Magnetoviscosity of dilute magnetic fluids in oscillating and rotating magnetic fields

The magnetic field dependent viscosity (magnetoviscosity) of dilute suspensions of spherical magnetic particles suspended in a Newtonian fluid subjected to both magnetic and shear flow fields was studied through Brownian dynamics simulations. Results are presented for the response of these suspensions to oscillating magnetic fields and magnetic fields that are corotating or counter-rotating with respect to the fluid vorticity. A decrease in negative values in the intrinsic magnetoviscosity is observed for oscillating and corotating magnetic fields, whereas an increase is observed for counter-rotating magnetic fields. The frequency corresponding to zero viscosity and the minimum value in the negative viscosity are lower for corotating magnetic fields than for oscillating magnetic fields. In the negative magnetoviscosity regions the particles in a corotating magnetic field rotate faster than in an oscillating magnetic field. It is estimated that the flow due to corotating particles could be strong enough to obtain a negative effective viscosity in dilute suspension. Moreover, it is shown that the commonly accepted constitutive equation for the antisymmetric stress describes well the intrinsic magnetoviscosity of the suspension.