the polymer chains seems to be responsible for the observed differences. Copyright 2000 Academic Press.     

Flow properties of freshly prepared ettringite suspensions in water at 25 degrees C

The rheology of a complex, heterogeneous mineral colloid was rationalised using models devised for model rod systems. Mixing a calcium hydroxide slurry with an aluminium sulphate solution produces a suspension of rod-shaped ettringite particles. Ettringite rod suspensions exhibit non-Newtonian flow behaviour, which depends on the shape of the particles, their size distribution, concentration and surface properties as well as the suspension medium characteristics. We have measured the shear viscosity of suspensions of ettringite rods with a median aspect ratio, r(i) approximately 8, at 25 degrees C as a function of particle volume fraction, phi, in the range 0.0001-0.08. It was found that the viscosity of the suspensions increased with phi, and showed a marked change of slope at phi approximately 0.01, which we identified as the minimum overlap concentration phi(*). Above phi(*), the system is in the semi-dilute regime. At phi>phi(*), when Pe(rot)>1, hydrodynamic interactions between rods become increasingly significant, and we observe shear-thinning behaviour. The high effective hydrodynamic volume of rotating rods, resulting in much lower values of the maximum packing fraction, phi(c), than for spheres, dominates the rheological behaviour of ettringite suspensions.     

Kinetics of the shear-induced isotropic-to-lamellar transition of an amphiphilic model system: a nonequilibrium molecular dynamics simulation study

The shear-induced isotropic-to-lamellar phase transition in the amphiphilic systems in the vicinity of the quiescent order-to-disorder transition point is investigated by the large-scale parallel nonequilibrium molecular dynamics simulations of simple amphiphilic model systems. There is a shear-induced upward shift of the ordering temperature. The initial isotropic phase orders into a lamellar phase perpendicular to the shear vorticity. The phase diagram as a function of temperature and shear rate is established. The dependency of the ordering transition on interaction strength and shear rate is rationalized by the competition between shear rate and chain relaxation. The time evolution of morphology reveals that the shear-induced ordering proceeds via nucleation and growth, a signature of a first-order phase transition. At low shear rate, a single ordered domain grows after an incubation period. With increasing shear rate ordering speeds up, but eventually develops in a lamellar system with disordered shear bands. The time dependence of the order parameter follows that of the mean-squared end-to-end distance, shear viscosity, and bulk pressure, and follows an Avrami scheme with an Avrami exponent between 2 and 4.     

Lamellar lyomesophases under shear as studied by deuterium nuclear magnetic resonance

The lamellar phases of two aqueous ethylene oxide surfactants, tetra(ethylene oxide) dodecyl ether and hexa(ethylene oxide) dodecyl ether, have been investigated by deuterium nuclear magnetic resonance spectroscopy during shear. The evolution of the shear-induced NMR line shapes and their dependence on surfactant concentration, shear geometry and shear history is analyzed. Two of three previously described shear-induced states (Diat O, Roux D, Nallet F (1993) J Phys II France 3: 1427–1452), namely the state of aligned lamellae with the layer normal parallel to the velocity gradient, which occurs at low shear rates, and the vesicular state at intermediate shear rates are found and identified by their characteristics NMR line shapes.     

Formation and Characterization of a Useful Biological Microemulsion System Using Mixed Oil (Ricebran and Isopropyl Myristate), Polyoxyethylene(2)oleyl Ether (Brij 92), Isopropyl Alcohol,and Water

A ‘biological microemulsion’ prepared using mixed oil (ricebran oil and isopropylmyristate), mixed amphiphiles (Brij 92 and isopropyl alcohol), and water has been studied with respect to phase behavior, shear viscosity, droplet dimension, and energetics of formation. The viscosity measured at different shear rates and temperatures has been analyzed for understanding the internal consistency of the system, and the activation parameters for its viscous flow. The hydrodynamic diameter, diffusion coefficient, and polydispersity of the dispersed droplets of both water-in-oil and oil-in-water samples have been determined from dynamic light scattering (DLS) experiments. The energetics of formation have been evaluated from calorimetric measurements.     

Experimental study on the rheological behavior of nanolubricant-containing MCM-41 nanoparticles with viscosity measurement

In this study, the rheological behavior and viscosity of a stable nanofluid, which is prepared with the suspension of MCM-41 nanoparticles in SAE40 engine oil as base fluid, would be presented. Two-step method has been used to stabilize the nanoparticles in engine oil. To obtain structural and morphological properties of the synthesized nanoparticles, small-angle X-ray scattering, N₂ adsorption/desorption analysis and scanning electron microscopy have been done. Then, viscosity of nanofluids has been measured in temperature range of 25–55 °C, shear rates up to 13,000 s^?1 and different concentrations (0 mass%, 0.5 mass%, 1 mass%, 3 mass% and 5 mass% of MCM-41 nanoparticles). For all the samples, the shear stress versus shear rate diagrams showed that SAE40 oil has Newtonian behavior, in which adding mesoporous silica nanoparticles causes non-Newtonian or pseudoplastic behavior. The results declared that viscosity decreases with increasing temperature and increases with an enhancement in concentration. Furthermore, based on experimental results, an accurate correlation has been proposed to predict the viscosity of SAE40/MCM-41 nanolubricants.

     

Rheological study of an aromatic polyamide-hydrazide

Rheological and rheo-optical data are reported for the poly(terephthalamide of p-aminobenzhydrazide), X-500, in dimethyl sulfoxide solutions in the concentration range 0.2 to 6.0 g/100 ml. The objective of these measurements is to seek evidence of shear-induced isotropic → nematic phase transition. Three types of measurement, Couette, cone and plate, and capillary rheometer, all indicate that this system exhibits flow instabilities at high shear rates and concentrations. In this region both the viscosity and the primary normal stress difference decrease with time under shear. In the capillary rheometer the apparent viscosity is smaller for shorter capillaries and, for short capillaries, there is a range of shear rates in which plug flow and a coiled extrudate are observed. These instabilities may arise from the existence of a mixture of isotropic and nematic phases. At lower shear rates, where the flow behavior appears normal, the concentration dependence of the flow birefringence increases at a critical polymer concentration C This value is in reasonable accord with the concentration corresponding to the change of slope of logη (measured at low shear rate) vs. logC_p. Both the latter measurements appear to be sensitive to the onset of the phenomenon, which may be due to a shear-induced transition or the rheological effect of chain entanglements.     

Viscoelastic properties of liquid crystal polymers: Collective motions and nuclear spin relaxation

Transverse deuterium (²H) nuclear spin relaxation experiments have been performed on a ²H labelled main chain liquid crystal polymer. Relaxation rates are determined as a function of temperature and pulse frequency using a modified Carr-Purcell-Meiboom-Gill pulse train. The results are analysed in terms of a hydrodynamic model for fluctuations of the liquid crystal director. Analytic expressions are employed which relate the transverse spin relaxation rate to the anisotropic viscoelastic parameters of the polymer and allow estimates to be obtained for the effective viscosity and average elastic constant of the polymer. The molecular weight dependence of the viscoelastic parameters has been investigated and is found to be consistent with theoretical predictions for highly extended liquid crystal polymers.