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Nov 2005

Volume 49, Issue 6, pp. i-1553


A few words of appreciation

Morton M. Denn

J. Rheol. 49, i (2005); http://dx.doi.org/ ( pages)

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Abstract Unavailable
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99.10.Fg Publisher's note

Viscoelastic coalescence of thermotropic liquid crystalline polymers: The role of transient rheology

Eric Scribben, Aaron P. R. Eberle, and Donald G. Baird

J. Rheol. 49, 1159 (2005); http://dx.doi.org/10.1122/1.2039827 (17 pages)

Online Publication Date: 21 Oct 05

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The coalescence in air of two polymeric drops into a single drop (also referred to as sintering) was investigated for two thermotropic liquid crystalline polymers. Initial coalescence via elastic contact was ruled out based on the magnitude of the equilibrium compliance values and the process was, therefore, believed to be driven by surface tension and resisted by means of viscous flow. Remarkably the viscous coalescence model developed for Newtonian fluids (an extension of the Frenkel and Eshelby approach) agreed well under some conditions of temperature with coalescence data (i.e., observation of neck growth under a microscope). On the other hand, the extension of the Newtonian model to the viscoelastic case by incorporating the upper convected Maxwell model (UCM) assuming steady state stresses always underpredicted the rate of coalescence. The viscous neck growth model using the UCM constitutive equation was extended to the transient stress case in order to incorporate the slow growth of viscosity at the startup of flow. The unsteady state UCM approach represented a qualitative improvement over the Newtonian and steady state UCM formulations because it predicted accelerated coalescence, relative to the Newtonian model, by increasing the relaxation time. However, the model was unable to quantitatively predict the experimental coalescence rates, as it overpredicted the acceleration of coalescence.
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47.50.-d Non-Newtonian fluid flows
66.20.-d Viscosity of liquids; diffusive momentum transport
61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions

Study of electrorheological properties of poly(p-phenylene) dispersions

Anna Krztoń-Maziopa, Henryk Wyciślik, and Janusz Płocharski

J. Rheol. 49, 1177 (2005); http://dx.doi.org/10.1122/1.2048740 (16 pages)

Online Publication Date: 21 Oct 05

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Samples of powdered poly(p-phenylene) lightly doped with ferric chloride were dispersed in silicone oil and the observed electrorheological (ER) effect was studied. The conjugated polymers were obtained in three different synthetic procedures resulting in materials of different crystallinity, which was then additionally modified by annealing in vacuum. The polymer samples were carefully characterized and their electric conductivity and permittivity, doping level, x-ray diffractograms, Fourier-transform infrared spectra, and grain size distribution were determined. The influence of these properties on the magnitude of the ER phenomenon was examined. It was found that the ER activity of the suspensions depended strongly on the crystallinity of a dispersed polymer. This observation was correlated with the ionic conductivity of the material leading to the conclusion that the ER effect in suspensions of FeCl3 doped polyphenylene resulted from bulk polarization processes relying on movement of ions within the polymer matrix. The electronic conductivity typical of doped conjugated polymers contributed predominantly to increase of current flowing through an ER suspension under electric field and not to the ER effect itself.
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83.80.Gv Electro- and magnetorheological fluids
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
61.41.+e Polymers, elastomers, and plastics
72.80.Le Polymers; organic compounds (including organic semiconductors)
61.72.S- Impurities in crystals
81.40.Gh Other heat and thermomechanical treatments
66.30.H- Self-diffusion and ionic conduction in nonmetals
77.22.Ch Permittivity (dielectric function)
78.30.Jw Organic compounds, polymers
82.70.Kj Emulsions and suspensions

The determination of creep and relaxation functions from a single experiment

A. Nikonov, A. R. Davies, and I. Emri

J. Rheol. 49, 1193 (2005); http://dx.doi.org/10.1122/1.2072027 (19 pages) | Cited 3 times

Online Publication Date: 21 Oct 05

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The creep compliance and relaxation functions used in characterizing the mechanical response of linear viscoelastic solids are traditionally found by conducting two separate experiments. Alternatively, one of the functions may be determined from a single experiment while the other is obtained through interconversion. All direct interconversion methods, however, require the solution of an ill-posed problem. The goal of this paper is to present the theoretical framework for developing a new apparatus, based on “spring loading,” which facilitates the determination of both creep and relaxation functions from a single experiment. There is no need for interconversion. Questions of stability with respect to the measured data are discussed and a stable numerical algorithm is presented.
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81.40.Lm Deformation, plasticity, and creep
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.Hg Creep
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances

