Microstructure and rheology of ferrofluids

Experimental studies made for different ferrofluid samples under shear flow have shown that increasing the magnetic field strength yields an increase of the fluids viscosity, the so-called magnetoviscous effect, while increasing shear rate leads to a decrease of the magnitude of the viscosity (shear thinning). The change of the viscosity with magnetic field strength is theoretically explained as an effect of chain-like structure formation in ferrofluids whereas its magnitude depends on the particle-particle interaction. Both effects, the shear thinning and the magnetoviscous effect, can therefore be related to the microstructure and microstructure dynamics of ferrofluids. Using a specially designed rheometer, ferrofluids having different magnitude of the magnetoviscous effect were investigated by small-angle neutron scattering. Correlated to the structure formation in the fluid, the scattered intensity shows a variation with the magnetic field and shear rate only in the case of the fluids with a high magnetoviscous effect. The presented results show that there is a strong connection between the rheological behaviour of ferrofluids and their microstructure.