Rheologica Acta最新文献

Flow-induced disentanglement (FID or CCR-D) and chain tumbling are two molecular mechanisms typically observed in non-equilibrium molecular dynamics simulations of entangled polymer melts under fast shear. As regards quantitative performance, classical tube models exhibit limitations at fast rates presumably due to the negligence of the aforementioned mechanisms. CCR-D or tumbling inclusion is reported in some revised tube models. For example, in Desai–Larson’s (DL) work (J Rheol 58:255–279, 2014), which focuses on uniaxial elongation, FID is coupled to the alignment and stretch status of the chains. In Costanzo et al. (Macromolecules 49:3925–3935, 2016), tumbling is accounted for via incorporation of a semi-empirical tumbling function in the stretch equation. Nevertheless, CCR-D is neglected. Here, we include tumbling in the DL differential constitutive set and we assess its performance at shear and relaxation following shear. Model predictions are compared against data on various polystyrene melts as obtained by a cone-partitioned-plate fixture.

We suggest a new interpretation for pH-dependent temporal evolution in the microstructure and rheology of silica suspensions based on interparticle interactions, which differs from the conventional explanation including the catalytic effect of hydroxyl ions and charges. The temporal evolution of silica suspensions under various pH conditions was investigated through rheometry and diffusing wave spectroscopy (DWS) analysis. The transition from liquid to solid was observed to be the most rapid at pH 5 compared to other pH conditions (pH 3, 7, 9). This phenomenon could not be adequately explained by the conventional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory that predicts the liquid-to-solid transition occurs more rapidly at the lower pH condition due to the lower surface charge. As an alternative, we employed an elaborated DLVO theory that additionally considers the hydration force, arising from the hydrophilic nature of the silica surface. The pH dependency was interpreted using the elaborated DLVO theory, which showed that the strong short-range nature of the hydration force significantly reduced the attraction range at pH 3, leading to the retardation and decline in structural and rheological development. The impact of pH and resulting alterations in interparticle interaction on the microstructure were further investigated using rheological scaling theory.

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Liquid crystalline polymer (LCP) solutions undergo uniaxial elongation in fiber spinning, yielding highly oriented fibril-structured fibers with enhanced orientation and mechanical properties. This study explores how initial fiber orientation and Frank elasticity influence the dynamics and stability of the isothermal spinning process for LCP solutions. The simplified Larson-Doi mesoscopic model is employed, capable of capturing elastic stress emerging from domain structure evolution. Two main factors, inlet orientation and the Ericksen number as a parameter representing Frank elasticity, significantly affect steady-state fiber orientation profiles and the onset of draw resonance instability, as determined through linear stability analysis. The sensitivity of spinline flow to a sinusoidal disturbance is assessed using the frequency response method. Changes in stability onset concerning these two main factors are reasonably correlated with the extensional behavior of the LCP solution in the spinline and the results of the frequency response.

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This work investigates transient non-colloidal suspension flows in cone-and-plate, plate-plate, and cylindrical geometries to assess particle motion’s impact on viscosity measurement. Mass and momentum conservation equations model the two-phase liquid–solid flow, with both phases treated as continuous in an Euler-Euler approach. Findings demonstrate rheometric flow induces particle motion, affecting suspension homogeneity and viscosity measurement over time. Both buoyancy and inertia effects drive particle motion, with buoyancy dominating at low shear rates and inertia at high shear rates. Particle volume fractions, shear rates, and liquid viscosity notably influence viscosity measurements. Measurements with concentric cylinders are the least affected by particle motion. Additionally, we propose a time limit and a critical Reynolds number in which particle motion does not affect the measurement of the suspension viscosity.

We present linear viscoelastic data with anionically synthesized and critically fractionated polybutadiene (rich in vinyl content) rings having about Z = 22 entanglements. These rings are experimentally as pure as currently possible. They exhibit a power-law stress relaxation G(t) that is well-described by the state-of-the-art fractal loopy globule (FLG) model (power-law exponent of − 3/7). Previously reported data with polystyrene rings, prepared by anionic synthesis in dilute solution and purified by liquid chromatography at the critical condition, having Z = 14 entanglements, showed a power-law G(t) as well. Recent developments with different synthetic methods yielding not so well-characterized rings with a very large number of entanglements (up to 300), suggest that a rubbery plateau emerges in the linear viscoelastic response for Z > 15. Our work confirms the power-law G(t) with the FLG exponent with another chemistry and contributes to the current discussion about different regimes of rheological behavior, indicating that a possible deviation from the power-law FLG type of behavior toward rubbery plateau may occur for Z > 22. To fully capture the experimental G(t) data, the FLG model is complemented by two additional relaxation modes which are attributed to ring-ring (RR) and ring-linear (RL) threading, in accordance with recent reports in the literature. The faster RR mode likely reflects a new mechanism of stress relaxation not described by FLG, and the slower RL mode is attributed to synthetic and material handling imperfections (for example, due to thermal treatment). However, it does not change the punchline of the work: no rubbery plateau for entangled rings with up to 22 entanglements.

