Viscoelastic properties of MR fluids

Mechanical properties of magnetorheological (MR) fluids are classified into pre-yield and post-yield regions according to whether the shear stress is below or above the yield stress. MR fluids within the pre-yield region exhibit viscoelastic properties; and these properties are important for understanding MR suspensions, especially for vibration damping applications. MR suspensions composed of reduced iron powders dispersed in silicone oil are utilized to study the viscoelastic properties with the help of a strain-controlled rheometer with plate-plate configuration. Two types of experiments, i.e. strain-amplitude sweep mode and frequency-sweep mode, were carried out to investigate the viscoelastic properties of MR fluids: (a) strain-amplitude sweep mode, the strain amplitude was swept from 0.0001 to 0.001 at a fixed driving frequency of 10 Hz. (b) Frequency-sweep mode, the driving frequency was swept from 1 Hz to 100 Hz at a constant strain amplitude of 0.001. The results show that both storage modulus G´ and loss modulus G´´ decrease with the increment of the strain amplitude, however, the loss factor increases with the increment of the strain amplitude. On the other hand, both the storage modulus and loss modulus increase with the increment of frequency, which is different from the loss factor. Moreover, the effects of magnetic field and volume fraction on the viscoelastic properties are also investigated. The higher the magnetic field, the higher the storage modulus and loss modulus, and the lower the loss factor. The higher the volume fraction, the higher the storage modulus and the loss factor.