Korea-Australia Rheology Journal最新文献

The relationship between microstructure changes and rheological properties in suspensions containing boehmite particles, which are well applied in various industrial wash-coating processes, was investigated by changing pH condition. The boehmite particles in suspensions were either well dispersed or aggregated depending on the pH, owing to the relative contributions of repulsive interaction between particles as well as hydrolysis and condensation reactions. Four groups of boehmite suspensions were classified as very low, intermediate, almost zero charge, and high pH regimes based on their colloidal behaviors, and their microstructural differences were investigated using transmission electron microscopy (TEM) and multi-speckle diffusing wave spectroscopy (MSDWS). For gel-like suspensions of three groups, various rheological properties such as shear viscosity, viscoelastic modulus, yield stress, and recovery behavior were extensively compared, and the results clearly demonstrated that a suspension with high yield stress was not fully recovered into the original state when disturbed at high shear rates.

The present work investigates the effect of plate gap on the rheological properties of shear thickening fluid (STF) and proposes a phenomenological model to predict the viscosity curve of STF for different values of plate gap and temperature. Multiwalled carbon nanotube (MWCNT) reinforced silica-based STF (MWCNT/SiO₂-STF) containing 0.8 wt% MWCNT and 20 wt% SiO₂ nanoparticles was prepared using polyethylene glycol as a dispersion medium and tested for its steady and dynamic rheological behavior at different plate gaps. The peak viscosity of MWCNT/SiO₂-STF follows the characteristic behavior of an initial increase followed by a subsequent decrease corresponding to the increase in plate gap. A maximum viscosity of 198.89 Pa s was recorded at a plate gap of 1.0 mm. Although significant shear thinning in the dynamic rheological response of MWCNT/SiO₂-STF was noticed at a 1.0 mm gap, the storage and loss modulus were better than those at 0.5 mm gap. The proposed model based predicts the shear thinning and thickening behavior of STF at low and high shear rates for different values of plate gap with reasonable accuracy. The model also provides a very good fit for the viscosity of STF at different temperatures. Thus, the proposed model is suitable for numerical simulations as well as theoretical analysis in the vibration control field.

We present the structural, dynamical, and rheological behaviors of theta chains according to the solution type and the molecular architecture, using extensive bead-rod Brownian simulations, and compare them with the corresponding pure ring chain. Through detailed analysis of the properties of theta chains in each solution system, we found that theta chains have their own chain structure for each solution system and, accordingly, a characteristic dynamical behavior in each solution. For example, due to the absence of intra and intermolecular interactions in the free-draining solution system, the theta chain can have a fully stretched structure and can rotate like a conventional linear chain. On the other hand, due to the presence of intra and intramolecular interactions in the dilute solution system, the theta chain can have a spread (or extended) chain configuration that is aligned parallel to the flow-vorticity plane under shear flow, resulting in a two dimensional (2D) planar sheet-like chain structure. Thus, the theta chain alternates between the 2D planar sheet structure and the folded sheet structure during the chain rotation and tumbling cycle. We also found that the structural, dynamical, and rheological properties of the theta chains are highly dependent on the existence of a bridged linear part of the theta chain irrespective of the solution type. Since the bridged part of the theta chain physically prevents chain extension induced by a strong shear flow, the theta chains tend to have a less deformed chain structure than the corresponding pure ring chain. Depending on the length of the branched part of the theta chain, the rotation and tumbling behavior of the theta chain are greatly modified.

Colloidal dispersions have been frequently encountered in a wide range of industrial applications, such as foods, paints, and Li-ion electrode slurries. Therefore, it is essential to understand the rheological and flow characteristics of colloidal dispersions to improve the quality and optimize the processing conditions of colloidal products. The shear viscosity of a colloidal dispersion deviates from Newtonian behavior, exhibiting shear thinning and/or shear thickening as the volume fraction of the colloidal particles increases. However, there are not many reports on the non-Newtonian flow phenomena caused by the normal stress differences of colloidal dispersion due to their small magnitude. Recently, these normal stress differences in colloidal dispersions with a constant shear viscosity lead to a single-line focused streams of micron-sized particles along the centerline of microchannels. In this study, the lateral migration of single micron-sized particles suspended in poly (N-isopropylacrylamide) microgel dispersions with a shear-thinning viscosity was investigated. The micron-sized particles migrated toward the centerline or between the centerline and wall of a microchannel depending on the volume fraction of the colloidal particles and the flow conditions. The current findings are expected to contribute to our understanding of the non-Newtonian fluid dynamics in colloidal dispersions and flow-induced particle segregation phenomenon.

