Korea-Australia Rheology Journal最新文献

We investigated the effect of surfactant concentration on the rheological behavior of suspensions containing multi-walled carbon nanotubes (MWNTs) dispersed in N-methyl-2-pyrrolidone (NMP) mixed with nonionic surfactant, polyoxymethylene sorbitan monooleate (Tween 80). The surfactant concentration ranged from 0 to 10 wt.% relative to the MWNT particles. As the surfactant concentration increased, the shear viscosities initially decreased, reaching a minimum at 6 wt.%, and then increased. Similar trends were observed in the behavior of the yield stress and the power-law index. In oscillatory shear tests, storage modulus increased with the surfactant concentration up to 6 wt.% and then decreased. Aggregation structure was analyzed using a scaling theory, based on the suspension’s elasticity obtained from linear viscoelastic measurements. The dependence of elastic modulus on MWNT concentration at different surfactant concentrations provided fractal dimension of average aggregate. It was found that the addition of surfactant lowered the fractal dimension of average aggregates from 1.9 to 1.2–1.3. The fractal dimension at the surfactant concentration of 6 wt.% showed slightly lower values than those of 2 wt.% and 10 wt.%. which is the same trend as the rheological properties. These findings were further supported by a UV–visible-NIR spectroscopy method, which measured the light absorbance of individual MWNTs in diluted suspensions. The UV–visible-NIR spectroscopy analysis showed maximum absorbance at a surfactant concentration of 6 wt.%, indicating optimal dispersion of the MWNTs. This behavior aligned with the observed trends in shear viscosity and fractal dimension.

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The study aims to analyse the importance of electroosmotic force on the solute dispersion in a microchannel where the carrier fluid is defined by non-Newtonian Casson rheology. The Electric-double layer (EDL) is considered near the channel walls. Flow unsteadiness due to the electric force leads to a highly non-linear momentum equation, which is solved using a regular perturbation method. However, Gill’s generalized dispersion technique is used to discuss the solute dispersion mechanism. The velocity profile, along with the advection, dispersion coefficients and mean concentration, is discussed with different rheological and controlled parameters. The impact of key control parameters, i.e., thickness of the EDL, yield stress, and Péclet number, on the dispersion coefficient, advection coefficient, and mean concentration is examined. A wide range of parameters is considered based on the experimental and physical database from different literatures. Although the transport coefficients are evaluated analytically, numerical tests have also been conducted, producing results that match very well. The existence of electroosmotic force (higher Debye–Hückel electro-osmotic parameter, κ) raises the advection coefficient, and this impact is more noticeable for lower κ values. The dispersion coefficient ((K_{2})) diminishes nonlinearly with increasing κ and eventually converges to a specific value with larger κ. The mean concentration variation is more pronounced at lower values of κ, showing a 50% increase as κ rises from 10 to 50, while the variation is only 7% when κ changes from 50 to 100. Understanding dispersion phenomena controlled by electroosmotic force and pulsatility is a challenging task, mainly due to its nonlinearity, and as a result, this area has not been extensively explored. This type of study may have wide applications in biomedical engineering, human blood flow analysis, and beyond. The present simulation will thus be valuable in understanding mass transfer processes.

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Schematic diagram of the proposed geometry

This article describes a new rheological model that considers the structural changes of Simple Yield Stress Fluid (SYSF). These fluids exhibit an elastic behavior below a threshold stress and non-Newtonian viscous behavior governed by a model similar to the generalized Casson model proposed by Benhadid et al. [1] above the yield stress. This model represents the complex rheological behavior of SYSFs across the whole shear rate range while taking into account the structural changes found in this type of fluid. This approach is based on the development of a generic model that will encompass most classical models as particular cases by describing the complete flow curve of viscoplastic (VP) materials with an extension to ElastoViscoPlastic fluids (EVP). Thus, the apparent viscosity determined from the proposed model describes a smooth flow curve without discontinuities, where the yield stress is clearly expressed with a maximum Newtonian viscosity (viscosity at zero shear rate (eta_{0})), which approaches a Newtonian viscosity (eta_{infty }) at high shear rates.

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We propose a systematic method to solve viscoelastic model for viscometric flows. The method is to generalize integration factor for tensor differential equation. If the analytical solution for viscometric flow exists, then it is easier to check the validity of the constitutive equation than use of numerical solution. Our ansatz gives the analytical solutions of quasi-linear viscoelastic models, such as the upper-convected, lower-convected, and corotational Maxwell models, because they become sets of linear ordinary differential equations for viscometric flows such as simple shear and elongational flows. We expect that the integration factor method can improve the numerical algorithms for nonlinear viscoelastic models and we are in preparation of a new numerical algorithm based on the integration factor method.

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In this study, low-reflective multilayer films (LRMFs) and high-reflective multilayer films (HRMFs) were designed and manufactured using colorless polyimide (CPI) as a low refractive index layer and Si₃N₄ as a high refractive index layer. A numerical simulation was conducted to design a nanolayered structure with different reflectivity characteristics at specific wavelengths. Rheological analysis was performed to determine a solution concentration needed for coating CPI at the nanoscale via the wet process. A layer of 89 nm was found to be formed stably using a 5wt% CPI solution at 5000 rpm. The results revealed that LRMFs exhibited less than 1% reflectivity at the central wavelength, while HRMFs showed more than 40% reflectivity at the central wavelength. Morphological analysis confirmed that the designed nanostructures were successfully fabricated. This research has provided a new way to fabricate optical film through the wet process.

