[en] In a dense granular system, particles interact in networks containing many particles and interaction times are long compared with the particle binary collision time. In these systems, the streaming part of the granular stress is negligible. We only consider the collisional stress in this paper. The average behavior of particle contacts is studied. By following the statistical method developed recently by the authors [Zhang and Rauenzahn, J. Rheol. 41, 1275 (1997)], we derive an evolution equation for the collisional stress. This equation provides guidance to collateral numerical simulations, which show that the probability distribution of particle contact times is exponential for long contact times. This can be explained by network interactions in a dense granular system. In general, the relaxation of the collisional stress is a combined effect of the decay of the contact time probability and the relaxation of collisional forces among particles. In the numerical simulations, the normal force between a pair of particles is modeled as parallel connect of a spring and a dashpot. In this case, the relaxation of the force magnitude conditionally averaged given a specific contact time is negligible, and the major contribution to the stress relaxation is from the exponential decay of the contact time probability. We also note that the probability decay rate is proportional to the imposed strain rate. Consequently, in a simple shear flow with a constant particle volume fraction, as the shear rate approaches zero, the shear stress approaches a finite value. This value is the yield stress for that particle volume fraction. Hence, the evolution equation of the collisional stress predicts viscoplasticity of dense granular systems. (c) 2000 Society of Rheology

[en] Rheology and small-angle neutron scattering are used to probe the structure of nonionic surfactant mixtures in water. Small amounts of a C₁₄ diol (Surfynol registered 104) cause enormous structural and rheological changes when added to aqueous solutions of an ethylene oxide-propylene oxide-ethylene oxide triblock copolymer (Pluronic registered P105). The C₁₄ diol is only soluble up to 0.1 wt % in pure water, but can be added in large quantities to aqueous solutions of the copolymer. The hydrophobic diol incorporates into the existing copolymer micelles and causes a cascade of changes in the micelle structure, with resultant changes in rheology. Particularly striking is the spherical to worm-like micelle transition, where the viscosity changes by a factor of more than 10⁴

[en] This study focuses on development of a nuclear magnetic resonance (NMR) imaging based viscometric technique using pulsed gradient NMR for characterizing fluid materials under steady tube flow conditions. By simultaneously measuring velocity profiles and pressure gradients it is possible to characterize complex fluids locally. Shear viscosity - shear rate data, ranging from one to over two decades of shear rate from one combined velocity profile/pressure drop per unit tube length measurement, are provided for five fluids that exhibit both Newtonian and shear thinning characteristics. The resolution of the velocity data controls the accuracy of the measured shear viscosity whereas the radial resolution prescribes the number of shear viscosity - shear rate data points and the minimum shear rate. Velocity profile measurements with a velocity resolution of 1 mm/s and radial resolutions which provided from 22 to 110 spatially resolved velocity data points accurately characterized fluids for shear rates greater than 0.1 s^-1. Dynamic yield values measured from velocity data for a microfibrous cellulose solution were within 2% of those measured with a conventional rheometer using the controlled shear rate method and a vane attachment. copyright 1999 Society of Rheology

[en] Using a Couette viscometer, we investigated the dependence of the steady-shear viscosity of simulated neutralized current acid waste on pH and different acids. For a fixed weight percent of waste solids, the relative viscosity decreased as pH was lowered. The magnitude of the relative viscosity varied for the different acids. The values of random maximum packing for these different slurries were obtained using a floc-agglomerate structural model proposed by Tsutsumi et al. (1994). As the weight percent of waste solids is normalized by the value of maximum packing, the relative viscosity data approximately fall onto a single master curve for a specific shear rate. A direct relationship between the macroscopic rheological properties (steady shear viscosity) and the suspension microstructure was established through the packing behavior of a suspension. A comparison of the strongly flocculated and noncolloidal suspensions indicates that besides the normalized weight fraction, the dependence of shear rate must be considered to estimate the relative viscosity for strongly flocculated suspensions. copyright 1996 Society of Rheology