Thixotropy: Build-up and breakdown curves during flow

Konraad Dullaert and Jan Mewis

J. Rheol. 49, 1213 (2005); http://dx.doi.org/10.1122/1.2039868 (18 pages) | Cited 10 times

Online Publication Date: 21 Oct 05

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Thixotropic systems are usually characterized by measuring the stress transients resulting from sudden changes in shear rate. By using a procedure that takes into account the transfer function of the transducer on a strain-controlled rheometer, the initial part of the transients could be accessed. In this manner complete transient stress curves and accurate constant-structure curves could be recorded on a model system consisting of fumed silica particles dispersed in a Newtonian medium. In agreement with previously obtained stress jump data [Dullaert and Mewis (2005b)], an initial viscoelastic response is detected that precedes the structural time effects in either build-up or breakdown experiments. The effect of flow history on the characteristic times and on the shape of the transient curves is investigated. It is shown that common assumptions in thixotropic models, such as a structure that can be characterized by a single parameter or the existence of exponential stress transients, do not generally hold. Although the material response is quite complex, the time constants of the stress transients obey a simple scaling law. The present data set is very suitable for model evaluation. Here, two representative models, those of Houska and of Coussot, have been tested. The initial parts of the curves are not well described. The time constants for the thixotropic structure build-up are reasonably well predicted, although shear history effects are not properly modeled. For the breakdown curves larger errors are recorded.
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83.60.Pq Time-dependent structure (thixotropy, rheopexy)
47.50.-d Non-Newtonian fluid flows
82.70.Kj Emulsions and suspensions
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids

Effect of block number on multiblock copolymer lamellae alignment under oscillatory shear

Lifeng Wu, Timothy P. Lodge, and Frank S. Bates

J. Rheol. 49, 1231 (2005); http://dx.doi.org/10.1122/1.2048744 (22 pages) | Cited 1 time

Online Publication Date: 21 Oct 05

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The effect of block number on the alignment behavior was systematically studied for lamellae forming poly(styrene-b-isoprene) (SI) diblock to SISISISISIS undecablock copolymers under oscillatory shear. For SI and SIS, both perpendicular and parallel orientations were induced at low strain amplitudes, while only the parallel orientation was produced at large strain amplitudes. For SISI, SISIS, and SISISIS, the perpendicular orientation was induced at low strain amplitudes or high frequencies, whereas the parallel orientation was obtained at large strains and low frequencies. Only the perpendicular alignment was generated with SISISISISIS. The viscoelastic properties of these oriented materials are correlated with the alignment behavior. The parallel configuration in SISI, SISIS, and SISISIS multiblock copolymers is speculated to derive from predominantly looping block conformations, which permit layer-by-layer sliding. Structural evolution during alignment was investigated for SIS and SISIS at low strains using an in situ rheo-small-angle x-ray scattering instrument.
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83.80.Uv Block copolymers
83.80.Tc Polymer blends
61.41.+e Polymers, elastomers, and plastics
83.60.Bc Linear viscoelasticity

Axisymmetric slump and spreading of cohesive plastic soft materials: a yield stress measurement by consisto-rheometry

J-M. Piau

J. Rheol. 49, 1253 (2005); http://dx.doi.org/10.1122/1.2048747 (24 pages)

Online Publication Date: 21 Oct 05

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Empirical consistometry and slump tests can become true rheometrical tests when basic constitutive equation parameters can be calculated from experimental data files. One-dimensional formulas have been obtained successively within the two complementary frameworks of solid mechanics plasticity theory which is fit for moderate slump values, and that of fluid mechanics shallow-water theory, fit for significant spreading. They relate the slump value and/or the spreading radius to the dimensionless ratio of the shear yield stress s and the compression stress ρgH at the basis of the cylinder which was initially released. The influence of the aspect ratio H∕2R and that of the extensional stresses have been taken into account. Both are noticeable. The overall slump regime is found to have good potential as a rheometrical test because the shape of the sample at rest depends only on its volume, not on any other geometrical property. Flow smoothes imperfections which may become more or less disturbing during the other regimes. A discussion of the two theories and comparison with experimental data enables a third matched theory to be suggested and the free surfaces obtained to be modeled over the whole range of relative yield stresses and sample volumes.
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83.60.La Viscoplasticity; yield stress
83.80.-k Material type
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
68.08.Bc Wetting

