Rheologica Acta最新文献

Understanding the interactive behavior of Janus particles (JPs) is a growing field of research. The enhancement in binding energy, in comparison to homogenous particles, and the dual characteristic of JPs open up new possibilities for novel applications. In many such applications, interfacial materials become subjected to flows that produce dilational and shear stresses. Therefore, it is important to understand the impact that the Janus character brings to interfaces. In this work, we study the microstructure of two-dimensional (2D) JP monolayers formed at the air–water interface and examine the shear viscoelasticity with an interface rheometer that was adapted for in situ surface pressure control via a Langmuir trough. We extend concepts from bulk rheology to data obtained from interfacial rheology as a tool to understand and predict the monolayer’s viscoelastic behavior. Finally, by calculating the time relaxation spectrum from the measured 2D dynamic moduli, we conclude that a phenomenon similar to glass transition is taking place by analogy.

Shear thickening fluids (STFs) exhibit a liquid–solid-like transition under impact because of the formation and evolution of shear jamming in the STFs. This study aims to develop a computational fluid dynamics (CFD) model to simulate the shear jamming formation and evolution in a concentrated STF under impact for the optimum design of the STF applications. The STF was defined with a strain rate–dependent viscosity and compressibility. In the CFD model, the interface between air and STF was modelled by the volume of fluid method to solve the multiphase flow problem. In addition, the impact penetration process of an impactor was reproduced by the change of the fluid domain shape with a dynamic mesh method. The shear jamming was demonstrated clearly by a high strain–rate region caused by the impact. The numerical results were comparable to the experimental observations of shear jamming evolution using a high-speed camera. Furthermore, the numerical results showed that the effect of the STF’s dimensions (depth and diameter) on the expansion rate of the shear jamming was insignificant.

The phenomenon of discontinuous shear thickening (DST) is observed in suspensions of solid particles with a very high-volume fraction. For suspensions of ferromagnetic particles, this transition can also be triggered by the application of a magnetic field as discussed by Bossis et al. (2016). Here, we explore the rheological behavior of a bidisperse suspension made of magnetic-carbonyl iron (CI) and non-magnetic-calcium carbonate (CC) particles with a brush-like coating of the same superplasticizer molecule. We highlight the synergetic effect of non-magnetic particles whose inclusion in the percolated frictional network amplifies the effect of the magnetic field on the remaining fraction of magnetic particles. In plate-plate geometry, a small fraction of ferromagnetic particles (about 5%) is sufficient to trigger the transition by the application of a magnetic field and optimum for the increase of viscosity. The progressive interpenetration of the coating layers of polymer and the demagnetizing field can explain this behavior.

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Abstract

Extensional rheology of a variety of linear and branched polymer melts is investigated using entry flow measurements and 15:1 axisymmetric contraction flow simulations. Using a Cogswell model analysis, we show that log−log plots of entrance pressure drop versus wall shear stress display three distinct power-law regimes, the intermediate one of which is observed beyond a critical stress associated with the onset of chain stretching effects. Our observations suggest that this stress threshold is a chain architecture-dependent property characteristic of entangled polymers. Converging flow methods are used to analyze the excess pressure losses to predict the uniaxial extensional viscosity. As the temperature is increased, the progressive shift of the kink to higher strain rates seen in the flow curves can be captured by a proposed Trouton ratio model, where the characteristic time of the fluid is assumed to follow the empirical William–Landel–Ferry (WLF) equation. Experimental pressure drops in converging flows for Weissenberg numbers up to about 10⁵ are used to evaluate predictions of an extended generalized Newtonian fluid (GNF-X) model, where a weighted viscosity for mixed flows has recently been derived and a weighting function classifies flows intermediate between shear and shearfree flows. Judging from its success in predicting the nonlinear extensional response of both linear and branched polymers, as well as its ability to differentiate the respective flow patterns, the GNF-X model should be useful for simulations of commercial polymer processing.

