SynopsisThe structure development in dispersions of magnetic barium ferrite particles in cyclohexanone with polyvinylchloride wetting resin was tested by oscillatory rheological measurements and orthogonal superposition of steady and oscillatory shear. The optimum dispersion is achieved at the resin concentration c ϭ c 0 , which corresponds to a minimum in the viscoelastic modulii. At c Ͻ c 0 the system is highly elastic, brittle ͑G Ј drops sharply and G Љ goes through a strong maximum at high strain͒, thixotropic and its relaxation spectrum, H(), is dominated by long relaxation times, , which indicates the existence of a strong network. At c Ͼ c 0 the system becomes less elastic, more flexible, less thixotropic and H() is dominated by short , which indicates that the structure formation in this case is dominated by small aggregates rather than a network. By superimposing steady and oscillatory shear we show that the steady shear dramatically break the structure and shifts H() to very short .
The transverse susceptibility, χt, of a suspension of single-domain ferromagnetic particles in a viscous fluid is calculated. The yield stress is introduced to describe the structure formation. Magnetic interactions are taken into account through the mean-field model. Calculations are compared to experiments in which a dc field, H, is slowly increased. The experiments were performed on a model iron oxide dispersion. The function χt(H) goes through a maximum which decreases and shifts to a higher field when the pigment concentration is increased. This is attributed to an increase of the yield stress and the mean-field interaction parameter in our model.
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