Rheometer

About: Rheometer is a research topic. Over the lifetime, 5759 publications have been published within this topic receiving 125849 citations.

Shear-induced diffusion and rheology of noncolloidal suspensions: Time scales and particle displacements

Abstract: The shear-induced self-diffusion and rheology of concentrated suspensions of noncolloidal hard spheres have been studied experimentally. The combined results provide an interesting physical picture. The projection of the trajectories of individual particles on the vorticity (z)–velocity (x) plane were determined through particle tracking. The particle trajectories turned out to be very useful for gaining qualitative insight into the microscopic particle motion. However, the technique is less suitable to obtain quantitative information. For a quantitative analysis of the particle displacements we measured the evolution of the ensemble averaged displacements as a function of time. The statistical analysis revealed two diffusion regimes, where 〈ΔzΔz〉 ∼ Δt. For large strain values (Δt>1) long-time self-diffusion was observed. The associated diffusion coefficient ∞ is in excellent agreement with literature data on shear-induced self-diffusion. On very short times (Δt≪1) a novel diffusive regime was discovered, characterized by a diffusion coefficient 0, which is significantly smaller than ∞ and grows monotonically with ϕ. 0 is detected on time scales on which the particle configuration is not changed significantly and thus it must represent the fluctuating motion of particles in the “cage” formed by their nearest neighbors. Finally, the rheology was studied with steady shear and oscillatory rheometry. The dynamic measurements in a controlled stress rheometer revealed that the viscoelastic response of the suspension is determined mainly by the amplitude of deformation. At small strain amplitudes γ0<1, the response is linear and a dynamic viscosity η′ is found, which is in excellent agreement with the high frequency limit η∞′ as reported in literature for colloidal hard sphere suspensions. Around γ0 = 1 the “cage” around a particle is deformed and a shear-induced microstructure is built. This leads to O(a) displacements of the particles and the viscoelastic response becomes strongly nonharmonic. Although the effect persists at large amplitudes, it becomes relatively small for γ0≫1. The microstructure is rearranged immediately after flow reversal and remains unchanged for the larger part of the period of oscillation. As a result a pseudolinear viscoelastic regime is found with a viscosity close to steady shear viscosity. Experiments show a correlation between the time scales controlling the 0/∞ diffusive behavior and the ones controlling the shear-induced changes in particle configuration as probed by the rheological measurements.

Dynamic macroscopic heterogeneities in a flexible linear polymer melt.

Abstract: This article describes shear-experiments on flexible linear polymer melts (polystyrene) with molecular weights (4,000 and 17,500 g / mol ) lower than that of critical entanglement. The technique used is a shear piezoelectric rheometer, enabling the complex shear-modulus to be measured in a broad frequency domain (ranging from a few hundredths Hz to some kHz), for weak imposed strains (∼10 −4 ), and for thicknesses between 15 and 100 μm . The results obtained show that the behavior of the shear modulus progressively shifts from a liquid-type behavior to a solid-type as sample-thickness decreases from 100 to 15 μm . This unexpected change in behavior, which is only observed for strong anchoring conditions of the polymer on the substrate, indicates the presence of macroscopic heterogeneities (elastic clusters). We show that these clusters are associated with the glass transition and suggest that they are due to long-range density-fluctuations which are frozen as a result of their ultra-slow relaxation times and thus display an elastic response.

Shear and extensional properties of short glass fiber reinforced polypropylene

Abstract: Shear and extensional properties of a commercial short glass fiber reinforced polypropylene were carefully investigated using commercial rheometers and a novel on-line rheometer. This on-line slit rheometer, installed on an injection molding press, has been designed to measure the steady shear viscosity, the first normal stress difference, and the apparent extensional viscosity of polymer melts and composites for high strain rates up to 105 s−1 in shear and 200 s−1 in extension. Our results show that the steady-state viscosity measurements using the on-line rheometer are in excellent agreement with those obtained using commercial rheometers. The steady-state and the complex viscosities of the composites were found to be fairly close to that of the matrix, but the Cox-Merz rule was not verified for the composites at high rates. The elasticity of the composites was found to be equal to that of the polypropylene matrix. The apparent extensional viscosity was obtained from the pressure drop in the planar converging die of the slit rheometer using the analyses proposed by Cogswell [1] and Binding [2]. The extensional viscosity of the polypropylene was found to be much larger than the shear viscosity at low strain rates with a Trouton ratio of about 40 that decreased rapidly with increasing strain rate down to the value of 4 at 200 s−1. The extensional viscosity of the composites was also found to be close to that of the matrix, with values 35 and 5% larger for the 30 and 10 wt% reinforced polypropylenes, respectively. These results are compared with the predictions of the Goddard model [3], which are shown to overpredict our experimental results. POLYM. COMPOS. 26:247–264, 2005. © 2005 Society of Plastics Engineers.

Shear Flow of a Granular Material

Abstract: The shear flow of a granular material between parallel plates is treated by means of the Boltzmann equation with pseudo-Maxwellian grains. The moments for reverse reflection boundary conditions are found explicitly. The shearing stress is found to depend quadratically on the shear rate.

Extensional rheology of concentrated emulsions as probed by capillary breakup elongational rheometry (CaBER)

Abstract: Elongational flow behavior of w/o emulsions has been investigated using a capillary breakup elongational rheometer (CaBER) equipped with an advanced image processing system allowing for precise assessment of the full filament shape. The transient neck diameter D(t), time evolution of the neck curvature κ(t), the region of deformation ldef and the filament lifetime tc are extracted in order to characterize non-uniform filament thinning. Effects of disperse volume fraction ϕ, droplet size dsv, and continuous phase viscosity ηc on the flow properties have been investigated. At a critical volume fraction ϕc, strong shear thinning, and an apparent shear yield stress τy,s occur and shear flow curves are well described by a Herschel–Bulkley model. In CaBER filaments exhibit sharp necking and tc as well as κmax = κ (t = tc) increase, whereas ldef decreases drastically with increasing ϕ. For ϕ < ϕc, D(t) data can be described by a power-law model based on a cylindrical filament approximation using the exponent n and consistency index k from shear experiments. For ϕ ≥ ϕc, D(t) data are fitted using a one-dimensional Herschel–Bulkley approach, but k and τy,s progressively deviate from shear results as ϕ increases. We attribute this to the failure of the cylindrical filament assumption. Filament lifetime is proportional to ηc at all ϕ. Above ϕc,κmax as well as tc/ηc scale linearly with τy,s. The Laplace pressure at the critical stretch ratio ec which is needed to induce capillary thinning can be identified as the elongational yield stress τy,e, if the experimental parameters are chosen such that the axial curvature of the filament profile can be neglected. This is a unique and robust method to determine this quantity for soft matter with τy < 1,000 Pa. For the emulsion series investigated here a ratio τy,e/τy,s = 2.8 ± 0.4 is found independent of ϕ. This result is captured by a generalized Herschel–Bulkley model including the third invariant of the strain-rate tensor proposed here for the first time, which implies that τy,e and τy,s are independent material parameters.