Rheologica Acta最新文献

Unlike the continuous relaxation spectrum (CRS), discrete relaxation spectra (DRS) are nonunique. This means that the linear viscoelastic response of a material can be described by two or more distinct DRS. Constraints like parsimony and consistency help us to infer meaningful DRS, but are not sufficient to induce uniqueness as it is an inherent property of discretization. Using parsimonious DRS from two different programs (DISCRETE and pyReSpect) on data drawn from experiments, simulations, and theory, we demonstrate that nonuniqueness does not hinder the two most common applications of relaxation spectra, viz. characterization and interconversion. Furthermore, information for reconstructing the CRS underlying the data is embedded in the DRS. Therefore, for most practical considerations, we find that the nonuniqueness of the DRS does not matter.

This work presents a physics-based two-dimensional model for simulating displacement flows of power-law fluids in Hele-Shaw cells. The model is derived by approximating fully developed velocity profiles across the gap-wise direction and averaging the mass and momentum conservation equations, resulting in a two-dimensional formulation that efficiently captures complex fluid dynamics. Implemented in OpenFOAM, this approach achieves computational speeds over 200 times faster than comparable 3D simulations, while preserving the accuracy of displacement dynamics. Validated against 3D DNS results and experimental data, this 2D model accurately replicates observed flow phenomena. Simulations of over 70 cases examined the effect of the ratio of friction pressure gradients (RFG) between fluid pairs on interface stability. Results show that RFGs below unity maintain a flat interface, while higher values induce viscous fingering. In cases with RFG closer to unity, a longer duct or extended displacement time is required for significant finger growth.

It has been reported that the length (molecular weight) of oligomeric solvents has a significant influence on nonlinear rheology of the corresponding polymer solutions. While different explanations, such as flow-induced reduction of monomeric friction or flow-induced phase separation, have been proposed, we show that the influence of oligomeric solvents on nonlinear shear and extensional rheology can be ignored when the length of the oligomers is long enough. We compared three polystyrene (PS) solutions, 600 k-4 k-50%, 600 k-10 k-50%, and 600 k-8a4k-50%, all containing the same weight fraction of the same long PS chains but different styrene oligomeric solvents. The first two contain linear oligomers with different length, while the last two contain oligomers with a similar span length but different molecular architectures (linear and star, respectively). All the solutions show identical nonlinear rheological behavior in startup shear and extensional flows until steady state, and also the same stress relaxation behavior after step shear strain.

The linear and nonlinear rheological behavior of filled polymers has been a research focus for decades. In this study, multi-walled carbon nanotubes (MWCNTs) were incorporated into a linear polypropylene (PPC) and co-extruded with a long-chain branched polypropylene (PPH) to form a multilayer system with a layered distribution of MWCNTs. The nonlinear shear and extensional rheological behaviors of the product films were then characterized in both machine direction (MD) and transverse direction (TD). Interestingly, the number of layers and layer thickness had a significant impact on rheological behavior. When fewer layers with thicker dimensions were present, strain hardening during extension was decreased in the filled system compared with the neat polymer multilayer system. Conversely, when the number of layers increased and the layer thickness decreased, strain hardening in the filled system was notably enhanced, particularly in the transverse direction (TD) during extensional rheology tests. This behavior is attributed to the PPC/MWCNTs layers were confined by the PPH layers effectively as the number of layers increased and layer thickness decreased close to or below the average length of the MWCNTs. In the multipliers, this confinement synergized with the extrusion flow, enhancing the orientation of MWCNTs in the machine direction (MD). In comparison with the multilayer systems composed of only LLDPE and MWCNTs, the neat LLDPE layer showed less impacts to the LLDPE/MWCNTs layer and the MWCNTs orientation. In addition, MWCNTs orientation effects to the elongational viscosities were more significant at lower Hencky strain rates. The enhancement of the MWCNTs orientation was further confirmed and studied by morphology analysis.

Graphical abstract

We derive an operating limit line for the non-ideal artifacts caused by machine stiffness (instrument compliance) which causes measured apparent viscoelastic moduli to be systematically lower than the true values. The limit is represented as a maximum measurable apparent shear modulus (G_{max }), or tensile modulus (E_{max }), which can be shown explicitly on plots of viscoelastic moduli independent of the applied displacement, load, or frequency. Uncorrected data should be much lower than these limits. Corrected data can be above these limits and credible. These interpretations are supported by studying how correction equations can be re-written in terms of (G_{max }) or (E_{max }) and how error propagates in the corrections. We also show how the dynamic compliance representation leads to simpler corrections and how machine stiffness can be calibrated from apparent dynamic compliance measurements of a single sample at two different geometry conditions. Equations are provided for rotational rheometers as well as linear displacement dynamic mechanical analyzers. Used as an operational limit line, (G_{max }) or (E_{max }), the method can assess the credibility of data from others—even without access to their primary data of displacement, force, torque, or amount of correction, which are rarely reported. The method can also anticipate future issues before data are taken, e.g., to understand operational limits when selecting instruments and test geometries.

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.