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.