Anisotropy of the magnetoviscous effect in a cobalt ferrofluid with strong interparticle interaction

•The magnetoviscous effect in a ferrofluid with strong interaction is anisotropic.•The strongest effects are found in a magnetic field parallel to the shear gradient.•In strong magnetic fields the microstructure of the fluid is stable against shearing.•In weak fields the fluid behavior indicates the presence of heterophase structures.

The anisotropy of the magnetoviscous effect (MVE) of a cobalt ferrofluid has been studied in a slit die viscometer for three orientations of the applied magnetic field: in the direction of the fluid flow (Δη1), the velocity gradient (Δη2), and the vorticity (Δη3). The majority of the cobalt particles in the ferrofluid exhibit a strong dipole–dipole interaction, which corresponds to a weighted interaction parameter of λw≈10.6. Thus the particles form extended microstructures inside the fluid which lead to enhanced MVE ratios Δη2/Δη1>3 and Δη3/Δη1>0.3 even for strong shearing and weak magnetic fields compared to fluids which contain non-interacting spherical particles with Δη2/Δη1≈1 and Δη3/Δη1=0. Furthermore, a non-monotonic increase has been observed in the shear thinning behavior of Δη2 for weak magnetic fields <10 kA/m, which cannot be explained solely by the magnetization of individual particles and the formation and disintegration of linear particle chains but indicates the presence of heterophase structures.