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Nov 2004

Volume 48, Issue 6, pp. 1187-1423


Inception of shear flow for ferromagnetic dispersions

Meihua Piao, D. E. Nikles, A. M. Lane, and J. M. Wiest

J. Rheol. 48, 1187 (2004); http://dx.doi.org/10.1122/1.1803575 (8 pages)

Online Publication Date: 04 Nov 04

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Rheological data are presented for shear flows of concentrated dispersions of acicular ferromagnetic particles; such dispersions are widely used in the manufacture of data storage materials. 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. © 2004 The Society of Rheology.
Show PACS
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Mm Magnetic liquids
47.50.-d Non-Newtonian fluid flows
83.50.Ax Steady shear flows, viscometric flow
83.10.Bb Kinematics of deformation and flow
66.20.-d Viscosity of liquids; diffusive momentum transport
83.60.Fg Shear rate dependent viscosity
47.55.Kf Particle-laden flows
83.80.Gv Electro- and magnetorheological fluids

Influence of particle modulus on the rheological properties of agar microgel suspensions

S. Adams, W. J. Frith, and J. R. Stokes

J. Rheol. 48, 1195 (2004); http://dx.doi.org/10.1122/1.1795193 (19 pages) | Cited 5 times

Online Publication Date: 04 Nov 04

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The linear viscoelastic and steady shear flow properties 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 shear-thickening behavior at high stresses, while those containing the softest particles continue to shear thin. A high-shear 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. © 2004 The Society of Rheology.
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83.80.Kn Physical gels and microgels
83.60.Bc Linear viscoelasticity

Rheological properties of whey protein/gum arabic coacervates

Fanny Weinbreck, Roland H. W. Wientjes, Hans Nieuwenhuijse, Gerard W. Robijn, and Cornelus G. de Kruif

J. Rheol. 48, 1215 (2004); http://dx.doi.org/10.1122/1.1795191 (14 pages) | Cited 1 time

Online Publication Date: 04 Nov 04

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Complex coacervation in whey protein (WP)/gum arabic (GA) mixtures occurred in a specific pH 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 pH values. The viscosity of the WP/GA coacervate exhibited a surprisingly low shear-rate dependence, especially at pH 4.0, from 0.3 to 30 s−1 and a shear thinning above 30 s−1, indicating a structural change. Hysteresis in the flow curve was measured at the pH values at which the electrostatic interactions 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 viscous modulus G″ were up to ten times higher than the values of the elastic modulus G′, indicating the highly viscous character of the coacervates. pH 4.0 appeared to be the pH at which the coacervate phase was the most concentrated in biopolymer (Cp=32%), and at which the highest viscosity was measured. By decoupling the effect of biopolymer concentration and electrostatic interactions, it appeared that the high viscosity of the WP/GA coacervates was mainly due to the strong electrostatic interactions between WP and GA at low pH. The weaker the electrostatic interaction was, the lower the viscosity, especially at pH 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 electrostatic interactions with WP. © 2004 The Society of Rheology.
Show PACS
83.80.Kn Physical gels and microgels
87.14.E- Proteins
87.15.B- Structure of biomolecules
82.45.-h Electrochemistry and electrophoresis
64.75.-g Phase equilibria
82.70.Gg Gels and sols
83.60.Fg Shear rate dependent viscosity
66.20.-d Viscosity of liquids; diffusive momentum transport

Rheo-optical studies of the response of entangled polymer solutions to step changes in shear rate

J. P. Oberhauser, K. Pham, and L. G. Leal

J. Rheol. 48, 1229 (2004); http://dx.doi.org/10.1122/1.1807845 (21 pages) | Cited 4 times

Online Publication Date: 04 Nov 04

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Experimental birefringence data are reported for an entangled 4.86 wt % polystyrene solution involving a narrow molecular weight distribution, 8.42×106 g/mol 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 O(1) and thus provide data that can be used to explore the effects of modest chain stretching on the dynamics. Although the long-time behavior was found to be qualitatively similar to a startup flow, the short-time 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 short-time dynamics. © 2004 The Society of Rheology.
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83.60.Fg Shear rate dependent viscosity
83.80.Rs Polymer solutions

A successive fine-graining scheme for predicting the rheological properties of dilute polymer solutions

