Irreversible Effects in Magnetorheological Fluids

Abstract

The effect of residual structure on properties of MR fluids, based on CI particles, was investigated using a magneto-sweep technique and initial magnetic susceptibility measurements. The shear rate, particle size, and the CI surface properties were among the experimental variables. A mechanical hysteresis, associated with the irreversible residual structure formation, was observed via measurements of both shear stress and first normal stress difference. The hysteresis was most pronounced at the creeping flow as compared to high shear conditions. Regardless of experimental settings, the level of the hysteresis was the same at high shear rate, while the fluids containing larger particles and higher CI concentrations demonstrated stronger residual effect at slow flow. The hysteresis loop was slightly wider for the silica-treated particles suggesting that chemical (non-magnetic) interaction might contribute to the fluid irreversible behavior. Existence of some residual structure has been also shown in measurements of initial magnetic susceptibility of MR fluids. After exposure to a sequence of DC magnetic fields, fluids exhibited higher susceptibility presumably caused by the formation of residual structures of larger size than those in the initial state.