Investigation Of The Viscous Properties Of Magnetic Fluids

In this paper the rheological properties of magnetic fluid (MF) were systematically studied, especially, the viscous properties of MF with different carrier and different solid content.1. The equipment constant of capillary rheometer was measured. The viscosities were used to evaluate different equations for the MF viscosities. It was found that the MF viscosities deviated away from the Einstein equation basically due to the influence of spatial distribution and aggregation of Fe3O4 magnetic nanoparticles (MNPs). By taking account of the coating layer on MNPs and the aggregation of MNPs in MFs, a modified equation was proposed to predict the MF viscosities. Predicted viscosities were compared with experimental viscosities of MFs, the differences were acceptable. Moreover, the influence of external magnetic field on the viscosity was also taken into account. The predicted viscosities were in good agreement with the experimental data when the total volume fraction was lower than 15%. Combining hydrokinetics theory, the equation was modified to fit for the effect of applied magnetic field. When the intensity of magnetic field was not strong, the new equation also could be used.2. The relationships between flow rate and MF viscosity that had different solid content and different dosage of surfactants were investigated. The effects of different solid volume fraction, dosage of surfactants and temperature on viscosities were studied. The different proportion of two surfactants obtained the different solid contents. Using 3 g of oleate sodium and 3 g of PEG, the solid content of the MF was the maximum and the viscosity was the lowest. However, when 3 g oleate sodium and 5 g PEG were adopted, the solid content of the MF was the least and the viscosity of the MF was the maximum. For low-concentration MFs, the distance between particles was large and the interaction among particles could be neglected. The dominate influence on MF viscosity was the carrier liquids. With the increasing of temperature, the viscosity decreased. Under applied magnetic field, the higher the solid content was, the higher the viscosity was.3. The viscous properties of high-concentration MFs obtained by a high-energy ball mill were investigated using a rotating rheometer. With prolonging of shearing process under the same shear rate, the viscosities of MFs had almost never changed. So, it could be seen that MFs had good stability for time. With the increasing of shear rate, the MFs showed severe shear-thinning behavior. The higher solid contents were, the more obviously the shear-thinning behavior showed and the higher the viscosities were. Because of the hysteresis phenomenon, viscosity had restored at some extent but could not recover completely. Similarly, the same result happened under applied magnetic field. The different constitutive models were used to analyze experimental data and obtained the correlation non-Newton parameter. Whether under an applied magnetic field or not, Herschel-Bulkley (H-B) model could also be used to describe the viscous properties.4. The viscous properties of the MFs coated by SiO2 were measured, such as viscosity versus shear rate, shear stress versus shear rate, shear stress versus intensity of magnetic field. The Bingham model and H-B model were used to describe the viscous properties. Considering the effects of surfactants on shear stress, the H-B model was modified. Taking account of the effects of particles/aggregates size, the thickness of the surfactant coating on the MNPs surface, the intensity of magnetic field and temperature, the empirical equation for predicted the plastic viscosities was obtained. The effects of the intensity of an applied magnetic field, solid content related to yield stress were investigated.5. The rheological properties of the MFs coated by acrylic acid were tested. The MNPs were prepared by co-precipitation with surfactant and by aging under an applied magnetic field. It was found that different preparation methods could obtain different rheological properties. The MF with total solid content of 25% showed shear-thickening behavior due to the difference of the degree of twist and stretch of macromolecule link.