Volume 48, Issue 6, November 2004
Index of content:

Inception of shear flow for ferromagnetic dispersions
View Description Hide DescriptionRheological data are presented for shear flows of concentrated dispersions of acicular ferromagnetic particles; such dispersions are widely used in the manufacture of data storagematerials. In steady shear flow these dispersions are strongly shear thinning. Upon the inception of shear flow, they show stress overshoots for both the shear stress and the primary normal stress difference. The data can be rationalized in terms of a temporary junction network, resulting from the strong magnetic interactions between the particles, if the network consists of relatively weak magnetic links connecting strongly bound flocs of particles. Application of a shear deformation results in disruption of the initially sample spanning network. The overshoot in the shear stress occurs when the flocs become arranged in the shear planes. The overshoot in the primary normal stress difference occurs as a result of deformation of the strongly bound flocs.

Influence of particle modulus on the rheological properties of agar microgel suspensions
View Description Hide DescriptionThe linear viscoelastic and steady shear flowproperties of high phase volume suspensions of a range of agar microgel particles have been measured and are found to depend upon the deformability (or modulus) of the particles. Agar concentrations in the range 0.5–5 wt % are utilized, giving a range of particle modulus spanning 2.4–185 kPa. On increasing the particle modulus, in suspensions with phase volumes above maximum packing, the storage modulus increases by two orders of magnitude although the loss tangent (tan δ) also increases due to increasing viscous dissipation. The flow properties of the suspensions at high shear stresses also showed significant differences due to changing particle rigidity. The suspensions containing the hardest particles are found to display limited evidence of shearthickening behavior at high stresses, while those containing the softest particles continue to shear thin. A highshear plateau in the viscosity is observed for suspensions with particles of medium rigidity. The suspensions containing the stiffer particles also have a considerably higher viscosity for the same degree of space filling. Empirical expressions linking the viscoelastic and flow properties to the particle modulus have been derived.

Rheological properties of whey protein/gum arabic coacervates
View Description Hide DescriptionComplex coacervation in whey protein (WP)/gum arabic (GA) mixtures occurred in a specific window between 2.5 and 4.8. After phase separation, a concentrated polymer (also called coacervate) phase was obtained, whose viscoelastic properties were studied at various values. The viscosity of the WP/GA coacervate exhibited a surprisingly low shearrate dependence, especially at 4.0, from 0.3 to and a shear thinning above indicating a structural change. Hysteresis in the flow curve was measured at the values at which the electrostaticinteractions were the strongest. Hysteresis was due to a slow structural rearrangement of the coacervate phase and, with time, the initial viscosity was completely recovered, showing that structural changes were reversible. In frequency sweep experiments, the values of the viscousmodulus were up to ten times higher than the values of the elastic modulus indicating the highly viscous character of the coacervates. 4.0 appeared to be the at which the coacervate phase was the most concentrated in biopolymer and at which the highest viscosity was measured. By decoupling the effect of biopolymer concentration and electrostaticinteractions, it appeared that the high viscosity of the WP/GA coacervates was mainly due to the strong electrostaticinteractions between WP and GA at low The weaker the electrostaticinteraction was, the lower the viscosity, especially at 4.5 and 3.0. The viscous behavior of the coacervates showed parallels with that of concentrated latex dispersions. WP/GA particles would consist of a GA polymer chain (as in latex) but now crosslinked by the electrostaticinteractions with WP.

Rheooptical studies of the response of entangled polymer solutions to step changes in shear rate
View Description Hide DescriptionExperimental birefringence data are reported for an entangled 4.86 wt % polystyrene solution involving a narrow molecular weight distribution, polymer in tricresyl phosphate. The average number of entanglements per chain was estimated to be approximately 22. The flows considered involve step changes in shear rate from one steady value to another (including the case in which the two shear rates have the same sign and that in which the sign changes and the flow direction is reversed). Comparisons were also made for these flows with data obtained for the same polymer/solvent combination, but at 3.00 wt % where the estimated number of entanglements per chain was approximately 10. The range of shear rates was chosen to span Weissenberg numbers based on the longest Rouse time of and thus provide data that can be used to explore the effects of modest chain stretching on the dynamics. Although the longtime behavior was found to be qualitatively similar to a startup flow, the shorttime dynamics was significantly different. Qualitative considerations based on our current ideas about the dynamics of entangled polymers do not offer a full explanation of these shorttime dynamics.

A successive finegraining scheme for predicting the rheological properties of dilute polymer solutions
View Description Hide DescriptionA new method for refining the predictions of a finitely extensible beadspring chain model with hydrodynamicinteractions is introduced. The suggested successive finegraining procedure involves the representation of a long but finite macromolecule of Kuhn steps with a coarsegrained beadspring chain model, and subsequently "fine graining" the representation by progressively increasing the number of beads, N. Extrapolating the results obtained using exact Brownian dynamics simulations for several values of N to the limit leads to improved estimates for the behavior of the actual polymer chains. The extrapolated results for the extensional viscosity are found to be in good agreement with the recent experimental observations of Gupta et al. (2000). The scaling of the extensional viscosity with molecular weight is also examined.

