Rheologica Acta最新文献

We investigate the shear flow of a sludge mimicking slurries produced by the nuclear industry and constituted of a dispersion of non-Brownian particles into an attractive colloidal dispersion at a total solid volume fraction of about 10 %. Combining rheometry and ultrasound flow imaging, we show that, upon decreasing the shear rate, the flow transitions from a homogeneous shear profile in the bulk to a fully arrested plug-like state with total wall slip, through an oscillatory regime where strong fluctuations of the slip velocity propagate along the vorticity direction. When the shear stress is imposed close to the yield stress, the shear rate presents large, quasi-periodic peaks, associated with the propagation of local stick-and-slip events along the vorticity direction. Such complex dynamics, reminiscent of similar phenomena reported in much denser suspensions, highlight the importance of local flow characterization to fully understand sludge rheology.

A rheological model was developed to predict the aging behavior, i.e., the increase of viscosity over time, for plastisols containing a fraction of plasticized processed PVC. The model is valid for low-concentration plastisols, composed of a bimodal virgin resin and a monomodal processed PVC powder which is initially plasticized with at least 33 wt% of diisononyl phthalate (DINP). The swelling rate was determined for individual spherical particles of each type and accounts for variations of the transfer surface during diffusion of DINP. The viscosity of this multimodal suspension was subsequently determined according to the Farris approach (Trans Soc Rheol 12:281–301, 1968). Predictions of the model are congruent with experimental data and a subsequent variational study aimed at completing these observations by investigating the effects of parameters that cannot be varied experimentally. Results demonstrate that the aging phenomenon depends on several factors such as amount and initial plasticizer concentration of processed PVC but also initial radius and size distribution as well as shape and deformability of particles. Limitations of the model are also discussed.

Graphical abstract

In this work, we study the rheology of suspensions of polystyrene (PS) spheres at concentrations ranging from 8 to 66% (v/v) in a matrix of molten ethylene vinyl acetate copolymer (EVA) at temperatures spanning through the glass transition temperature of the PS spheres. As a result of the significant difference between the softening temperatures of these blend components, the melt order plays a crucial role in the generation of the blend morphology, rheology, and optimization of processing conditions. When the PS is rigid (below ({T}_{g})), the suspensions showed significant Mullins and Payne effects, disappearing at higher temperatures where PS was sufficiently softened. The origin of these effects is postulated to be due to the extensive filler-filler contacts and radial alignment of the PS beads. Near the glass transition of PS, the PS spheres became deformable and viscosity of the suspension showed an unusual variation as a function of PS loading. The Krieger-Dougherty model was used to predict viscosity variation with volume fraction. We demonstrate that the deformability of the “solid” PS spheres significantly affects the reduced suspension viscosity at both intermediate volume fractions of 0.42 to 0.44, where there was an anomalous dip in viscosity, and at higher volume fractions beyond 0.60, where the increase in viscosity was not as prounounced as that for suspensions of rigid particles.

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).

Graphical Abstract

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.

Graphical abstract

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.