Rheologica Acta最新文献

The particle migration in circular Couette-Poiseuille flows of non-colloidal suspensions is theoretically studied to elucidate the effects of the coupled Couette and Poiseuille flows on the particle distribution. The flow is driven by both simple shear imposed on the inner cylinder and an axial pressure gradient between two infinite co-axial cylinders. We consider concentrated suspensions with bulk particle volume fractions ranging from ({phi }_{mathrm{b}})= 0.2 ~ 0.4, a ratio of the annular gap to the particle radius of (epsilon mathrm{ = 60}), and a radius ratio (i.e., the ratio of inner and outer radii) of (eta mathrm{ = 0.877}). The suspension-balance model is employed with rheological constitutive laws to describe the particle dynamics and predict the particle distributions. The axial flow rate is varied while maintaining a constant shear rate to examine its influence on particle migration. The parabolic axial velocity enhances the local shear rate near the inner and outer walls, causing particles to migrate to the middle of the gap undergoing shear-induced migration. As the axial flow rate increases, an increasing number of particles are transferred to the middle from near the walls. However, a sharp peak, which has been observed in Poiseuille flows, does not appear in the particle distribution. In addition, friction coefficients, which measure the torque acting on the inner cylinder, are evaluated as the axial flow rate is varied. The results reveal that friction coefficients only depend on the axial flow rate.

Startup shear stress data of a well-defined set of binary polystyrene blends consisting of monodisperse blend components were reported recently by Parisi et al. (J Non-Newtonian Fluid Mech 315:105028, 2023). They presented convincing evidence that in the fast flow of melts with narrow molar mass distribution, shear stress undershoot is observed after the overshoot and before approaching the steady state. For blends with broad relaxation time spectra, no undershoot was found. We analyze this data set by comparison with predictions of the rotation zero stretch (RZS) model (Wagner et al. Rheol Acta 63:573–584, 2024), which is generalized here to the multi-mode MRZS model for polymer blends. We confirm that the steady-state shear viscosity of the monodisperse blend components as well as of the binary blends agrees with the viscosity predicted by the Doi-Edwards independent alignment model. As long as there is no undershoot, the RZS model (monodisperse melts) and the MRZS model (binary blends) result in a quantitative description of the full startup curve of the shear stress growth ({sigma }_{12}^{+}(dot{gamma },t)) including overshoot and steady state, based solely on the linear viscoelastic characterization. The shear stress undershoot observed at higher shear rates in melts with narrow molar mass distribution is not described by the RZS or MRZS model. However, the analysis of the experimental data shows clear evidence that undershoot occurs only if after the overshoot, the decreasing shear stress at a higher shear rate undercuts the shear stress at lower rates, i.e., only if (partial {sigma }_{12}^{+}(dot{gamma },t)/partial dot{gamma }<0). For blends with broad relaxation time spectra, (partial {sigma }_{12}^{+}(dot{gamma },t)/partial dot{gamma }cong 0) and no undershoot is observed. The hypothesis is made that undershoot is due to transient shear banding, which is initiated in shear stress regimes characterized by (partial {sigma }_{12}^{+}(dot{gamma },t)/partial dot{gamma }<0) and which disappears at larger strains when the shear stress growth ({sigma }_{12}^{+}(dot{gamma },t)) approaches the steady state ({sigma }_{12}(dot{gamma })) with (partial {sigma }_{12}(dot{gamma })/partial dot{gamma }>0).

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This research presents an assessment of the rheological and anti-aging characteristics of 70# grade asphalts sourced from various manufacturers. By utilizing a range of experimental techniques at both macroscopic and microscopic levels—including gel permeation chromatography (GPC), differential scanning calorimetry (DSC), dynamic shear rheometer (DSR), multi-stress creep recovery (MSCR), linear amplitude scanning (LAS), and bending beam rheometer (BBR) tests—the study investigates the performance of asphalt. Although all five 70# asphalts are classified under the same grade, their performance exhibits significant variation. A direct correlation has been established between microscopic attributes, such as molecular weight distribution and thermal stability, and macroscopic performance, which includes rheological behaviors and anti-aging properties. This study enhances our understanding of the factors influencing asphalt performance and provides guidance for selecting asphalt materials with improved characteristics for highway construction.

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The flow of polymer melts through microscale systems is crucial in several additive manufacturing processes, such as extrusion, injection molding, and polymer three-dimensional (3D) printing. This study conducts a numerical investigation of the flow dynamics of low-density polyethylene (LDPE) polymer melts through a straight microchannel with sidewall cavities. Specifically, it examines the effects of flow rate (quantified by the Weissenberg number) and sidewall cavity aspect ratio (the ratio of cavity width to height) on the transition of the flow field from steady and laminar to unsteady and chaotic due to elastic instability. The findings indicate that flow field fluctuations in polymer melt flows, induced by elastic instability, increase progressively with the Weissenberg number. However, beyond certain Weissenberg number values, the fluctuation intensity is unexpectedly suppressed, indicating a suppression of elastic instability at high Weissenberg numbers. Additionally, as the cavity aspect ratio increases, the flow field fluctuations increase. Nevertheless, the differences in fluctuation become minimal at high Weissenberg numbers. Not only this non-monotonic transition in the flow field but also the vortex dynamics within the system depend strongly on the Weissenberg number and cavity aspect ratio. Various vortices appear in the present flow system, particularly within the cavity region, such as the central primary vortex, corner vortex, and lip vortex. The size, shape, appearance, and disappearance of these vortices are significantly influenced by the Weissenberg number and cavity aspect ratio. Moreover, the study explores the impact of adding another cavity to the microchannel sidewall on this flow transition, and it finds that the additional cavity does not affect the onset of the flow transition. However, it does introduce some differences in vortex dynamics.

