Influence of temperature and magnetic field on rheological behavior of ultra-sonicated and oleic acid coated cobalt ferrite ferrofluid

The research work examines the influence of magnetic field and temperature on the magneto viscous and magneto viscoelastic nature of cobalt ferrite ferrofluid (FF). Steady rotational and oscillatory rheological measurements which examined the rheological responses of the fluid have been performed. A steady state flow was established through the application of suitable relaxation modulus and attempt was made to regulate unwanted oscillatory behavior by application of damping function. The particles were ultra-sonicated and coated with oleic acid to minimize agglomeration and improve fluid stability and its rheological behavior. The fluid is underdamped at high shear rate and high magnetic field due to damping factor that is less than one but overdamped behavior was obtained at low shear rate. Storage modulus (G') and loss modulus were used to examine the viscoelastic behavior of the fluid. This fluid is accurately viscoelastic, because it exhibits of both elastic and viscous behaviors. Storage modulus is higher than loss modulus as a result of dominant magnetostatic forces at low strain rate. A crossover point was formed at critical strain due to overlapping of loss and storage modulus curves at high strain rate, this effect indicates the formation of phase transition. This occurred due to dominant hydrodynamic forces over magnetostatic forces. Magnetic effect revealed a steady increase in viscous effect which is as a result of formation of enhanced chain-like structure and strong particle aggregation. Enhanced viscous effect was shown in the presence of low temperature, low angular frequency and high applied magnetic field.