Bidisperse polymeric mixtures for independent control of viscosity and elasticity

 It is known that the zero shear viscosity of a polydisperse melt of linear polymers depends only on its weight-average molecular weight, whereas its recoverable compliance increases with polydispersity. These facts can be exploited to design model viscoelastic fluids using mixtures of short and long chains of the same homopolymer (bidisperse mixtures). The composition required to obtain a bidisperse mixture with the desired viscosity can be calculated from the molecular weights of the components, and the known relationship between viscosity and weight-average molecular weight. The terminal viscoelastic properties of such a bidisperse mixture are estimated from theoretical predictions for the compliance of bidisperse mixtures available in the literature. These predictions suggest that the elasticity of bidisperse mixtures can be varied independent of their viscosity by appropriately choosing the molecular weights of their components and their composition. This strategy is applied here on bidisperse mixtures of monodisperse 1,4-polyisoprene, which are shown to display second-order fluid behavior over a reasonable range of accessible shear rates. The same procedure is also applied to mixtures of PDMS polymers which are not particularly monodisperse. Rheological measurements show that the elasticity of these polyisoprene and PDMS mixtures can indeed be varied without changing their viscosity. Such materials are ideally suited to study structure-rheological properties relationships in blends of immiscible viscoelastic fluids. Received: 12 April 2001 Accepted: 28 August 2001     

A thermodynamic approach to the rheology of highly interactive filler-polymer mixtures. Part II. Comparison with polystyrene/nanosilica mixtures

The dynamic properties as a function of frequency and strain amplitude, steady-state viscosity as a function of shear rate, and transient shear stresses at startup and cessation of shear flow of polystyrene (PS)/fumed silica mixtures of various concentrations were investigated. An abrupt change in the viscoelastic properties was noticed at a concentration above 1% by volume. Observations by means of scanning electron microscopy (SEM) indicate the presence of a three-dimensional network through the bridging of filler particles by the adsorbed polymer. The viscoelastic behavior is simulated utilizing a theory proposed in Part I (Havet and Isayev 2001) based on a double network created by the entangled polymer matrix and the adsorbed polymer with filler concentration taken into account through the bridging density of polymer-filler interactions and a hydrodynamic reinforcement. The steps taken for determining the model parameters required to carry out the simulation are described. The major features of the rheological behavior of highly interactive polymer-filler mixtures are captured qualitatively and in some cases, quantitatively predicted.     

Numerical and experimental investigation of turbulent characteristics in a drag-reducing flow with surfactant additives

Cetyltrimethyl ammonium chloride (CTAC) surfactant additives, because of their long-life characteristics, can be used as promising drag-reducers in district heating and cooling systems. In the present study we performed both numerical and experimental tests for a 75 ppm CTAC surfactant drag-reducing channel flow. A two-component PIV system was used to measure the instantaneous streamwise and wall-normal velocity components. A Giesekus constitutive equation was adopted to model the extra stress due to the surfactant additives, with the constitutive parameters being determined by well-fitting apparent shear viscosities, as measured by an Advanced Rheometric Expansion System (ARES) rheometer. In the numerical study, we connected the realistic rheological properties with the drag-reduction rate. This is different from previous numerical studies in which the model parameters were set artificially. By performing consistent comparisons between numerical and experimental results, we have obtained an insight into the mechanism of the additive-induced drag-reduction phenomena.

Our simulation showed that the addition of surfactant additives introduces several changes in turbulent flow characteristics: (1) In the viscous sublayer, the mean velocity gradient becomes gentler due to the viscoelastic forces introduced by the additives. The buffer layer becomes expanded and the slope of the velocity profile in the logarithmic layer increases. (2) The locations where the streamwise velocity fluctuation and Reynolds shear stress attain their maximum value shifted from the wall region to the bulk flow region. (3) The root-mean-square velocity fluctuations in the wall-normal direction decrease for the drag-reducing flow. (4) The Reynolds shear stress decreases dramatically and the deficit of the Reynolds shear stress is mainly compensated by the viscoelastic shear stress. (5) The turbulent production becomes much smaller and its peak-value position moves toward the bulk flow region. All of these findings agree qualitatively with experimental measurements.