[en] Blends containing 75 wt. % of an amorphous polylactide (PLA) with two different molecular weights and 25 wt. % of a poly[(butylene adipate)-co-terephthalate] (PBAT) were prepared using either a Brabender batch mixer or a twin-screw extruder. These compounds were selected because blending PLA with PBAT can overcome various drawbacks of PLA such as its brittleness and processability limitations. In this study, we investigated the effects of varying the molecular weight of the PLA matrix and of two different mixing processes on the blend morphology and, further, on droplet coalescence during shearing. The rheological properties of these blends were investigated and the interfacial properties were analyzed using the Palierne emulsion model. Droplet coalescence was investigated by applying shear flows of 0.05 and 0.20 s"−"1 at a fixed strain of 60. Subsequently, small amplitude oscillatory shear tests were conducted to investigate changes in the viscoelastic properties. The morphology of the blends was also examined using scanning electron microscope (SEM) micrographs. It was observed that the PBAT droplets were much smaller when twin-screw extrusion was used for the blend preparation. Shearing at 0.05 s"−"1 induced significant droplet coalescence in all blends, but coalescence and changes in the viscoelastic properties were much more pronounced for the PLA-PBAT blend based on a lower molecular weight PLA. The viscoelastic responses were also somehow affected by the thermal degradation of the PLA matrix during the experiments.

[en] The soft glassy rheology (SGR) model has successfully described the time dependent simple shear rheology of a broad class of complex fluids including foams, concentrated emulsions, colloidal glasses, and solvent-free nanoparticle-organic hybrid materials (NOHMs). The model considers a distribution of mesoscopic fluid elements that hop from trap to trap at a rate which is enhanced by the work done to strain the fluid element. While an SGR fluid has a broad exponential distribution of trap energies, the rheology of NOHMs is better described by a narrower energy distribution and we consider both types of trap energy distributions in this study. We introduce a tensorial version of these models with a hopping rate that depends on the orientation of the element relative to the mean stress field, allowing a range of relative strengths of the extensional and simple shear responses of the fluid. As an application of these models we consider the flow of a soft glassy material through a dilute fixed bed of fibers. The dilute fixed bed exhibits a range of local linear flows which alternate in a chaotic manner with time in a Lagrangian reference frame. It is amenable to an analytical treatment and has been used to characterize the strong flow response of many complex fluids including fiber suspensions, dilute polymer solutions and emulsions. We show that the accumulated strain in the fluid elements has an abrupt nonlinear growth at a Deborah number of order one in a manner similar to that observed for polymer solutions. The exponential dependence of the hopping rate on strain leads to a fluid element deformation that grows logarithmically with Deborah number at high Deborah numbers. SGR fluids having a broad range of trap energies flowing through fixed beds can exhibit a range of rheological behaviors at small Deborah numbers ranging from a yield stress, to a power law response and finally to Newtonian behavior

[en] The microrheology of dry soap foams subjected to quasistatic, simple shearing flow is analyzed. Two different monodisperse foams with tetrahedrally close-packed (TCP) structure are examined: Weaire-Phelan (A15) and Friauf-Laves (C15). The elastic-plastic response is evaluated by using the Surface Evolver to calculate foam structures that minimize total surface area at each value of strain. The foam geometry and macroscopic stress are piecewise continuous functions of strain. The stress scales as T/V^1/3, where T is surface tension and V is cell volume. Each discontinuity corresponds to large changes in foam geometry and topology that restore equilibrium to unstable configurations that violate Plateau's laws. The instabilities occur when the length of an edge on a polyhedral foam cell vanishes. The length can tend to zero smoothly or abruptly with strain. The abrupt case occurs when a small increase in strain changes the energy profile in the neighborhood of a foam structure from a local minimum to a saddle point, which can lead to symmetry-breaking bifurcations. In general, the new structure associated with each stable solution branch results from an avalanche of local topology changes called T1 transitions. Each T1 cascade produces different cell neighbors, reduces surface energy, and provides an irreversible, film-level mechanism for plastic yield behavior. Stress-strain curves and average stresses are evaluated by examining foam orientations that admit strain-periodic behavior. For some orientations, the deformation cycle includes Kelvin cells instead of the original TCP structure; but the foam does not remain perfectly ordered. Bifurcations during subsequent T1 cascades lead to disorder and can even cause strain localization. (c) 2000 Society of Rheology

