Rheological behavior and microstructure of magnetic dispersions with varying nonmagnetic particle content

We examine the frequency, stress, and strain dependencies of the dynamic shear moduli of magnetic dispersions containing mixtures of varying amounts of magnetic cobalt-modified γ-Fe2O3 and nonmagnetic cobalt-modified α-Fe2O3 particles, and we interpret these results in terms of a network structure in the dispersions. The storage modulus exhibits a plateau, providing evidence for such a network. The critical strain amplitude indicating the transition from linear to nonlinear behavior is independent of magnetic particle fraction. This implies that the topology of the network structure formed by magnetic interactions between the particles is independent of nonmagnetic particle content. However, the critical stress amplitude indicating the transition to nonlinear behavior does depend on magnetic particle fraction. This indicates that the strength of the network depends on nonmagnetic particle content. The loss modulus exhibits a minimum when considered as a function of frequency. This minimum can be rationalized in terms of network elasticity at low frequencies and local viscous behavior at high frequencies. Furthermore, the ‘network elasticity’ extracted from small-amplitude oscillatory measurements scales the magnetic particle fraction dependence of shear stress–shear rate data for steady shear flow onto a single master curve.