Viscosity predictions for model miscible polymer blends: Including self-concentration, double reptation, and tube dilation

Jeffrey C. Haley and Timothy P. Lodge

J. Rheol. 49, 1277 (2005); http://dx.doi.org/10.1122/1.2039847 (26 pages)

Online Publication Date: 21 Oct 05

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The viscosities of miscible blends of 1,4-polyisoprene (Mn=78 kg∕mol)/poly(vinylethylene) (Mn=10 kg∕mol and 120 kg∕mol) and polystyrene (Mn=30 kg∕mol)/poly(vinyl methyl ether) (Mn=105 kg∕mol) were measured as a function of temperature and composition. These results, along with literature data for poly(ethylene-alt-propylene)/head-to-head polypropylene [Gell (1996)] and molecular weight blends of 1,4-polybutadiene [Wang et al. (2003)], were compared to the predictions of a previously published model [Haley and Lodge (2004a)]. The agreement between theory and experiment is generally good, though some quantitative discrepancies are apparent. A new tube dilation model that predicts the terminal relaxation time of the slower moving component in a blend as a function of composition is presented. This model enables improved viscosity predictions, and in several cases produces near quantitative predictions.
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66.20.-d Viscosity of liquids; diffusive momentum transport
61.25.H- Macromolecular and polymers solutions; polymer melts

Aging properties of semidilute aqueous solutions of polyethylene oxide seeded with silica nanoparticles

Jing Wang, Linda Heuer, and Daniel Joseph

J. Rheol. 49, 1303 (2005); http://dx.doi.org/10.1122/1.2072007 (14 pages)

Online Publication Date: 21 Oct 05

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Rheological properties of semidilute aqueous solutions of polyethylene oxide (PEO) seeded with silica nanoparticles are studied as a function of aging time. The viscosity, dynamic moduli, and extensional properties of solutions of 0.5% PEO of 4 million g∕mol molecular weight seeded with 10–20 nm silica particles in concentrations of 1%, 2%, and 3% by weight are greatly enhanced by the nanoparticles. The aforementioned properties were measured every week over a period of up to 18 weeks. The degradation of rheological properties with aging time in this period is greatly reduced and, in some cases is completely suppressed, by nanoparticles. The evolution of properties of these nanosolutions is not perfectly understood but is possibly determined in a resolution of the competing effects of adsorption of PEO onto the silica with cleavage of the polymers due to oxidation and other effects at work in static samples.
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83.80.Rs Polymer solutions
66.20.-d Viscosity of liquids; diffusive momentum transport
82.35.Np Nanoparticles in polymers
83.50.Jf Extensional flow and combined shear and extension
61.25.H- Macromolecular and polymers solutions; polymer melts
82.30.Nr Association, addition, insertion, cluster formation

Quantitative prediction of transient and steady-state elongational viscosity of nearly monodisperse polystyrene melts