Graphical Abstract

Stress relaxation modulus for entangled ring polybutadiene (exhibiting power-law decay) and its linear precursor (exhibiting rubbery plateau), along with fits to the data: tube model for linear chains, and fractal loopy globule (FLG) with slow modes (RR and Tsalikis et al.) for the ring.

We consider the elongational rheology of model polystyrene topologies with 2, 3 and 4 stars, which are connected by one (2-star or “Pom-Pom”), two (3-star) and three (4-star) linear backbone chains. The number of arms of each star varies from q_a = 3 to 24, the molecular weight of the arms from M_w,a = 25 kg/mol to 300 kg/mol, and the backbone chains from M_w,b = 100 kg/mol to 382 kg/mol. If the length of the arm is shorter than the length of the backbone, i.e. M_w,a < M_w,b, and despite the vastly different topologies considered, the elongational stress growth coefficient can be modeled by the Hierarchical Multi-mode Molecular Stress Function (HMMSF) model, based exclusively on the linear-viscoelastic characterization and a single nonlinear parameter, the dilution modulus. If the length of the arms of the stars is similar or longer than the length of the backbone chain (M_w,a ≥ M_w,b) connecting two stars, the impact of the backbone chain on the rheology vanishes and the elongational stress growth coefficient is dominated by the star topology showing similar features of the elongational stress growth coefficient as those of linear polymers.

Graphical Abstract

Food rheology is key to manage the swallowing safety of people affected by swallowing disorders (dysphagia). Simple approaches to assess the flow properties of texture-modified drinks are widely used, but relatively poorly understood. This study focuses on the International Dysphagia Diet Standardisation Initiative (IDDSI) flow test, adopted by caregivers worldwide. This test considers the gravity-driven flow in a vertical syringe. Newtonian liquids and non-Newtonian fluids obtained using a commercial starch-based thickener were considered in this study. An accurate theoretical description of the flow is proposed for Newtonian and power-law fluids considering the effect of fluid properties and of the syringe geometry. A rheological map is proposed, based on the results of several thousand simulations, to capture quantitatively the effect of rheological properties and density on the IDDSI classification, highlighting the important effect of the fluid density which is usually ignored. The sensitivity of the IDDSI results with respect to the syringe outlet diameter is discussed, as well as the different average shear rates at which different IDDSI levels are tested. The rheological map also shows quantitatively that different combinations of the fluid rheological properties and density can result in the same IDDSI classification, suggesting interesting directions for future clinical research.

The transition from concentric primary flow to non-tangential secondary flow of blood was investigated using experimental steady shear rheometry and numerical modelling. The aims were to: assess the difference in secondary flow in a Newtonian versus shear-thinning blood analogue; and measure the secondary flow in the blood. Both experiments and numerical modelling showed that the transition from primary to secondary flow was the same in a Newtonian fluid and a shear-thinning blood analogue. Experiments showed whole blood transitioned to secondary flow at lower modified Reynolds numbers than the Newtonian fluid; and transition was haematocrit dependent with higher RBC concentrations transitioning at lower modified Reynolds numbers. These results indicate that modelling blood as a purely shear-thinning fluid does not predict the correct secondary flow fields in whole blood; non-Newtonian effects beyond shear-thinning behaviour are influential, and incorporating effects such as multiphase contributions and viscoelasticity, yield stress and thixotropy is necessary.

In quasi-two-dimensional conditions, different invasion patterns can be observed when a fluid displaces another fluid with a higher viscosity. The transition from interfacial instability to fracture during gas invasion remained poorly understood, and classification criteria for different invasion patterns demanded further improvement. In this study, single-point compressed gas injection experiments were conducted in magnesium lithium phyllosilicate (MLPS) suspensions in a rectangular Hele-Shaw cell. Interestingly, with the increase of the injection pressure and the decrease of the concentration of MLPS suspension, the gas invasion pattern underwent a transition from viscous elastic fracture, elastic fracture, elastic viscous fingering to viscous fingering, in which viscous elastic fracture was observed for the first times. We detailly discuss the characteristics and occurrence conditions of each invasion pattern. Furthermore, by analyzing the velocity field of each invasion pattern, it is found that the relationship between the velocity direction around the gas and the gas growth direction varies with different invasion patterns. A simple and effective quantitative indicator is constructed to distinguish the different invasion patterns. Following the identification of invasion patterns, a further investigation was conducted into the relationship between invasion patterns and experimental conditions. By utilizing the relationships among injection conditions and material rheological properties, two dimensionless numbers, Bingham number and Weissenberg number, are conducted, which have an impact on the various invasion patterns and invasion process. A unified phase diagram based on the Bingham number and Weissenberg number was also proposed to incorporate the possible gas invasion patterns in the MLPS suspension.