Results Of Rheological Experiments Research Articles

Viscosity of yttrium oxide (Y2O3) and magnesium-aluminum spinel (MgAl2O4) nanopowder suspensions in ethyl alcohol

We report results of rheological experiments on suspensions of nanopowders. In this work we have used two ceramic nanopowders such as magnesium-aluminum spinel (S30CR Baikowski – MgAl 2 O 4 ) and yttrium oxide (Y 2 O 3 ) suspended in ethyl alcohol. Rheological studies have been carried out using several experimental systems including Haake Mars 2 rheometer (Thermo Electron Corporation, Karlsruhe, Germany) and Rheo-NMR (Bruker BioSpin, Rheinstetten, Germany). Measurements of dynamic viscosity in the range of shear rates from 0,01 s -1 to 2000 s -1 and the temperature range from -15°C to 20°C has been conducted. Most of the samples exhibited a non-Newtonian nature.

Effect of an Anionic Polyelectrolyte Adsorption on the Colloidal Stability of Aqueous 3Y-TZP Suspensions

The effects of an anionic polyelectrolyte on the dispersion of 3Y-TZP suspensions in aqueous medium have been systematically investigated. The results of rheological experiments show that at CD = 0.5 wt% (CD : dispersant concentration), the dispersant could afford well-dispersed 3Y-TZP suspensions over a wider range of solids loading, owing to the stronger electrostatic repulsion between particles resulting from the monolayer coverage of the particle. But as CD ≤ 0.5 wt% or CD ≥0.5 wt%, the stable 3Y-TZP suspensions can only be achieved at a relatively lower solids loading, which are ascribed to the reduced repulsion barrier between particles owing to the lower ζ values of particles and the higher ionic strength in the system, respectively.

Preparation and Characterization of Agarose-Gelatin Blend Hydrogels as a Cell Encapsulation Matrix: An In-Vitro Study

Cell encapsulation represents an alternative nonviral technique to treat multiple diseases, leading to a reduction or even absence of administration of immunosuppressants. Hydrogels have been introduced as novel materials suitable for cell encapsulation. This study involves agarose–gelatin blend hydrogels with four different weight percentage ratios (100:0, 75:25, 50:50, 25:75) of agarose to gelatin. Prepared blend hydrogels were assessed in terms of rheological behavior (gel point by using complex viscosity), cell attachment (hemocytometer), cell viability and cytotoxicity (3-(3,4-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliium bromide, MTT assay), and mechanical and integral stability (Bradford test and shear force rupture assay, respectively). Based on the obtained rheological experimental results, the sol-gel transition point for 50:50 was in the physiological condition range (35°C–37°C). The percent of nonattached cells on the surface of the hydrogel decreased from 92% for the 100:0 sample to 46.3% for the 50:50 sample, and the cell viability was more than 95%. A good structural integrity was achieved for samples with weight ratio of 50:50; 20.195% gelatin was released during the 24 h in phosphate buffer solution at 25°C and the mechanical stability of agarose–gelatin microcapsules under shear force were improved about 14% rather than pure agarose microcapsule.

Thixotropic rheology of concentrated alumina colloidal gels for solid freeform fabrication

An engineering model for the thixotropic rheology of alumina (Al2O3) concentrated colloidal gels is proposed and compared to rheological experimental results. Concentrated colloidal gels used in extrusion-based solid freeform fabrication are often modeled as simple shear-thinning fluids with finite yield stress such that their rheological properties depend only on instantaneous shear rate and not on shear history. Although this is a satisfactory view for steady state flow, it lacks the detail to consider phenomena associated with transient processes such as die entry and exit in extrusion processes or the shape evolution of an extruded filament as it comes to rest in a low shear environment. The rheological model proposed here considers that both elastic and viscous properties of an aqueous Al2O3 gel vary with microstructure changes of colloidal aggregates and that the aggregate structure is a function of both shear rate and time. A first-order structural kinetics assumption is used to quantify the microstructure evolution of the gel network during shear flow, where shear-induced attrition and diffusion limited aggregation are the rate limiting mechanisms. A constitutive flow equation is developed by incorporating the structural kinetics to describe unsteady shear-thinning behavior. The data collected through shear rate step-change measurements are used to determine 12 model parameters in the proposed rheological model. The model is compared and validated through hysteresis-loop experiments with satisfactory agreement. This model is quasiempirical in the sense that details of interparticle pair-potentials are not used in the formulation. Nevertheless, the model could have practical uses in simulation of the macroscopic flow behavior of colloidal gels during extrusion processes. Specifically, the model offers predictions of elastic and viscous property evolution of colloidal gels as a function of shear history.

