The influence of dipolar particle interactions on the magnetization and the rotational viscosity of ferrofluids

Abstract

The effect of the dipolar particle interactions on the behavior of ferrofluids under a shear flow is not yet well understood. The equilibrium magnetization in the absence of flow is studied in Paper I [A. P. Rosa, G. C. Abade, and F. R. Cunha, “Computer simulation of equilibrium magnetization and microstructure in magnetic fluids,” Phys. Fluids 29(9), 092006 (2017)]. In this paper, we present the results of magnetization and rheology in terms of a rotational viscosity obtained by applying Brownian dynamics simulations for a periodic magnetic suspension, where the many body long-range dipole-dipole interactions are calculated by the Ewald summation technique. The dependence of these macroscopic properties on the dipolar interactions is explored in ferrofluids undergoing both weak and strong shear flows in the presence of a uniform magnetic field. Through the simulations, the suspension microstructure is also analyzed in order to characterize the interplay between the structure and the investigated macroscopic properties. We show that for weak shear flows the dipole-dipole interactions produces a magnetization increasing. In contrast, a decrease in the ferrofluid magnetization with the shear rate is substantially intensified as the dipolar interactions are accounted for. Therefore, for strong shear flows, the dipolar interactions always have an effect of decreasing magnetization. In addition, while the dipolar particle interactions produce an increase in the rotational viscosity for weak flows, variations in the same property are not perceptible under the condition of strong flows. The numerical simulations show chain-structure formation oriented in the direction of the magnetic field (i.e., perpendicular to the direction of the shear) for weak flows, which explains the remarkable increasing of the suspension rotational viscosity as a function of the applied magnetic field and of the dipolar interactions parameters. A detailed comparison shows that our simulation results of magnetization and the rotational viscosity are in excellent agreement with approximate theoretical predictions reported in the literature for the case of noninteracting particles.