In order to expand the range of applications of magnetic fluid, water-based magnetic fluid with different surfactants is synthesized and the property of fluidity has been intensively studied. Three kinds of surfactants (sodium dodecyl sulfate, oleic acid and polyethylene glycol) are used to synthesize three different magnetic fluids with the same magnetic nanoparticles and carrier liquid. Their microstructures and thermal stability of magnetic nanoparticles coated with different surfactants are characterized by transmission electron microscopy and thermogravimetric analysis. Results show that water-based magnetic fluid with sodium dodecyl sulfate agglomerates more obviously while magnetic fluid with polyethylene glycol keeps good dispersion. In addition, magnetic measurements reveal that all these magnetic fluids exhibit typical superparamagnetic behavior and the magnetic fluid using polyethylene glycol as surfactant maintains the specific saturation magnetization of ∼60 emu g −1 . What is more, shear stress and viscosity of these three magnetic fluids are measured by the rotational rheometer. The shear stress increases with the increasing shear rate while the viscosity decreases with the increasing shear rate. This indicates that water-based magnetic fluid has the property of pseudoplastic fluid. What is more, the viscosity of the magnetic fluid with polyethylene glycol as surfactant is lower than other two magnetic fluids and is prone to reach a steady state at a low shear rate in a short time.
The self-suspension of magnet in magnetic fluid has been widely used in micromechanical systems, sensors, and dampers. The magnetic field associated with the ring magnet is obtained by numerical calculation and simulation through which the axial magnetic levitation force is calculated, and the numerical calculation, simulation, and experimental results agree with each other. The influence of the radial eccentricity of the ring magnet on the axial magnetic levitation force is studied, the ring magnet will experience a maximum axial magnetic levitation force without radial eccentricity. With the increase of radial eccentricity and the decrease of the distance between the bottom of the ring magnet and container, the axial magnetic levitation force will continue to decrease. But it is worth noting that the magnitude of the change caused by radial eccentricity is negligible compared to that of the axial magnetic levitation force.
In this research, a new engine oil-based Fe3O4/Ag magnetic fluid is prepared applying the method of modified chemical co-precipitation. This preparation method is green without toxic gases being released. The nanoparticles are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and vibration sample magnetometer (VSM). These characterizations demonstrate that the Fe3O4/Ag nanoparticles modified by oleic acid are successfully synthesized and uniformly dispersed in the engine oil. The hysteresis loop of this new magnetic fluid shows that it has no remnant magnetism and maintains superparamagnetic properties. Shear stress and viscosity of both Fe3O4magnetic fluid and Fe3O4/Ag magnetic fluid are measured by the rotational rheometer. The shear stress increases with the increasing shear rate while the viscosity decreases with the increasing shear rate. What’s more, the viscosity of Fe3O4/Ag magnetic fluid significantly increases compared with the traditional magnetic fluid.
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