Magnetorheological (MR) fluids are utilized to develop a variety of electromechanical devices that have potential applications in the automotive, medical, aerospace, and other areas. Since yield stress is the most important design parameter of an MR fluid and its devices, it is dependent on the types of operational modes. This paper thoroughly examines the effects of the operational modes of MR fluids such as shear mode, and squeeze mode along with the impact of mixed-mode operation on the performance of MR fluids and devices. The study found that mixed-mode operation results in higher yield stress and offers better performance control for MR fluid devices compared to single-mode operation under the same working conditions. Several factors impact the performance of MR fluid devices in various operational modes discussed in the paper such as geometry, initial gap thickness, temperature, pressure, velocity, applied compressive strain, response time, magnetic particle composition, magnetic field, and other working factors. Therefore, there is a necessity to thoroughly examine the rheological and mechanical behaviors of MR fluids and the performance of MR devices in different operational modes and working circumstances, highlighting the experimental and theoretical findings conducted by researchers.

Concentrated emulsions and foams and microgels are comprised of deformable particles making these materials display complex rheological behavior that includes a yielding transition from an elastic solid to viscous fluid. Most studies of this class of soft matter involve shear flows and only a few report both shear and normal stresses. Here, we report measurements of the shear stress and two normal stress differences for a Carbobol microgel, which is usually classified as simple yield stress fluid, subjected to constant shear rate flows. Similar to our previous study, the shear stress evolves through the yield point in a manner indicative of simple yield stress fluid behavior while the normal stress differences evolve in a reproducibly chaotic manner. We also find that the evolution of the stresses is dependent on the whether the microgel has been in a state of relaxation or recovery prior to the measurement.

The mechanisms of flow in suspensions of soft particles above the glass-transition volume fraction and in the jammed state were probed using orthogonal superposition rheometry (OSR). A small amplitude oscillatory shear flow is superimposed orthogonally onto a steady shear flow, which allows monitoring the viscoelastic spectra of sheared jammed core–shell microgels during flow. The characteristic crossover frequency ω_c, deduced from the viscoelastic spectrum, provides information about the shear-induced structural relaxation time, which is connected to the microscopic yielding mechanism of cage breaking. The shear rate evolution of the crossover frequency is used to achieve a superposition of all spectra and get a better insight of the flow mechanism. Despite their inherent softness, the hybrid core–shell microgels exhibit similarities with hard sphere-like flow behavior, with the main difference that for the microgels, the transition from a glassy to a jammed state introduces a volume fraction dependence of the scaling of ω_c with shear rate. We further check the application of the Kramers–Kronig relations on the experimental low strain amplitude OSR data finding a good agreement. Finally, the low frequency response at high strain rates was investigated with open bottom cell geometry, and instrumental limits were identified. Based on these limits, we discuss previous OSR data and findings in repulsive and attractive colloidal glasses and compare them with the current soft particle gels.

We report the linear and nonlinear rheological properties of human serum albumin (HSA) hydrogels with and without urea using small (SAOS) and large (LAOS) amplitude oscillatory shear tests. In SAOS tests, pure HSA and HSA-urea hydrogels exhibited a predominantly solid-like behavior (G' ≫ G''), with viscoelastic properties proportional to HSA concentrations. As urea concentration increased, the viscoelastic properties of the hydrogels decreased and the frequency dependence declined, indicating a sparser cross-linking network. Under LAOS flow, both pure HSA and HSA-urea hydrogels exhibited intra- and inter-cycle strain-stiffening, which became more pronounced with increasing urea concentration. The presence of urea delayed the onset of the nonlinear behavior in HSA-urea hydrogels, in proportion to the urea concentration, decreasing the hydrogel strength. The degree of nonlinearity, quantified by the intrinsic nonlinear parameter Q₀ from Fourier-transform rheology, decreased with increasing urea concentration. Additionally, critical strain amplitudes obtained from LAOS tests indicated that the yield strain of the HSA-urea hydrogels increased with urea concentration. The intracycle behavior was analyzed by sequence of physical processes methods. Molecular dynamics simulations were performed to analyze the hydrogels behavior at the atomic level. Significant changes in the hydrogel network were attributed to the efficient insertion of urea into the HSA hydrogen bond network, which forms the cross-linking network. Thus, the hydrogen bonding between urea and HSA, as well as urea’s contribution to HSA denaturation, affects the gelation, linear, and nonlinear viscoelastic properties of the HSA hydrogels.