Influence of large size magnetic particles on the magneto-viscous properties of ferrofluid

We compare the magneto-viscous behavior in a shear flow of three different types of magnetic suspension in the presence of a magnetic field. The first suspension contains magnetite particles of average size 10 nm dispersed in transformer oil. The second one is made of large sized magnetite particles having 30 nm particle size dispersed in transformer oil. The third suspension is a mixture of the first and second fluids in different weight proportions. The size and size distribution have been confirmed by transmission microscopy and small angle neutron scattering experiments. The rheological properties of the first two suspensions were measured for varying shear and field values. The flow behavior of the nanosized dispersed ferrofluid is described with Bingham's yield stress model and it varied from 2.2 to 5.5 Pa on increasing the field from 0 to 1 T. The large sized particle dispersed fluid exhibits magneto-viscous behavior with increasing field. The value of Bingham's yield stress obtained is nearly 15 times higher than that of the small size dispersion. On mixing these two fluids with different weight fractions, the Bingham yield stress value increases by a factor of three compared with that of the large sized particle dispersed fluid. The Mason number provides a good scaling of data in the steady simple flow regime. The observed yielding behavior is due to the formation of a longer chain structure in the system under the field and in-field microscopy confirms the same. The present study shows that the addition of large sized magnetic particles in magnetic fluid increases the yield stress as well as the fluid stability under a field.