One of the most important features of ferrofluids is the possibility to change their physical properties, especially their viscosity, by means of moderate magnetic fields. This capability makes ferrofluids very useful in the fields of engineering, medicine and fundamental research. Rheological experimental results, as well as theoretical studies, correlate the change of the viscosity of a sheared ferrofluid under the influence of a magnetic field, the so-called magnetoviscous effect, to the internal structure formation under certain external conditions. To obtain information about the microstructure of ferrofluids, experiments using the small-angle neutron scattering (SANS) technique have been carried out. Three magnetite-based ferrofluids with different particle-particle interactions, and thus various magnitudes of the magnetoviscous effect, were investigated. Using a specially designed rheometer, SANS experiments were performed for different shear rates and magnetic field strengths in order to observe the modification of the microstructure in ferrofluids and to associate the SANS information with their macroscopical behaviour. The scattering patterns obtained show a good agreement with the qualitative model that explains the magnetoviscous effect.

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