Hollow structured magnetic particles of CoFe2O4 and their magnetorheological characteristics

Magnetorheological (MR) fluids as intelligently magneto-responsive materials are composed of magnetic particles dispersed in nonmagnetic liquid carriers, whose state can be rapidly changed from a liquid-like to solid-like under an external magnetic field. When no magnetic field is applied, magnetic particles are randomly dispersed in liquid carriers and behave as like a Newtonian fluid. On the other hand, with an applied magnetic field, they can form a fibrillar structure within milliseconds aligning along the magnetic field direction due to a magnetic-polarization interaction between particles, exhibiting significantly enhanced rheological properties of shear viscosity, dynamic modulus, and yield stress. The field-induced transformation of MR fluids is similar to that of electrorheological (ER) fluids when exposed to an eclectic field. Due to the field-controllable properties, MR fluids have been widely developed in a range of engineering devices, such as shock absorbers, brakes, active dampers, etc. However, a mismatch could occur in MR fluids because the densities of magnetic particles (iron, iron oxide and iron alloys etc.) are much higher than that of nonmagnetic fluids (mineral oil, silicone oil, hydrocarbon, paraffin etc.). Therefore, a major concern in MR fluids today is to continue to improve dispersion stability. In order to solve the problem, many efforts have been made such as addition of surfactants, magnetic nanoparticles, thickening agents and coating the magnetizable particles by organic or inorganic materials. Recently, owing to the good dispersion stability, hollow structured magnetic materials have been employed for preparation of MR fluids. However, no any information about CoFe₂O₄ nanoparticle-based MR fluids has been reported. In this study, the hollow structured CoFe₂O₄ nanospheres were prepared by a facile hydrothermal method and its MR effect was investigated.