The principle of causality constrains the real and imaginary parts of the complex modulus (G^{*} = G^{prime } + i G^{prime prime }) via Kramers–Kronig relations (KKR). Thus, the consistency of observed elastic or storage ((G^{prime })) and viscous or loss ((G^{prime prime })) moduli can be ascertained by checking whether they obey KKR. This is important when master curves of the complex modulus are constructed by transforming a number of individual datasets; for example, during time-temperature superposition. We adapt a recently developed statistical technique called the ‘Sum of Maxwell Elements using Lasso’ or SMEL test to assess the KKR compliance of linear viscoelastic data. We validate this test by successfully using it on real and synthetic datasets that follow and violate KKR. The SMEL test is found to be both accurate and efficient. As a byproduct, the parameters inferred during the SMEL test provide a noisy estimate of the discrete relaxation spectrum. Strategies to improve the quality and interpretability of the extracted discrete spectrum are explored by appealing to the principle of parsimony to first reduce the number of parameters, and then to nonlinear regression to fine tune the spectrum. Comparisons with spectra obtained from the open-source program pyReSpect suggest possible tradeoffs between speed and accuracy.

In this study, we investigate how the initial solvent concentration can influence the final structure and property of the polymer nanocomposites (PNCs). To produce the PNCs, nanoparticles (NPs) and polymers are first required to disperse in a good solvent and then the dispersing solvent quickly evaporates. Previous studies found that controlling the evaporation rate of solvents or drying conditions of solution can change the structure of PNCs; however, the colloidal stability of the NP-polymer mixtures depending on the solvent concentrations has not been much considered. In the NP-polymer colloidal mixture as a precursor system of PNC, the microstructure of the NP dispersion is determined by the net interaction between particles, which may sensitively vary depending on the polymer/solvent concentration. The evaporation of the solvent accompanying the PNC manufacturing process results in a continuous change in the component concentration, which means that the interaction between particles can be continuously changed. We found that the varying initial concentrations in NP-polymer mixtures with different amount of the solvent indeed changes the initial dispersion state of the NPs, which ultimately determined the final microstructure and the physical properties of the PNCs.

Blood is a rheologically complex suspension, in which the soluble fraction contains proteins, total cholesterol and triglycerides. The blood rheological behavior is strongly affected by the concentration of these components. This work evaluates the total cholesterol effect on the rheological behavior of Wistar rat blood by means of an in vitro study. Twenty-one rats were divided into 3 groups, each one had an assigned diet with different fat content: reference group (RG) with 3%, medium fat content group (MG) with 4.5% and high-fat content group (HG) with 6.5%; in the latter group, mixed-vegetable fat was added. From each group, intraocular representative blood samples were taken with time lapse of 15 days between each sampling followed by biochemical and hemo-rheological tests. The first analysis detected changes in total cholesterol levels, attributed to the rat metabolism, formation of adipose tissue and competition for food. The second test consisted in steady simple-shear and linear oscillatory flow. The linear viscoelastic spectra reveal that the viscous modulus is larger than the elastic modulus (Gʺ > G′), with simple-shear viscosity exhibiting shear-thinning behavior. An important finding is a pseudo-solid-like behavior at low frequencies (1 rad/s) akin to the presence of yield stresses in the high-fat content group after 30 days, revealing strong interactions between total cholesterol levels and blood cells. The hemo-rheological tests represent a promising alternative to identify pathologies present in the blood (total cholesterol, digestive problems, and diabetes).

Circulating tumor cells (CTCs) detection has become one of the promising solutions for the early diagnosis of cancers. Thus, the separation of CTCs is of great importance in biomedical applications. In addition, microfluidic technology has been an attractive approach to the manipulation of biological cells. This study presents the parametric investigations relevant to the volumetric throughput of a microfluidic platform with the dielectrophoresis (DEP)-based cell manipulation technique for the continuous CTCs separation. A low potential voltage at an appropriate frequency was applied to slanted planar electrodes to separate CTCs from normal cells in blood samples due to mainly the cell size difference. The performance of the separation process was analyzed by evaluating the cell trajectories, purity, and recovery rates. Several inlet flow rates of buffer and cell sample fluid streams were examined. Various channel configurations with different outlet and height dimensions were also investigated to enhance the isolation of CTCs. During the simulation, the size and shape of cells were assumed as fixed-sized, solid spheres. The results showed that CTCs could be separated from blood cells, including white blood cells (WBCs), red blood cells (RBCs), and platelets (PLTs) with recovery and purity factors up to 100% at the cell sample throughput of 10 µL/min by utilizing a suitable microchannel design. The current study significantly contributes valuable insights into the design of the microchip devices to effectively and selectively isolate different cancerous cells in biofluids.

Graphical abstract