Thixotropy and rheopexy of aggregated dispersions with wetting polymer
View Description Hide DescriptionDifferent types of thixotropy of aggregated dispersions of magnetic metal, nonmagnetic iron oxide and carbon black particles in organic solvents with a polymer containing pendant wetting groups are studied. Thixotropic behavior depends on the type of aggregation which determines the type of structural recovery in and after shear. Metal particles form denser aggregates combined into flocs or networks recoverable in or after shear, which results in positive thixotropy with upshear stressvsshearrate curves running above downshear curves. By contrast, iron oxide and carbon black particles form looser aggregates which reflocculate slower in shear and do not reflocculate at all at rest after shear, which results in rheopexy with recovery strongly accelerated by shear and negatively thixotropic loops. Viscoelasticmoduli of the latter systems are determined by their shear history, e.g., decreases with the preceding shear rate and does not recover. The effect of the wettingpolymer on timedependant phenomena is twofold: It suppresses thixotropy by consolidating aggregates, but slows down recovery, so that rheopexy is maximized at certain polymertopigment ratios. Furthermore, two types of rheopexy are discriminated. Recovery rate of carbon black dispersions is controlled by the current shear rate only, resulting in thixotropic loops strongly dependent on sweep time. On the other hand, recovery rate of iron oxide dispersions has a long memory of shear history, which leads to thixotropic loops almost insensitive to sweep time. The difference is attributed to slow shearinduced dilation/shrinkage of aggregates built from elongated iron oxide particles, as opposed to stable aggregates of carbon black.

Slip and flow in pastes of soft particles: Direct observation and rheology
View Description Hide DescriptionMicrogelpastes and concentrated emulsions are shown to exhibit a generic slip behavior at low stresses when sheared near smooth surfaces. The magnitude of slip depends on the applied stress. Well above the yield stress, slip is negligible compared to the bulk flow. Just above the yield stress, slip becomes significant and the total deformation results from a combination of bulk flow and slip. At and below the yield stress, the bulk flow is negligible and the apparent motion is entirely due to wall slip. By directly imaging the deformation of pastes and from rheological measurements, we show that slip is characterized by universal scaling properties, which depend on solvent viscosity, bulk shear modulus, and particle size. A model based on elastohydrodynamic lubrication between the squeezed particles and the shearing surface explains these properties quantitatively.

Effect of attractions on shear thickening in dense suspensions
View Description Hide DescriptionShear thickening in dense suspensions is investigated as the strength of interparticle attraction is increased. Starting with hard spheres, attractions are induced through depletion interactions up to and beyond the point where the suspensionsgel and the zero shear rate viscosity diverges. For a range of volume fractions we find that the stress at the onset of shear thickening is a weakly increasing function of the strength of attraction but the extent of shear thickening is diminished. For attractions of sufficient strength to gel the suspension,shear thickening is completely absent. These observations are discussed in terms of a time scale argument for hydrocluster formation and how the shear rate where hydroclusters are formed is influenced by interparticle attractions. For this purpose we develop methods to deconvolute hydrodynamic and thermodynamic contributions to the viscosity and find that the shear rate characterizing the onset of hydrocluster formation is a decreasing function of strength of attraction. Shear thickening is lost because the thermodynamic contributions to the viscosity increase more rapidly than do the hydrodynamic contributions. This conclusion is supported by extension of force balance models for hydrocluster formation to the case where particles experience attractive interactions. These results suggest gelation and shear thickening may be linked as localization phenomena where, in the former case the localization is thermodynamically driven while in the later case, localization is hydrodynamically driven.

Nonlinear shear rheology of polystyrene melt with narrow molecular weight distribution—Experiment and theory
View Description Hide DescriptionMeasurements of the shear viscosity and the first and second normal stress coefficients are shown at 175 °C for a nearly monodisperse polystyrene melt with (PS 200 k). Tests are performed on a conepartitioned plate shear rheometer and cover a range of Weissenberg numbers from 0.13 to 40. Experimental problems encountered are the axial compliance of the rheometer and the normal force capacity of the transducer. The later limits the maximum shear rate to Experimental data are compared with the models of Öttinger [termed thermodynamically consistent reptation model (TCR), Öttinger, H. C., J. Rheol. 43, 1461–1493 (1999)] for the convective constraint release parameter 1, and 2 and Mead, Larson, and Doi [termed Mead, Larson, and Doi (MLD), Mead, D. W., R. G. Larson, and M. Doi, Macromolecules31, 7895–7914 (1998)] for The steady state and transient values of and agree qualitatively well between both models and the experiment. The predicted normal stress ratio is sensitive to the magnitude of in the TCR model, similar to the extinction angle. The MLD model yields and Ψ values lower than both experiments and the TCR model with From a comparison with the chain stretch time (0.065 s) it can be shown that the overshoot of and are linked to chain orientation, whereas is associated with chain stretching. The magnitude of the overshoot for all shear rates increases as for PS 200 k. In comparison, a polydisperse polystyrene melt shows stronger shear thinning of and an increase of the magnitude of the overshoot as

