| The magneto-optical and rheological behaviors of magnetic fluids and magnetorheological (MR) fluids have been investigated. A magneto-optical apparatus was constructed which enabled us to investigate the birefringence and dichroism of ferrofluids at various levels of applied magnetic field. Specifically, the effects of the film thickness of oil-based ferrofluids and the concentration of surfactant in the oil-based ferrofluids on their magneto-optical behavior were investigated. A commercial magneto-rheological instrument (Physica MCR 301, Anton Paar) equipped with a cone-and-plate fixture was employed to investigate the transient and steady-state shear flow of both ferrofluids and MR fluids as a function of shear rate at various levels of applied magnetic fields. The rheological investigation has enabled us to determine the effect of applied magnetic field on the shear viscosity and yield stress of ferrofluids and MR fluids. A special ferrofluid was prepared by filtering out nearly all of the surfactant and small particles in an oil-based ferrofluid. We then compared its magneto-optical and rheological behaviors with those of an unfiltered ferrofluid. Further, we have found that the ferrofluid with a lower concentration of surfactant gave rise to larger birefringence and yield stress, and stronger shear thinning behavior than the ferrofluid containing a higher concentration of surfactant. This observation has lead us to conclude that an increase in unbound surfactant in a ferrofluid hindered chain formation of magnetic particles, leading to a decrease in the optical and rheological behaviors of the ferrofluid. Optical microscopy confirmed no visible chain formation of magnetic particles in the ferrofluid having a high concentration of surfactant owing to weak yield stress, birefringence, and shear thinning. On the other hand, we observed from optical microscopy that the filtered ferrofluid gave rise to larger yield stress, birefringence, and stronger shear thinning behavior. Thus, using optical microscopy we were able to explain the magneto-optical and rheological behaviors of the ferrofluids and magnetorheological fluids investigated. |