Seismogenic lavas and explosive eruption forecasting

Volcanic dome-building episodes commonly exhibit acceleration in both effusive discharge rate and seismicity before explosive eruptions. This should enable the application of material failure forecasting methods to eruption forecasting. To date, such methods have been based exclusively on the seismicity of the country rock. It is clear, however, that the rheology and deformation rate of the lava ultimately dictate eruption style. The highly crystalline lavas involved in these eruptions are pseudoplastic fluids that exhibit a strong component of shear thinning as their deformation accelerates across the ductile to brittle transition. Thus, understanding the nature of the ductile-brittle transition in dome lavas may well hold the key to an accurate description of dome growth and stability. Here we present the results of rheological experiments with continuous microseismic monitoring, which reveal that dome lavas are seismogenic and that the character of the seismicity changes markedly across the ductile-brittle transition until complete brittle failure occurs at high strain rates. We conclude that magma seismicity, combined with failure forecasting methods, could potentially be applied successfully to dome-building eruptions for volcanic forecasting.

Melt flow index on high molecular weight polyethylene: A comparative study of experiments and simulation

The flow that takes place in a melt flow index (MFI) experiment has been simulated for two grades of high molecular weight polyethylene. Both materials present very similar rheological behaviour in the characteristic times involved in the MFI measure so an exhaustive experimental work has been necessary in order to reveal slight but important molecular differences. The materials have been characterized through an integral viscoelastic model K-BKZ. It has been adopted an isothermal assumption and the shape of the free jet downstream is included in the calculations. Further considerations about flow morphology, i.e. die swell and vortex enhancement, are discussed. Rheological experimental results have pointed out a molecular difference that is directly reflected in the flow extrusion behaviour of the materials studied.

The microstructure of ferrofluids and their rheological properties

AbstractOne of the most important features of ferrofluids is the possibility to change their physical properties, especially their viscosity, by means of moderate magnetic fields. This capability makes ferrofluids very useful in the fields of engineering, medicine and fundamental research. Rheological experimental results, as well as theoretical studies, correlate the change of the viscosity of a sheared ferrofluid under the influence of a magnetic field, the so‐called magnetoviscous effect, to the internal structure formation under certain external conditions. To obtain information about the microstructure of ferrofluids, experiments using the small‐angle neutron scattering (SANS) technique have been carried out. Three magnetite‐based ferrofluids with different particle–particle interactions, and thus various magnitudes of the magnetoviscous effect, were investigated. Using a specially designed rheometer, SANS experiments were performed for different shear rates and magnetic field strengths in order to observe the modification of the microstructure in ferrofluids and to associate the SANS information with their macroscopical behaviour. The scattering patterns obtained show a good agreement with the qualitative model that explains the magnetoviscous effect. Copyright © 2004 John Wiley & Sons, Ltd.

Influence of pitch coal surface treatment on the beneficiation of high water content coal and the rheological properties of CWS

The influence of pitch coal surface treatment on the beneficiation of high water content coal and preparation of its highly concentrated coal-water slurry (CWS) has been investigated. Weak slurryability coal, containing a high water content, exhibited a high degree of hydrophilicity. Having been treated by pitch during milling, the surface of coal particles became increasingly hydrophobia Coal powders treated and untreated with pitch were beneficiated in the oil-water system for comparison. The value of ash reduction of the pitch treated coal powders was higher than that of the untreated ones after beneficiation. The untreated, treated and beneficiated coals were used to prepare highly concentrated CWS by adding additives. The results of rheological experiments show that the CWS of beneficiated coal after treatment with pitch had lower viscosities compared with the CWS of the untreated coal or treated coal without beneficiation.