[en] We examine the accuracy of dissipative particle dynamics (DPD) simulations of polymers in dilute solutions with hydrodynamic interaction (HI), at the theta point, modeled by setting the DPD conservative interaction between beads to zero. We compare the first normal-mode relaxation time extracted from the DPD simulations with theoretical predictions from a normal-mode analysis for theta chains. We characterize the influence of bead inertia within the coil by a ratio L_m/R_g, where L_m is the ballistic distance over which bead inertia is lost, and R_g is the radius of gyration of the polymer coil, while the HI strength per bead h* is determined by the ratio of bead hydrodynamic radius (r_H) to the equilibrium spring length. We show how to adjust h* through the spring length and monomer mass, and how to optimize the accuracy of DPD for fixed h* by increasing the friction coefficient (γ ≥ 9) and by incorporating a nonlinear distance dependence into the frictional interaction. Even with this optimization, DPD simulations exhibit deviations of over 20% from the theoretical normal-mode predictions for high HI strength with h* ≥ 0.20, for chains with as many as 100 beads, which is a larger deviation than is found for Stochastic rotation dynamics simulations for similar chains lengths and values of h*

[en] The rheology and three-dimensional microstructure of a concentrated viscoelastic solution of the triblock copolymer poly(ethylene oxide)₁₀₆-poly(propylene oxide)₆₈-poly(ethylene oxide)₁₀₆ (Pluronic F127) in the protic ionic liquid ethylammonium nitrate are measured by small angle neutron scattering (SANS) under flow in three orthogonal planes. This solution's shear-thinning viscosity is due to the formation of two-dimensional hexagonal close-packed (HCP) sliding layer structure. Shear-melting of the crystalline structure is observed without disruption of the self-assembled micelles, resulting in a change in flow properties. Spatially resolved measurements in the 1–2 plane reveal that both shear-melting and sliding are not uniform across the Couette gap. Melting and recrystallization of the HCP layers occur cyclically during a single large amplitude oscillatory shear (LAOS) cycle, in agreement with the “stick-slip” flow mechanism proposed by Hamley et al. [Phys. Rev. E 58, 7620–7628 (1998)]. Analysis of 3D “structural” Lissajous curves show that the cyclic melting and sliding are direct functions of the strain rate amplitude and show perfect correlation with the cyclic stress response during LAOS. Both viscosity and structural order obey the Delaware–Rutgers rule. Combining rheology with in situ spatiotemporally resolved SANS is demonstrated to elucidate the structural origins of the nonlinear rheology of complex fluids

[en] Variation of colloidal and interfacial interactions leads to a microstructural diversity in fumed silica dispersions exhibiting absolutely different sol- or gel-like rheological responses. In this study, fumed silicas with different surface areas (200–400 m²/g) and surface characteristics (hydrophilic or hydrophobic) are dispersed into moisture-cured polyurethane. The microstructures investigated using transmission electron microscope are associated perfectly with three different rheological behaviors: (i) Sols with well-dispersed silica aggregates, (ii) weak gels with agglomerate-linked networks, and (iii) strong gels with concentrated networks of large agglomerates. Though sols and gels are well distinguished by shear thickening or sustained thinning response through steady shear flow test, it is interesting that the sols and weak gels exhibit a uniform modulus plateau-softening-hardening-softening response with increasing dynamic strain at frequency 10 rad s⁻¹ while the strong gels show a sustained softening beyond the linear regime. Furthermore, the onset of softening and hardening can be normalized: The two softening are isoenergetic at mechanical energies of 0.3 J m⁻³ and 10 kJ m⁻³. On the other hand, the hardening is initiated by a critical strain of 60%. The mechanisms involved in the generation of the sol- and the gel-like dispersions and their structural evolutions during shear are thoroughly clarified in relation to the polyols, the characteristic and content of silica and the curing catalysts