Fluids near a critical point obey a generalized Cox–Merz rule
View Description Hide DescriptionUpon approaching the liquid–vapor critical point, the spontaneous density fluctuations in a smallmolecule fluid increase in both size and lifetime. Similar increases of the concentration fluctuations occur near the critical mixing point of a binary liquid mixture. The presence of these large fluctuations leads to an increase of the zeroshear viscosity, and their persistence in time leads to viscoelasticity and shear thinning. These rheological phenomena, which are already supported by theory and experiment, are shown here to obey a generalized form of the Cox–Merz rule. This relation formally equates the shear viscositymeasured at shear rate with the magnitude of the linear complex viscosity η^{*}(ω) measured at frequency ω. Comparisons of theoretical results and experimental data obtained elsewhere demonstrate that fluids near a critical point obey the somewhat generalized form with The demonstration is simplified by showing that the Carreau–Yasuda model in the form represents the theories for and η^{*}(ω) near critical points. (The product is a time constant, and is a universal critical exponent.).

Flow properties of hard structured particle suspensions
View Description Hide DescriptionA class of structured particles is developed such that the effects of particle interactions are adjusted enabling suspension mechanics to be studied without the influence of interparticle forces other than volume exclusion, Brownian and hydrodynamic interactions. Three grades of these nearhard structured particles are developed from fumed silica. These particles have the same structure as measured by the mass fractal dimension. Stress dependent viscosities and diffusivity measurements are reported as a function of particle concentration, providing an opportunity to determine the scaling behavior of these properties with the two characteristic sizes: the primary particle and aggregate sizes. Except at stresses exceeding the critical value demarking the onset of shear thickening,flow curves superimposed with each other and with hard sphere suspensions compared at the same effective packing fraction,

Ordering kinetics of bodycenteredcubic morphology in diblock copolymer solutions at low temperatures
View Description Hide DescriptionThe ordering kinetics of a bodycentered cubic (bcc) mesophase in squalane solutions of poly(styrenebethylenealtpropylene) (SEP) at low temperatures was studied using rheological and smallangle xray scattering methods. At temperatures more than 110 °C below the ordertodisorder transition temperature the evolution of the storage modulus, and the absolute value of the complex modulus, was found to be describable by the Avrami equation, assuming a “series” mechanical model for the combination of a soft initial phase and a growing hard phase. The ordering half time, of the SEP/squalane solutions decreased with temperature from 50 to 120 °C and increased with concentration from 5 to 10 wt %, indicating the diffusioncontrolled nature of the ordering process in these micellar systems at low temperatures. The temperature dependence of followed the Arrhenius relationship with the activation energy showing negligible concentration dependence. Furthermore, the elastic modulus of the bcc phase was found to increase almost linearly with temperature, which is qualitatively consistent with the assumption of entropic elasticity of the corona chains in the micellar bcc lattice.

The linear viscoelastic behavior of a series of molecular weights of the thermotropic mainchain liquid crystal polymers HBA/HNA 73/27
View Description Hide DescriptionIn this work the rheological properties of the thermotropic liquid crystalline polymer 1,4 hydroxybenzoic acid (HBA)/2,6 hydroxynaphatoic acid (HNA) 73/27 produced by Ticona have been investigated over the molecular weight range 30 000– > 51 000. The thermotropic mainchain liquid crystal polymers (TLCPs) show linear viscoelastic behavior at small strain amplitudes, as detected in strain sweep experiments performed at a constant frequency of 1, 5, 10, 30, 50, and 100 rad/s. The onset for nonlinearity significantly decreases (from ∼35% to ∼5%) as the applied frequency (ω) increases, independently of the molecular weight. When the results are plotted in terms of (the complex modulus normalized to the corresponding linear viscoelastic value) as a function of the stress amplitude, the different curves do not seem to collapse upon each other in the large frequency range investigated in this paper. Indeed, the stress onset for nonlinearity increases as the applied frequency (ω) increases at to at for opposite to the trend shown by the strain onset. The behavior of these TLCPs seems, therefore, intermediate between having a characteristic strain and stress level for the occurrence of nonlinearity. The nonlinear onset dependence on the frequency has been, then, analyzed by horizontally shifting the normalized profiles curves at different ω values and a master curve has been obtained. The frequency behavior of these TLCPs has also been studied showing that the lower molecular weight samples are characterized by a prevalent viscous behavior at high frequencies while decreasing ω the storage modulus tends to level off and dominates the loss modulus. On the other hand, the higher molecular weight samples and show a linear viscoelastic behavior that resembles that of conventional flexible polymers with a terminal regime at the lower frequencies and a transition to the rubberlike region at higher ω. Our results support, then, the hypothesis suggested in the literature [RomoUribe (2001)] of the existence of entanglements in the HBA/HNA 73/27 thermotropic copolyester, whose contribution increase with increasing the molecular weight.