R. Prabhakar, J. Ravi Prakash, and T. Sridhar

J. Rheol. 48, 1251 (2004); http://dx.doi.org/10.1122/1.1807841 (28 pages) | Cited 9 times

Online Publication Date: 04 Nov 04

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A new method for refining the predictions of a finitely extensible bead-spring chain model with hydrodynamic interactions is introduced. The suggested successive fine-graining procedure involves the representation of a long but finite macromolecule of NK Kuhn steps with a coarse-grained bead-spring 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 N−1→NK, 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. © 2004 The Society of Rheology.
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83.80.Rs Polymer solutions
83.10.Mj Molecular dynamics, Brownian dynamics
36.20.Cw Molecular weights, dispersity
47.50.-d Non-Newtonian fluid flows
66.20.-d Viscosity of liquids; diffusive momentum transport

Thixotropy and rheopexy of aggregated dispersions with wetting polymer

Andrei Potanin

J. Rheol. 48, 1279 (2004); http://dx.doi.org/10.1122/1.1807844 (15 pages)

Online Publication Date: 04 Nov 04

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Different 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 up-shear stress-vs-shear-rate curves running above down-shear curves. By contrast, iron oxide and carbon black particles form looser aggregates which re-flocculate slower in shear and do not re-flocculate at all at rest after shear, which results in rheopexy with recovery strongly accelerated by shear and negatively thixotropic loops. Viscoelastic moduli of the latter systems are determined by their shear history, e.g., G′ decreases with the preceding shear rate and does not recover. The effect of the wetting polymer on time-dependant phenomena is twofold: It suppresses thixotropy by consolidating aggregates, but slows down recovery, so that rheopexy is maximized at certain polymer-to-pigment 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 shear-induced dilation/shrinkage of aggregates built from elongated iron oxide particles, as opposed to stable aggregates of carbon black. © 2004 The Society of Rheology.
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83.60.Pq Time-dependent structure (thixotropy, rheopexy)
FREE

Slip and flow in pastes of soft particles: Direct observation and rheology

Steven P. Meeker, Roger T. Bonnecaze, and Michel Cloitre

J. Rheol. 48, 1295 (2004); http://dx.doi.org/10.1122/1.1795171 (26 pages) | Cited 18 times

Online Publication Date: 04 Nov 04

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Microgel pastes 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. © 2004 The Society of Rheology.
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47.45.Gx Slip flows and accommodation
82.70.Kj Emulsions and suspensions
82.70.Gg Gels and sols
47.80.-v Instrumentation and measurement methods in fluid dynamics
83.80.Kn Physical gels and microgels
47.55.Kf Particle-laden flows
66.20.-d Viscosity of liquids; diffusive momentum transport

Effect of attractions on shear thickening in dense suspensions

V. Gopalakrishnan and C. F. Zukoski

J. Rheol. 48, 1321 (2004); http://dx.doi.org/10.1122/1.1784785 (24 pages) | Cited 14 times

Online Publication Date: 04 Nov 04

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Shear 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 suspensions gel 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. © 2004 The Society of Rheology.
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83.80.Rs Polymer solutions
83.10.Bb Kinematics of deformation and flow
83.50.Ax Steady shear flows, viscometric flow
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
83.60.Df Nonlinear viscoelasticity
83.60.Fg Shear rate dependent viscosity
83.60.La Viscoplasticity; yield stress

Nonlinear shear rheology of polystyrene melt with narrow molecular weight distribution—Experiment and theory

Thomas Schweizer, Jan van Meerveld, and Hans Christian Öttinger

J. Rheol. 48, 1345 (2004); http://dx.doi.org/10.1122/1.1803577 (19 pages) | Cited 10 times

Online Publication Date: 04 Nov 04

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Measurements of the shear viscosity and the first and second normal stress coefficients are shown at 175 °C for a nearly monodisperse polystyrene melt with Mw=200 kg/mol (PS 200 k). Tests are performed on a cone-partitioned plate shear rheometer and cover a range of Weissenberg numbers (τdmath) 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 τdmath=40. 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 δ2=0, 1, and 2 and Mead, Larson, and Doi [termed Mead, Larson, and Doi (MLD), Mead, D. W., R. G. Larson, and M. Doi, Macromolecules 31, 7895–7914 (1998)] for δ2=1. The steady state and transient values of p21, N2, and N1 agree qualitatively well between both models and the experiment. The predicted normal stress ratio −N2/N1 is sensitive to the magnitude of δ2 in the TCR model, similar to the extinction angle. The MLD model yields ∣N2∣ and Ψ values lower than both experiments and the TCR model with δ2=1. From a comparison with the chain stretch time τs (0.065 s) it can be shown that the overshoot O of ∣N2∣ and p21 are linked to chain orientation, whereas O(N1) is associated with chain stretching. The magnitude of the overshoot for all shear rates increases as O(N1)<O(N2)<O(η) for PS 200 k. In comparison, a polydisperse polystyrene melt shows stronger shear thinning of −N2/N1 and an increase of the magnitude of the overshoot as O(N1)<O(η)<O(N2). © 2004 The Society of Rheology.
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83.80.Sg Polymer melts
82.70.-y Disperse systems; complex fluids
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
66.20.-d Viscosity of liquids; diffusive momentum transport
83.60.Fg Shear rate dependent viscosity