Regarding flow visualization, the violent streamwise vortices in the near wall region become dramatically suppressed, indicating that the additives weaken the ejection and sweeping motion, and thereby inhibit the generation of turbulence. The reduction in turbulence is accomplished by additive-introduced viscoelastic stress. Surfactant additives have dual effects on frictional drag: (1) introduce viscoelastic shear stress, which increases frictional drag; and (2) dampen the turbulent vortical structures, decrease the turbulent shear stress, and then decrease the frictional drag. Since the second effect is greater than the first one, drag-reduction occurs.     

Effect of viscoelasticity on the rotation of a sphere in shear flow

When particles are dispersed in viscoelastic rather than Newtonian media, the hydrodynamics will be changed entailing differences in suspension rheology. The disturbance velocity profiles and stress distributions around the particle will depend on the viscoelastic material functions. Even in inertialess flows, changes in particle rotation and migration will occur. The problem of the rotation of a single spherical particle in simple shear flow in viscoelastic fluids was recently studied to understand the effects of changes in the rheological properties with both numerical simulations [D’Avino et al., J. Rheol. 52 (2008) 1331–1346] and experiments [Snijkers et al., J. Rheol. 53 (2009) 459–480]. In the simulations, different constitutive models were used to demonstrate the effects of different rheological behavior. In the experiments, fluids with different constitutive properties were chosen. In both studies a slowing down of the rotation speed of the particles was found, when compared to the Newtonian case, as elasticity increases. Surprisingly, the extent of the slowing down of the rotation rate did not depend strongly on the details of the fluid rheology, but primarily on the Weissenberg number defined as the ratio between the first normal stress difference and the shear stress.In the present work, a quantitative comparison between the experimental measurements and novel simulation results is made by considering more realistic constitutive equations as compared to the model fluids used in previous numerical simulations [D’Avino et al., J. Rheol. 52 (2008) 1331–1346]. A multimode Giesekus model with Newtonian solvent as constitutive equation is fitted to the experimentally obtained linear and nonlinear fluid properties and used to simulate the rotation of a torque-free sphere in a range of Weissenberg numbers similar to those in the experiments. A good agreement between the experimental and numerical results is obtained. The local torque and pressure distributions on the particle surface calculated by simulations are shown.     

Thermo-rheological behavior of model protein–polysaccharide mixtures

The use of mixtures of pea protein isolate (PPI) and κ-carrageenan (κ-C) is increasing rapidly with the aim of increasing the stability and viscosity of food products. Recent works have studied their textural and thermal properties but few have studied the influence of the temperature and concentration on the rheological behavior of model systems. In the present work, we study the thermo-rheological properties in the linear and non-linear viscoelastic regimes, in both shear and extensional flows, of mixtures of PPI and κ-C with the aim of obtaining a model for the temperature-induced gelation of complex mixtures of globular vegetable proteins under linear and non-linear deformations. We analyzed the influence of temperature and protein-to-polysaccharide ratios and showed that there are strong changes in the mechanical properties. In shear flows, small-amplitude oscillatory shear was used to study the linear regime and large-amplitude oscillatory shear was used for the non-linear regime. In extensional flows, studies were carried out via the analysis of the dynamics of capillary thinning and breakup process in a filament-thinning rheometer.     

Rheological behavior of needle-like hydroxyapatite nano-particle suspensions

We describe here the preparation and rheological behavior of stable suspensions of needle-like hydroxyapatite nanoparticles dispersed in organic media, including methylethylketone (MEK), polycaprolactone (PCL) solutions in MEK, and PCL melt. These suspensions are the main ingredients in preparing certain biodegradable orthopedic materials that have some advantages over traditional implants. Rheological properties were experimentally determined at shear rates approaching those used in the processing methods such as roll coating, extrusion, and pultrusion. Analysis of the flow behavior suggests possible shear alignment at high Pe number (Pe ≈ 6,000). The linear viscoelastic properties and the paste-like behavior suggest the formation of a network as the particle content increases. These results are critical in designing a process for making composite materials containing highly oriented anisotropic particles.     