Manfred H. Wagner, Saeid Kheirandish, and Ole Hassager

J. Rheol. 49, 1317 (2005); http://dx.doi.org/10.1122/1.2048741 (11 pages) | Cited 15 times

Online Publication Date: 21 Oct 05

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Elongational behavior of four narrow molar mass distribution polystyrene melts of masses 50 000, 100 000, 200 000, and 390 000, g∕mol, respectively was investigated up to Hencky strains of 5. All melts show strain hardening behavior. For the two highest molar mass polystyrenes, strain hardening starts at elongation rates larger than the inverse reptation time, and the steady-state elongational viscosities decrease with increasing elongation rate according to a power law with a power-law index of approximately −1∕2 instead of −1 as predicted by the original Doi–Edwards tube model. Marrucci and Ianniruberto [Macromolecules 37, 3934 (2004)] have introduced an interchain pressure term arising from lateral forces between the chain and the tube wall into the Doi–Edwards model to account for the latter effect. Based on the molecular stress function theory allowing for a strain-dependent tube diameter, we show that the transient and steady-state elongational viscosities of the nearly monodisperse polystyrene melts can be modeled quantitatively by assuming affine chain deformation balanced by the interchain pressure term of Marrucci and Ianniruberto. The interchain pressure is governed by a tube diameter relaxation time τa, which is found to be larger than the Rouse time τR of the chain, and which is the only parameter of the model. For monodisperse polystyrene melts of sufficient low molar mass, τa is larger than the reptation time, and a maximum in the steady-state elongational viscosity is predicted.
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83.80.Sg Polymer melts
83.50.-v Deformation and flow
47.50.-d Non-Newtonian fluid flows
66.20.-d Viscosity of liquids; diffusive momentum transport
62.10.+s Mechanical properties of liquids

Time-dependent rheological behavior of bentonite suspensions: An experimental study

Karim Bekkour, Macaire Leyama, Adel Benchabane, and Olivier Scrivener

J. Rheol. 49, 1329 (2005); http://dx.doi.org/10.1122/1.2079267 (17 pages)

Online Publication Date: 21 Oct 05

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The thixotropic behavior of bentonite suspensions was studied using different experimental procedures. It was found that the bentonite dispersions exhibit a time-dependent non-Newtonian behavior. In addition to the major factors affecting the rheological properties in the dispersion were the shearing value and duration, the rest time preceding the measurements and the structural state of the dispersion. The Herschel–Bulkley model was found to correlate well with the behavior of bentonite suspensions. The time evolution of the shear stress at constant shear rates was correlated by the Bird–Leider equation. In order to take into account structural evolution of the suspensions, the rheological law was modified by the introduction of the phenomenological model of Tiu and Boger derived from Moore’s kinetics evolution. It was observed that at short rest times and low shearing conditions, this model describes satisfactorily the time-dependent behavior of the bentonite suspensions. The behavior laws and kinetics evolution were established through the determination of structure destructuration and reorganization rates values. The structure parameters were found to be dependent on the clay concentration, providing evidence that the scale characteristic times of the buildup and breakdown processes are also concentration dependent.
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82.70.Kj Emulsions and suspensions
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
83.60.Pq Time-dependent structure (thixotropy, rheopexy)
47.55.Kf Particle-laden flows
83.50.Ax Steady shear flows, viscometric flow
83.60.La Viscoplasticity; yield stress

Shear thickening in polymer stabilized colloidal dispersions

Lakshmi-Narasimhan Krishnamurthy, Norman J. Wagner, and Jan Mewis

J. Rheol. 49, 1347 (2005); http://dx.doi.org/10.1122/1.2039867 (14 pages) | Cited 3 times

Online Publication Date: 21 Oct 05

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A micromechanical model for the onset of shear thickening in polymer stabilized colloidal dispersions is derived. The model incorporates both the stabilizing forces of the polymer brush or “coat” and the associated modification of the hydrodynamic interactions. Comparison is made to simulations and experiments on well-characterized dispersions. The model is shown to provide a quantitative prediction for the onset of shear thickening in polymer coated colloidal dispersions as compared to simulations. For comparison with experiments, where independent characterization of some parameters is not available, the onset of shear thickening can be predicted with realistic assumptions for the model parameters. The model quantifies the sensitivity of the shear thickening transition to both the interaction potential and the hydrodynamic permeability of the polymer coat.
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82.70.Dd Colloids

Effects of molecular weight and chemical structure on phase transition of thermoplastic polyurethanes

