Negative viscosity induced in a ferromagnetic colloidal system subjected to both external shear flow and ac magnetic fields

摘要

We investigate numerically the effect of an external ac magnetic field on the theological characteristics of a ferromagnetic colloidal system which is subjected to a shear flow field. We carry out Brownian dynamics simulations, in which both translational and rotational motions of ferromagnetic particles are taken into account. The nondimensional parameters, which govern the problem, are the ratio of the magnetic dipole energy to thermal energy, lambda, the ratio of the interactive energy between the magnetic field and the dipole moment to thermal energy, zeta, and the ratio of the shear rate to the viscosity, Pe, which is called the Peclet number. We analyse the dependence of the apparent viscosity on the frequency of the external magnetic field for lambda = 1.5, zeta = 2 and Pe = 1. We find that when lambda is as small as 1.5, in which case large clusters are not formed in the system, the apparent viscosity increases at low frequencies and becomes almost zero at high frequencies. The viscosity starts decreasing and becomes negative at intermediate frequencies. [References: 7]