Ferrofluid Structure and Rheology

Abstract

Combined efforts of experimental, theoretical and simulation studies performed over the last years have significantly improved the understanding of structural and dynamical properties of ferrofluids. Experiments on newly synthesized cobalt ferrofluids – well characterized and with a narrow particle size distribution – show huge magnetoviscous effects, where the zero-field viscosity increases by a factor 10–100 due to an applied magnetic field. Such large variations in the viscosity are accompanied by dramatic structural changes that are detected by neutron scattering experiments under shear flow. Therefore, structure and rheology are intimately related in such ferrofluids. These results can be explained by the formation and breaking of chain-like aggregates of magnetic particles. Chain formation under equilibrium conditions is covered in this review as a starting point for a discussion of the dynamics of these aggregates in terms of the kinetic chain model. Also the case of weak dipolar interactions is reviewed here, where systematic approximations allow to discuss the effect of dipolar interactions on the equilibrium magnetization. These studies have been extended recently, giving valuable insights into the role of weak dipolar interactions on dynamical, viscous and viscoelastic properties of ferrofluids.