Structure and rheology of ferrofluids: simulation results and kinetic models

Magnetoviscous and viscoelastic phenomena in ferrofluids are intimately related to their internal structures. The available kinetic models describing the rheological behaviour rely on strong assumptions and simplifications of these structures. Using equilibrium and nonequilibrium computer simulations, here we discuss the validity of the crucial assumption of rigid, chain-like aggregates underlying the chain model. The simulation results support the existence of chain-like aggregates in strongly interacting ferrofluids, at least for sufficiently strong magnetic fields. In addition, shear-induced degradation of the clusters is observed, which apparently is related to strong shear thinning behaviour. For weakly interacting ferrofluids, only slightly anisotropic spatial structures are observed. In this regime, the simulation results of the magnetoviscous effect are in good agreement with the predictions of a dynamical mean-field theory. Further, we explore some first steps towards a unified kinetic model that is applicable in both, the weakly and strongly interacting regimes