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Complex chemical reactions and hydrodynamic dispersion feature in many aspects of hemodynamics. Motivated by examining the reactive phase change dispersion in perfusion, a mathematical study is presented for solute dispersion in incompressible laminar blood flow through a straight circular capillary with a permeable wall (enabling lateral movement across the vessel fenestrations in perfusion and associated with the presence of the endothelial layer). The boundary condition at the vessel wall is considered a reversible phase exchange process based on first-order chemical kinetics. Darcy’s law is deployed to feature the permeability nature of the capillary. A multiple-scale asymptotic analysis is developed, and a non-dimensional transverse averaged “macro-transport” equation for convective diffusion–dispersion is derived. Expressions are then presented for the advection coefficient and Taylor dispersion coefficient. Numerical evaluation of the impact of key control parameters i.e., permeability parameter, pressure parameter, retention parameter (α), Damköhler number (Da) on dispersion coefficient, advection coefficient and leading order concentration of the solute is conducted, and solutions are visualized graphically both for small and large times. The novelty of the present work is, therefore, the collective consideration of complex wall permeability and pressure difference in addition to boundary reaction and Darcian body force effects. The analysis shows that the dispersion coefficient is initially enhanced gradually with an increment in the retention parameter with its initial small value and thereafter exhibits a smooth decay. The hydrodynamic dispersion coefficient markedly decreases with higher values of the permeable parameter. A higher magnitude of dispersion coefficient is computed at the vessel inlet and then decreases towards the outlet. A boost in the leading order concentration of the solute is computed at small times but is stabilized and eventually remains invariant with time. The axial velocity is found to depend strongly on the axial position in the capillary. A displacement in concentration peaks is also observed which is attributable to advection along the axial direction, and the decreasing peaks with respect to time are due to the diffusion of the solute from the fluid phase to the vessel wall. Generally, it is also observed that retention enhances the chemical reaction effect, leading to a greater loss of solute over time. The simulations are relevant to chemo-hemodynamics and also may find applications in drug delivery (pharmacodynamics).

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Poly(ionic liquid)s are ion-containing polymers possessing ionic liquid structures on their repeating units. Owing to the unique physicochemical properties of ionic liquids, many existing studies have found that the properties of poly(ionic liquid)s are distinct from those of conventional ion-containing polymers, such as poly(sodium styrene sulfonate). A lot of scientific efforts have been made to understand the relationship between the chemical structure and the material properties of poly(ionic liquid)s, and several good review papers are available in the literature. The aim of this short review is to summarize key results on the viscoelastic properties of poly(ionic liquid)s in solution. We discuss in detail the counterion condensation and the charge screening in poly(ionic liquid) solutions.

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This study places a focus on flow-induced demixing of polyisoprene (PI) and poly(4-tert butyl styrene) (PtBS) polymer blends with high dynamic contrast. The rheological response is governed by the more immobilized PtBS chains. In contrast, the dielectric response at low frequency reflects exclusively the normal mode of PI chains having type-A dipole parallel along the chain backbone. These features enable selective detection of the component dynamics of PtBS or PI chains under either the small amplitude oscillatory shear or the nonlinear steady shear through a combination of rheological and dielectric techniques. We find the Cox–Merz rule fails significantly for the PtBS/PI blends, which is attributed to flow-induced demixing when the shear rate is higher than the relaxation rate of PtBS chains and lower than that of PI chains. The flow-induced demixing is further supported by varying the molecular parameters of PtBS, copolymerizing PtBS with polystyrene, as well as by the rheo-dielectric measurements that show a broadening of the PI relaxation mode distribution. This demixing could be partly driven by the increased conformational entropy of the PI chains after segregation from the surrounding restrictive PtBS chains.

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This review summarized the multiscale simulation (MSS) methods for polymeric liquids. Since polymeric liquids have multiscale characteristics of monomeric, mesoscopic, and macroscopic flow scales, MSSs that relate different hierarchical levels are adequate to reproduce flow properties accurately. Our review includes pioneering studies to the most advanced MSS studies on rheology predictions and flow simulations of polymeric liquids. We discuss two major types of MSS methods: the bottom-up and model-embedded MSS methods. The former method mainly connects all-atom molecular dynamics models and mesoscopic models to predict rheological properties. In contrast, the latter method, where a microscopic or mesoscopic model is embedded in a macroscopic computational domain, is designed to predict macroscopic flow properties. Finally, we also discuss MSS methods using machine learning techniques.

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This review provides an in-depth analysis of the thermal degradation of biodegradable polymers through rheological methods. Focusing on key techniques such as time sweep tests, frequency sweep tests, and nonlinear rheological analyses gained at higher shear tests, the review highlights how these approaches offer critical insights into polymer stability and degradation kinetics. It entails an understanding of how molecular weight reduction, a common degradation mechanism, significantly impacts the performance of biodegradable polymers, and how the use of appropriate fillers can enhance thermal stability by mitigating chain scission. The review also discusses the application of the Arrhenius equation in modelling thermal degradation, helping predict degradation rates and optimize processing conditions. Time sweep tests are particularly emphasized for their ability to monitor polymer stability under various environmental conditions, while frequency sweep tests provide insights into the effects of processing/thermal history on material degradation. Tests at higher shear rates, which simulate real-world processing conditions such as extrusion and injection moulding, are explored for their role in understanding how processing-induced shear forces accelerate polymer degradation. Various biodegradable polymers are considered in this review, with polylactic acid (PLA) being the dominant polymer studied across most research, providing a clear picture of its degradation behaviour and strategies for enhancing its thermal stability. Therefore, it is expected that this review will be a comprehensive guide for researchers and engineers looking to optimize the thermal stability and performance of biodegradable polymers in various industrial applications.

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