Dario Nichetti, Silvia Cossar, and Nino Grizzuti

J. Rheol. 49, 1361 (2005); http://dx.doi.org/10.1122/1.2071987 (16 pages)

Online Publication Date: 21 Oct 05

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The phase transition of a series of thermoplastic polyurethanes has been investigated by means of rheological and calorimetric techniques. Five different samples of varying molecular weight (MW) and hard-to-soft segment ratio have been studied. Thermal histories included isothermal annealing after cooling from the melt and heating cycles following isothermal annealing. The coupled use of the two different techniques allows for a quantitative description of the kinetics of phase transition and for a good understanding of the corresponding microstructural changes. At least for the family of polymers here investigated, such a process is a combination of two distinct stages, a faster one due to the activation of hard segment interactions, and a subsequent slower one, related to the microphase separation between soft and hard segments. A decrease of MW is found to moderately accelerate the phase transition kinetics, whereas an increase of the hard segment fraction shifts the phase transition to higher temperatures. When the critical gel behavior is considered, experiments clearly indicate the equivalence of either decreasing MW or increasing the hard segment content. Some practical implications of the combined effect of MW, hard segment fraction and phase transition temperature on the polymer processing behavior are also discussed.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
82.35.Jk Copolymers, phase transitions, structure
83.80.Sg Polymer melts
83.80.Uv Block copolymers
64.70.D- Solid-liquid transitions
61.41.+e Polymers, elastomers, and plastics
64.75.-g Phase equilibria
81.40.Gh Other heat and thermomechanical treatments
82.70.Gg Gels and sols

Effects of inertia on the rheology of a dilute emulsion of drops in shear

Xiaoyi Li and Kausik Sarkar

J. Rheol. 49, 1377 (2005); http://dx.doi.org/10.1122/1.2048748 (18 pages) | Cited 4 times

Online Publication Date: 21 Oct 05

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Effects of inertia on the rheology of dilute Newtonian emulsion of drops in shear flow are investigated using direct numerical simulation. The drop shape and flow are computed by solving the Navier-Stokes equation in two phases using Front-tracking method. Effective stress is computed using Batchelor’s formulation, where the interfacial stress is obtained from the simulated drop shape and the perturbation stress from the velocity field. At low Reynolds number, the simulation shows good agreement with various analytical results and experimental measurements. At higher inertia deformation is enhanced and the tilt angle of the drop becomes larger than forty-five degree. The inertial morphology directly affects interfacial stresses. The first and the second interfacial normal stress differences are found to change sign due to the change in drop orientation. The interfacial shear stress is enhanced by inertia and decreases with capillary number at lower inertia but increases at higher inertia. The total excess stresses including perturbation stress contribution shows similar patterns.
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82.70.Kj Emulsions and suspensions
83.80.Iz Emulsions and foams
47.55.D- Drops and bubbles
47.55.Kf Particle-laden flows
83.50.Ax Steady shear flows, viscometric flow
47.11.-j Computational methods in fluid dynamics
47.10.-g General theory in fluid dynamics

Shear alignment of a swollen lamellar phase in a ternary polymer blend

Kasiraman Krishnan, Frank S. Bates, and Timothy P. Lodge

J. Rheol. 49, 1395 (2005); http://dx.doi.org/10.1122/1.2072047 (14 pages) | Cited 2 times

Online Publication Date: 21 Oct 05

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The shear-induced structural changes in the swollen lamellar phase of a ternary polymer blend are investigated, and comparisons are made with the bicontinuous microemulsion (BμE) phase. The BμE, having 10 vol % copolymer, exhibits flow-induced phase separation under steady shear, whereas the swollen lamellar phase, with only 20 vol % copolymer and 80% homopolymers, goes into a perpendicular orientation at all accessible shear rates. This unexpected behavior, seen with small angle neutron scattering, is further characterized by rheological measurements. Oscillatory shear with large amplitude, on the other hand, induces perpendicular alignments at low frequencies, and a flipping of orientation to parallel alignment at higher frequencies. The lamellar phase also exhibits a shear-induced isotropic-lamellar-isotropic transition just above the order-disorder transition. The rheological properties show marked changes as the copolymer concentration is systematically increased from the BμE into the swollen lamellar region of the phase diagram. The orientation patterns observed in the lamellar phase are explained on the basis of fluctuations.
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83.80.Tc Polymer blends
64.70.K- Solid-solid transitions
61.41.+e Polymers, elastomers, and plastics
83.85.Hf X-ray and neutron scattering

Flow of a concentrated suspension through an abrupt axisymmetric expansion measured by nuclear magnetic resonance imaging

