Details of the rheology of concentrated suspensions are matters of important practical interest. For coagulated systems, rheological models based on fractal concepts have been proposed. The fractal dimension which is used to quantify the aggregate structure in these models has been measured in dilute suspensions but not in concentrated suspensions. The present study established the effect of shear stress and volume fraction on the structure of fractal aggregate in concentrated systems; the fractal dimension of the aggregate was established by adopting the rheological model of flocculated concentrated suspensions proposed by Mills and the fracture model of an aggregate in shear field proposed by Mühle. The result showed that the fractal dimension of an aggregate in a sheared suspension increases with volume fraction rather than with shear rate, γ. It is concluded that by increasing the concentration, the interpenetration of aggregates cause densification and thereby increases in the fractal dimension.
Silica sols are produced by the neutralization of sodium silicate with sulfuric acid. The produced silica sols gradually increase in viscosity and finally form gels through a sol-gel transition in which the pH of the sols strongly influences the gelation time. In this study, the rheological property of silica gels produced from alkaline sol was evaluated experimentally in comparison with that obtained from acid sol. Dynamic viscoelastic measurements, creep tests, and morphological measurements were conducted to compare the alkaline sol with the acid sol. The different properties of the gels may be related to the bonding manner and/or the sizes of the silica particles that form the network structure within the gels. The particle-bonding models are presented in this study.
Blend systems of Polystyrene-block-poly(ethylene-co-(ethylene-propylene)-block-polystyrene(SEEPS) tri-block copolymer and three kinds of paraffinic oil having different molecular weight (MW) were prepared. Styrene content of the SEEPS was 30 wt%. The oils used in this study were Oil-1 (MW=750 g/mole), Oil-2 (540 g/mole), and Oil-3 (410 g/mole). Dynamic viscoelastic properties, the storage (G') and loss modulus (G"), of the blends were measured as a function of temperature (T) and angular frequency (ω). SEEPS/oil = 100/0 and 75/25 (in weight) blends exhibited gel-like behavior over the entire experimental T window below 300°C. On the contrary, the viscoelastic properties of SEEPS/oil = 50/50 and 25/75 blends depended on the MW of oil. For the SEEPS/Oil-1 = 50/50 and 25/75 blends having high MW oil, the clear order-disorder transition (ODT) was observed showing a sudden change of G' at elevating T and the ODT temperature (TODT) was found to shift to lower T with increase in oil content. For the SEEPS/Oil-2 = 50/50 blend having medium MW oil, the ODT was observed very clearly, but could not be observed for the 25/75 blend. For the SEEPS/Oil-3 blends having low MW oil, the ODT was not observed showing that there is a rather gradual decrease in G' with T. It was found that the viscoelastic properties of SEEPS/oil blends changed with the content and MW of oil.
The pressure drops and frictional coefficients were measured and estimated for water, microbubble/water mixtures (MB water), and complex fluids (surfactant solutions and polymer solutions) in capillary flows. Good agreement for water was obtained between the experimental results and theoretical value for Hagen-Poiseuille flows. For MB water, the laminarization was suggested until 4000 of the Reynolds number. Both surfactant solutions and polymer solutions exhibited the same results as MB water. In the explaining this behavior, elasticity, electrical interaction, and size effect are considered. Consequently, it is strongly suggested that electrical interaction on capillary wall is a contributing factor. In addition, surface tension was investigated and supported the discussion.
Binders and plasticizers are key materials that decide the mechanical properties of composite propellants. Hydroxyl-terminated polybutadiene (HTPB) is commonly used as a binder for composite propellants, while polytetrahydrofuran (PTHF) is a useful plasticizer of the HTPB binder. In this study, the dynamic mechanical properties of cured HTPB containing PTHF were investigated, and the network structure of the blend was estimated to obtain useful data for the development of binders and plasticizers. The network density of the HTPB/PTHF blend was reduced by the addition of PTHF; furthermore, the temperature and frequency dependences of the modulus and loss tangent varied. It was inferred that a small amount of the added PTHF was incorporated into the network structure of the cured HTPB. And the PTHF incorporated into the network structure of the cured HTPB probably formed dangling ends in the network, thereby enhancing the mobility of the chain segment of the blends. The dynamic mechanical properties of the blends followed the time-temperature superposition principle, and the properties were estimated accurately by the Williams-Landel-Ferry approach. The apparent activation energy of relaxation of the HTPB/PTHF blends was smaller than that of HTPB alone.
Taylor-Couette instability of Giesekus fluids is investigated at large gaps using a temporal, linear instability analysis. Having superimposed axisymmetric, normal-mode perturbations to the base flow velocity and stress fields, an eigenvalue problem is obtained which is solved numerically using pseudo-spectral, Chebyshev-based, collocation method. The neutral instability curve is then plotted as a function of the Weissenberg number and also the mobility factor of the Giesekus model. Based on the results obtained in this work, it is concluded that at large gaps, a fluid's elasticity can have a stabilizing or destabilizing effect on the Couette flow depending on the Weissenberg number being smaller or larger than a critical value. The critical Weissenberg number increases by an increase in the gap size, and also by an increase in the mobility factor. Fluid's inertia is identified as the main source of instability.
We studied the dependence of sedimentation process in a flocculated kaolinite suspension on the diameter of cylindrical container in the measurement and volume fraction of clay in the suspension. The obtained results were as follows. The constant velocity in the slow settling process depends on the container diameter. In large containers (D=21.3, 28.2, 63.5 mm), it becomes lager as the container diameter increase. This phenomenon can be explained on the basis of a theory focusing on wall stress. However, there exists hardly difference in settling velocity among the small containers (D<13.9 mm) and the very slow sedimentation regardless of the container diameter causes. It is guessed that this phenomenon occurs by the flow which doesn't depend on the container diameter. On the other hand, the maximum settling velocity in the rapid settling process was almost constant regardless of container diameter. From this result, it became clear that the container diameter did not affect on the rapid settling process.