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Nanocomposite hydrogels were elaborated by the addition of citrated magnetic nanoparticles (MNPs) in sodium alginate aqueous solutions ionically crosslinked by in situ release of calcium ions from calcium carbonate (CaCO₃) with gradual hydrolysis of d-glucono-δ lactone (GDL). The sol-gel transition was studied by time-resolved mechanical spectroscopy (TRMS) in the linear viscoelastic region. The power law frequency dependence of the storage and loss moduli allowed to determine the gelation time (t_g), the power law relaxation exponent (Δ), and the gel stiffness (S) at the critical gel (gel at t_g) for different calcium and MNP concentrations. The effect of an applied magnetic field on these parameters was also studied for the first time. The obtained results show an effect of the concentration of both calcium and MNPs on the kinetics (t_g) and properties at the critical gel (S and Δ) obtaining faster kinetics and harder critical gels for higher calcium and lower MNP concentrations. Moreover, the application of the magnetic field allows to modulate the viscoelastic properties before the gel point, but no effect was observed on the structural properties of the critical gel. Finally, this work highlights how the shear viscoelastic, compressive, and swelling properties of totally gelled nanocomposite hydrogels can be successfully modulated when MNPs are introduced in the calcium alginate matrices with a good agreement between all these properties and with the properties of the critical gels.

The rheological properties of associating polymers are driven by a combination of entanglements and dynamic intermolecular interactions between chains. Binary mixtures of aminopolyolefin associating polymers with low and high molecular weight (M_w) were prepared at systematic mass fractions. The ability to tune rheological, mechanical, and adhesive properties was investigated. A transition from viscoelastic liquid to elastic solid is traversed between the limits of low and high M_w by controlling the ratio of the two components. As the fraction of high M_w increases, the mechanical properties (Young’s modulus, tensile strength) increase. Transitions between cohesive and adhesive bonding to low surface energy substrates are also observed, while the peel and lap shear strength can be tuned by the molecular weight. Due to the high density of polar amine groups, recoverable adhesion on poly(tetrafluoroethylene) is facilitated by the inherent self-healing ability of the associating polymers. The adhesion strength recovers monotonically with healing time. Controlling the ratio of low and high M_w could be used to accurately control material properties from two simple component feedstocks, replicating the behavior of an individual monomodal sample.

The use of structured measuring systems to prevent wall slip is a common approach to obtain absolute rheological values. Typically, only the minimum distance between the measuring surfaces is used for further calculation, implying that no flow occurs between the structural elements. But this assumption is misleading, and a gap correction is necessary. To determine the radius correction (Delta r) for specific geometries, we conducted investigations on three Newtonian fluids (two silicon oils and one suspension considered to be Newtonian in the relevant shear rate range). The results show that (Delta r) is not only shear- and material-independent, but geometry-dependent, providing a Newtonian flow behaviour in a similar viscosity range. Therefore, a correction value can be determined with only minute deviations in different Newtonian fluids. As the conducted laboratory measurements are very time-consuming and expensive, a CFD-approach with only very small deviations was additionally developed and compared for validation purposes. Therefore, simulation is an effective and resource-efficient alternative to the presented laboratory measurements to determine (Delta r) for the correction of structured coaxial geometries even for non-Newtonian fluids in the future.

The ubiquitous wall slip behavior of viscoplastic fluids renders the characterization of their yield stress values a challenge but also presents an opportunity. Here, a new process for the determination of the yield stresses of viscoplastic fluids is introduced and demonstrated on concentrated suspensions subjected to steady torsional flow, i.e., parallel-disk viscometry based on the understanding of apparent wall slip. Four viscoplastic suspensions (particles with a maximum packing fraction, ϕ_m, of 0.86 mixed with a Newtonian binder at the volume fraction, ϕ, range of 0.62 to 0.78) were used. It is demonstrated that a step change in the slope of the torque versus apparent shear rate (or the rotational speed) occurs at a critical torque that corresponds to the yield stress of the suspension. Below the critical torque the behavior is governed by apparent slip and plug flow while above the critical torque the behavior is governed by continuous deformation and apparent slip. The yield stresses of the four concentrated suspensions were verified by comparisons with those obtained from other methods including from wall slip velocities at various shear stresses.