Tracey Moraczewski, Haiying Tang, and Nina C. Shapley

J. Rheol. 49, 1409 (2005); http://dx.doi.org/10.1122/1.2079227 (20 pages) | Cited 6 times

Online Publication Date: 21 Oct 05

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The objective of this work is to characterize the parameters affecting the particle concentration distribution and flow field of a concentrated suspension undergoing steady flow in an abrupt axisymmetric 1:4 expansion. Of particular interest are the relationships between imposed operating conditions and the resulting spatial particle distribution and the interaction between particles and recirculating flow regions. Experiments were conducted to determine the effect of the bulk particle volume fraction, tube-particle radius ratio, and Reynolds number on observed concentration and flow patterns. Particle concentration and velocity profiles were measured by using nuclear magnetic resonance imaging. Results indicate that inlet concentration profiles formed in the upstream narrow tube greatly influenced behavior downstream, more so than direct interparticle collisions in the abrupt expansion. Also, particle depletion in recirculating flow regions was observed under all conditions studied and is consistent with the results of previous work expressed in terms of the tube-particle radius ratio. Finally, in all cases, the lengths of recirculating regions were greater for suspensions than for Newtonian fluids at equivalent Reynolds numbers, and increased with bulk particle volume fraction. The flow behavior is reminiscent of shear thinning in a single-phase material, but shear-induced particle migration is a much stronger effect.
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83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
82.70.Kj Emulsions and suspensions
47.55.Kf Particle-laden flows
83.85.Fg NMR/magnetic resonance imaging
76.60.Pc NMR imaging
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.27.N- Wall-bounded shear flow turbulence

Particle migration in a concentrated suspension flowing between rotating parallel plates: Investigation of diffusion flux coefficients

Dima Merhi, Elisabeth Lemaire, Georges Bossis, and Fadl Moukalled

J. Rheol. 49, 1429 (2005); http://dx.doi.org/10.1122/1.2079247 (20 pages) | Cited 6 times

Online Publication Date: 21 Oct 05

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This paper reports an experimental and numerical study conducted to investigate the behavior of macroscopic monomodal concentrated suspensions (40%) undergoing a creeping torsional flow between two rotating plates. An experimental technique based on the detection of tracers by measurement of light absorption is developed and used to quantify the time evolution of the particles concentration profiles. Contrarily to results reported in the literature, an outward migration of the particles is observed. This shear-induced migration is confirmed by viscometric measurements where an increase in the apparent viscosity of the suspension has been observed for long periods of shear. Moreover, this increase is found to depend solely on the value of the applied strain, which is consistent with a shear-induced migration phenomenon. Experimental results are reproduced using a semi-quantitative model involving the balance of three diffusion fluxes induced, respectively, by the gradient of viscosity (Jη), the gradient of the collision rate between particles (Jc), and the flow curvature (Jr). Steady and transient numerical profiles are obtained using a finite volume approach. The coefficients of the diffusion fluxes (Kη,Kc,Kr) are determined by optimizing the numerical profiles to fit the experimental data. The ratios of these coefficients (Kη∕Kc and Kr∕Kc) are found to be independent of the flow geometry with their absolute values being tightly coupled to the direction of particles drift. In particular, the migration coefficients in the direction of the velocity gradient (in a cylindrical Couette flow) are found to be almost five times larger than those along the direction of vorticity (in a rotating parallel-plate flow).
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83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
82.70.Kj Emulsions and suspensions
47.55.Kf Particle-laden flows
47.60.-i Flow phenomena in quasi-one-dimensional systems
66.10.C- Diffusion and thermal diffusion
47.15.G- Low-Reynolds-number (creeping) flows
47.27.N- Wall-bounded shear flow turbulence
83.50.Ax Steady shear flows, viscometric flow
47.80.-v Instrumentation and measurement methods in fluid dynamics
83.85.Jn Viscosity measurements
66.20.-d Viscosity of liquids; diffusive momentum transport
83.60.Fg Shear rate dependent viscosity
47.11.-j Computational methods in fluid dynamics
47.15.ki Inviscid flows with vorticity
47.32.C- Vortex dynamics