Fluids near a critical point obey a generalized Cox–Merz rule

Robert F. Berg

J. Rheol. 48, 1365 (2004); http://dx.doi.org/10.1122/1.1807843 (9 pages) | Cited 4 times

Online Publication Date: 04 Nov 04

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Upon approaching the liquid–vapor critical point, the spontaneous density fluctuations in a small-molecule 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 zero-shear 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 viscosity η(math) measured at shear rate math 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 η(kCMmath=ω)=∣η∗(ω)∣, with kCM=0.4. The demonstration is simplified by showing that the Carreau–Yasuda model in the form (1+bAγ∣mathτ∣)−p represents the theories for η(math) and ∣η∗(ω)∣ near critical points. (The product bAγτ is a time constant, and p=0.022 is a universal critical exponent.). © 2004 The Society of Rheology.
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66.20.-d Viscosity of liquids; diffusive momentum transport
64.70.F- Liquid-vapor transitions
83.60.Bc Linear viscoelasticity
83.60.Fg Shear rate dependent viscosity
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion

Flow properties of hard structured particle suspensions

W. E. Smith and C. F. Zukoski

J. Rheol. 48, 1375 (2004); http://dx.doi.org/10.1122/1.1807846 (14 pages) | Cited 1 time

Online Publication Date: 04 Nov 04

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A 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 near-hard 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, ϕ/ϕm. © 2004 The Society of Rheology.
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47.55.Kf Particle-laden flows
82.70.Kj Emulsions and suspensions
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
66.10.C- Diffusion and thermal diffusion
66.20.-d Viscosity of liquids; diffusive momentum transport
83.60.Fg Shear rate dependent viscosity

Ordering kinetics of body-centered-cubic morphology in diblock copolymer solutions at low temperatures

Zhizhong Liu, Montgomery T. Shaw, and Benjamin S. Hsiao

J. Rheol. 48, 1389 (2004); http://dx.doi.org/10.1122/1.1807842 (17 pages)

Online Publication Date: 04 Nov 04

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The ordering kinetics of a body-centered cubic (bcc) mesophase in squalane solutions of poly(styrene-b-ethylene-alt-propylene) (SEP) at low temperatures was studied using rheological and small-angle x-ray scattering methods. At temperatures more than 110 °C below the order-to-disorder transition temperature (TODT), the evolution of the storage modulus, G′, and the absolute value of the complex modulus, ∣G∗∣, 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, t0.5, of the SEP/squalane solutions decreased with temperature from 50 to 120 °C and increased with concentration from 5 to 10 wt %, indicating the diffusion-controlled nature of the ordering process in these micellar systems at low temperatures. The temperature dependence of t0.5 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. © 2004 The Society of Rheology.
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83.80.Qr Surfactant and micellar systems, associated polymers
83.80.Uv Block copolymers
83.80.Tc Polymer blends
83.80.Rs Polymer solutions
83.85.Hf X-ray and neutron scattering
61.25.H- Macromolecular and polymers solutions; polymer melts

The linear viscoelastic behavior of a series of molecular weights of the thermotropic main-chain liquid crystal polymers HBA/HNA 73/27

Elvira Somma and Maria Rossella Nobile

J. Rheol. 48, 1407 (2004); http://dx.doi.org/10.1122/1.1795194 (17 pages) | Cited 1 time

Online Publication Date: 04 Nov 04

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In 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 main-chain 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 (γc) 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 G∗/G0∗ (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 (σc∼700 Pa at ω=1 rad/s to σc∼2000 Pa at ω=100 rad/s for Mw=51 000), 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 G∗(γ) 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 (G″ > G′), while decreasing ω the storage modulus tends to level off and dominates the loss modulus. On the other hand, the higher molecular weight samples (Mw=51 000 and Mw > 51 000) 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 rubber-like region at higher ω. Our results support, then, the hypothesis suggested in the literature [Romo-Uribe (2001)] of the existence of entanglements in the HBA/HNA 73/27 thermotropic copolyester, whose contribution increase with increasing the molecular weight. © 2004 The Society of Rheology.
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83.60.Bc Linear viscoelasticity
83.80.Xz Liquid crystals: nematic, cholesteric, smectic, discotic, etc.
61.30.Vx Polymer liquid crystals
62.10.+s Mechanical properties of liquids
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