Small and large strain rheology of wheat gluten

 The rheological properties of wheat gluten were studied under both small and large deformation and compared with those of the parent flours. The limiting strain of linear viscoelastic behaviour of gluten doughs, 3 × 10⁻², was an order of magnitude larger than that of the flour doughs, 10⁻³. The role of starch in the lower limiting strain of flour doughs was indicated by the exponential decrease in the limiting strain of gluten-starch mixtures with greater quantities of starch. Large strain measurements showed gluten doughs possessed greater shear and elongational viscosities than flour doughs and these differences were greatest at lower shear and elongation rates (0.01 and 0.1 s⁻¹). The larger viscosities of flour and gluten doughs at the low strain rates help to stabilise and prevent the collapse of gas bubbles during bread fermentation and baking. Increasing starch levels in gluten-starch mixtures, at either constant or optimal water levels, lowered the elongational viscosity. Dynamic measurements were, however, more sensitive to the level of water added to the gluten-starch mixtures. The storage modulus decreased with increasing starch levels when constant water levels were used to prepare the mixtures, but when optimal water levels were used the storage modulus increased. Gluten and starch are major contributors to the large and small strain rheological properties of flour doughs; however, gluten-starch mixtures were unable to duplicate exactly the rheological properties of flour doughs, indicating that other flour components such as pentosans, lipids and water soluble proteins also influence dough rheology. Received: 20 March 2001 Accepted: 11 July 2001     

Shear-thickening behavior of polymethylmethacrylate particles suspensions in glycerine–water mixtures

The rheological behavior of polymethylmethacrylate (PMMA) particles suspensions in glycerine–water mixtures has been investigated by means of steady and dynamic rheometry in this work. The shear rheology of these suspensions demonstrates a strong shear thickening behavior. The variations of shear viscosity with the volume fraction and ratios of glycerine to water are discussed. The effect of volume fraction can be qualitatively explained using a clustering mechanism, which attributes the phenomena to the formation of temporary, hydrodynamic clusters. The influence of interactions between glycerine–water mixtures and PMMA particles on shear thickening is investigated by varying the ratio of glycerine to water. In addition, the reversible and thixotropic properties of suspensions of PMMA dispersed in glycerine–water (3:1) mixtures are also investigated, and the results demonstrate the excellent reversible and thixotropic properties of PMMA particle suspensions.     

A thermodynamic approach to the rheology of highly interactive filler-polymer mixtures: Part I – Theory

The rheological behavior of highly interactive filler-polymer mixtures is simulated utilizing a double network created by the entangled polymer matrix and the adsorbed polymer. Both networks are represented by a nonlinear viscoelastic constitutive equation. The dependence of rheological properties on filler concentration is taken into account through the bridging density resulting from polymer-filler interactions and a hydrodynamic reinforcement. The relative contribution of both networks is computed through the energy balance consistent with the thermodynamics of the polymer-filler chemical interactions and fluid mechanics. This self-consistent approach allows one to calculate the strain dependence of dynamic properties under oscillatory flow and shear rate dependence of stresses under steady simple shear flow and upon start up and cessation of shear flow. Received: 11 May 2000 Accepted: 8 March 2001     

Sedimentation of particles in shear flows of viscoelastic fluids with fibers

We study the effect of fiber additives on rheology and sedimentation of particle suspensions in a base viscoelastic suspending fluid in the case when the suspension is subjected to shear flow. We found experimentally that fiber additives (3–6 mm in length and 8–12 μm in diameter at a mass fraction of 0–0.4%) increase the suspension viscosity and retard the particle sedimentation significantly. At the same mass concentration, long and thin fibers reduce the sedimentation velocity and increase the viscosity to a much greater extent than short and thick fibers. We revealed that both rheology and sedimentation are controlled by a single conformational parameter (overlap parameter) defined as the number of fibers per unit volume multiplied by fiber length cubed.