Rheologica Acta最新文献

The transient rheological behaviour of semi-dilute kaolinite clay suspensions is investigated. Specifically, the flow curve hysteresis and step shear rate tests are used to investigate the shear behaviour of kaolinite suspensions. We found that there is a coupling effect between thixotropy and slip that dominates this transient rheological behaviour. It appears that the onset of solid-like slip in the system is a function of wall roughness and interfacial phenomena between the particles of the suspension and the wall. The coupled phenomena of slip and thixotropy are investigated using the application of sandpaper, varying the gap, and using different geometries. To illustrate the importance of slip in this system, we propose a Coupled Thixotropy and Slip (CTS) model that couples a thixotropic Structure Parameter Model (SPM) to an existing slip model.

We have found that the unique particle properties of fly ash can be applied to the modification of shear thickening fluid. In this paper, rheological properties and microscopic thickening mechanism of fly ash/silicon-based shear thickening fluid (subsequently abbreviated as FA/SiO₂-STF) are studied. Ultrasonic technology and mechanical stirring method were used to prepare FA/SiO₂-STF with different mass fractions of fly ash, and then rheometer was used to carry out steady-state rheological testing for FA/SiO₂-STF, and 4%FA/SiO₂-STF dynamic rheological test and temperature sensitivity testing, respectively. The thickening mechanism of FA/SiO₂-STF was analyzed by scanning electron microscope. The rheological test results show that the FA/SiO₂-STF with 4% fly ash content exhibits remarkable shear thickening effect. Finally, the relationship between the viscosity and shear rate of FA/SiO₂-STF is numerically described by a mathematical model, which can accurately reflect the viscosity thickening effect.

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.

Graphical Abstract

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.

Graphical Abstract

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.