“Microviscoelasticity” of colloidal dispersions

Aditya S. Khair and John F. Brady

J. Rheol. 49, 1449 (2005); http://dx.doi.org/10.1122/1.2085173 (33 pages) | Cited 12 times

Online Publication Date: 21 Oct 05

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We study the motion of a Brownian probe particle subjected to a small amplitude oscillatory external force and immersed in a colloidal dispersion, as a theoretical benchmark for particle-tracking microrheology experiments. The application of an external force on the probe drives the microstructure of the dispersion out of equilibrium; opposing this is the Brownian diffusion of the probe and colloidal “bath” particles. The degree to which the microstructure is driven away from equilibrium is given by the nondimensional external force on the probe, or Péclet number, Pe. The nonequilibrium microstructure of the dispersion is calculated for small departures from equilibrium, i.e., to first order in Pe, and to leading order in the bath particle volume fraction, accounting for excluded volume and hydrodynamic interactions between particles. The nonequilibrium microstructure is used to calculate the average velocity of the probe, from which one may infer the complex microviscosity (or modulus) of the dispersion via application of Stokes drag law. The microviscosity is computed over the entire range of oscillation frequencies, thereby determining the linear viscoelastic response of the dispersion. After appropriate scaling, our results are in qualitative agreement with traditional (macro-) rheology studies, suggesting that oscillatory-probe microrheology can be a useful tool to examine the viscoelasticity of hard-sphere colloidal dispersions and perhaps other complex fluids.
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82.70.Dd Colloids
83.60.Bc Linear viscoelasticity
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
83.10.Mj Molecular dynamics, Brownian dynamics
47.55.Kf Particle-laden flows
66.10.C- Diffusion and thermal diffusion
66.20.-d Viscosity of liquids; diffusive momentum transport
05.40.Jc Brownian motion

Microrheology of colloidal dispersions by Brownian dynamics simulations

Ileana C. Carpen and John F. Brady

J. Rheol. 49, 1483 (2005); http://dx.doi.org/10.1122/1.2085174 (20 pages) | Cited 12 times

Online Publication Date: 21 Oct 05

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We investigate active particle-tracking microrheology in a colloidal dispersion by Brownian dynamics simulations. A probe particle is dragged through the dispersion with an externally imposed force in order to access the nonlinear viscoelastic response of the medium. The probe’s motion is governed by a balance between the external force and the entropic “reactive” force of the dispersion resulting from the microstructural deformation. A “microviscosity” is defined by appealing to the Stokes drag on the probe and serves as a measure of the viscoelastic response. This microviscosity is a function of the Péclet number (Pe=Fa∕kT)—the ratio of “driven” (F) to diffusive (kT∕a) transport—as well as of the volume fraction of the force-free bath particles making up the colloidal dispersion. At low Pe—in the passive microrheology regime—the microviscosity can be directly related to the long-time self-diffusivity of the probe. As Pe increases, the microviscosity “force-thins” until another Newtonian plateau is reached at large Pe. Microviscosities for all Péclet numbers and volume fractions can be collapsed onto a single curve through a simple volume fraction scaling and equate well to predictions from dilute microrheology theory. The microviscosity is shown to compare well with traditional macrorheology results (theory and simulations).
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82.70.Dd Colloids
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
83.10.Mj Molecular dynamics, Brownian dynamics
83.60.Df Nonlinear viscoelasticity
47.55.Kf Particle-laden flows
83.50.-v Deformation and flow
65.20.-w Thermal properties of liquids
66.20.-d Viscosity of liquids; diffusive momentum transport
66.10.C- Diffusion and thermal diffusion
62.10.+s Mechanical properties of liquids
05.40.Jc Brownian motion

A sensitive method to detect very low levels of long chain branching from the molar mass distribution and linear viscoelastic response

E. van Ruymbeke, V. Stéphenne, D. Daoust, P. Godard, R. Keunings, and C. Bailly

J. Rheol. 49, 1503 (2005); http://dx.doi.org/10.1122/1.2048743 (18 pages) | Cited 3 times

Online Publication Date: 21 Oct 05

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We have developed a sensitive method to detect very low levels of long chain branching in sparsely branched polymers and applied it to high-density polyethylene samples with broad molar mass distribution obtained by Ziegler–Natta, Phillips, and metallocene catalysis. We compare experimental dynamic moduli with predicted values. The predicted moduli, which are only valid for truly linear chains, are computed from a known molar mass distribution by the application of a modified time-dependent diffusion reptation model described in [van Ruymbeke et al., Macromolecules 35, 2689 (2002)]. Discrepancies between experimental and predicted moduli are observed at branching levels below the usual detection limit of 13C nuclear magnetic resonance (about 1 branch per 10000 carbon atoms). Our method is easier to implement and the results more clearcut than those obtained by the zero-shear viscosity method. The sensitivity is comparable with the activation energy spectrum method described in [Wood-Adams et al., Macromolecules 33, 7489 (2000)]. The use of our method is however not restricted to thermorheologically complex polymers. The interest of polymer fractionation is also demonstrated for the detection of extremely low levels of long chain branching.
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83.80.Sg Polymer melts
83.60.Bc Linear viscoelasticity
61.25.H- Macromolecular and polymers solutions; polymer melts
82.30.Vy Homogeneous catalysis in solution, polymers and zeolites
66.10.C- Diffusion and thermal diffusion

Dynamic yield stress enhancement in bidisperse magnetorheological fluids

David Kittipoomwong, Daniel J. Klingenberg, and John C. Ulicny

J. Rheol. 49, 1521 (2005); http://dx.doi.org/10.1122/1.2085175 (18 pages) | Cited 3 times

Online Publication Date: 21 Oct 05

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Particle-level simulations are employed to investigate the rheological properties of bidisperse magnetorheological fluids. These suspensions are treated as nonlinearly magnetizable, neutrally buoyant, non-Brownian spheres immersed in a nonmagnetizable Newtonian continuous phase. We examine the effects of particle size ratio, composition, and field strength on the dynamic yield stress. The dynamic yield stress of bidisperse suspensions is larger than that of monodisperse suspensions at the same particle volume fraction. The smaller particles cause the larger particles to form more chainlike aggregates than those formed in monodisperse suspensions.
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75.50.Mm Magnetic liquids
82.70.Kj Emulsions and suspensions
83.80.Gv Electro- and magnetorheological fluids
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
83.60.Np Effects of electric and magnetic fields
83.60.La Viscoplasticity; yield stress
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Letter to the Editor: Comment on “Origin of concentric cylinder viscometry” [J. Rheol. 49, 807–818 (2005)]. The relevance of the early days of viscosity, slip at the wall, and stability in concentric cylinder viscometry

J-M. PIAU and M. PIAU

J. Rheol. 49, 1539 (2005); http://dx.doi.org/10.1122/1.2072087 (12 pages)

Online Publication Date: 21 Oct 05

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Abstract Unavailable
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83.85.Jn Viscosity measurements
47.80.-v Instrumentation and measurement methods in fluid dynamics
66.20.-d Viscosity of liquids; diffusive momentum transport
47.45.Gx Slip flows and accommodation
83.50.Lh Slip boundary effects (interfacial and free surface flows)
01.65.+g History of science

Authors’ Response

Prasannarao Dontula, Christopher W. Macosko, and L. E. Scriven

J. Rheol. 49, 1551 (2005); http://dx.doi.org/10.1122/1.2072107 (1 page)

Online Publication Date: 21 Oct 05

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Abstract Unavailable
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83.85.Jn Viscosity measurements
01.65.+g History of science
47.80.-v Instrumentation and measurement methods in fluid dynamics
47.45.Gx Slip flows and accommodation

Erratum: “Interplay of rheology and morphology in melt elongation and subsequent recovery of polystyrene/poly(methyl methacrylate) blends” [J. Rheol. 48, 1103 (2004)]

U. A. Handge and P. Pötschke

J. Rheol. 49, 1553 (2005); http://dx.doi.org/10.1122/1.2126589 (1 page)

Online Publication Date: 21 Oct 05

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Abstract Unavailable
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83.80.Tc Polymer blends
61.25.H- Macromolecular and polymers solutions; polymer melts
68.03.Cd Surface tension and related phenomena
66.20.-d Viscosity of liquids; diffusive momentum transport
99.10.Cd Errata
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