Effect of gap thickness on the viscoelasticity of magnetorheological fluids

In this work, the effect of confinement distance on the magnetorheological (MR) properties of a conventional MR fluid, constituted by 30 vol % of iron microparticles dispersed in a liquid carrier, is studied. With this aim a commercial magnetorheometer supplied with parallel-plate geometry was used. The distance between the upper and the lower plate (gap thickness) was tuned from 10 to 400 μm. The steady-state and the dynamic regimes of the MR fluid in the presence of applied magnetic fields were studied as a function of the gap length. The experimental results show that in the preyield regime there is a strong increase in the magnitude of the viscoelastic moduli and the shear stress as the gap thickness is increased. The physical reason for this effect might be the influence of gap thickness on the particle structures induced by the field. This hypothesis is corroborated by microscopic observations in diluted systems. These experiments show that the aspect ratio (length/diameter) of the field-induced structures increases with the gap thickness. Theoretical analysis shows that the increase in the storage modulus with gap thickness can be explained by a decrease in the demagnetizing factor of these structures and, as a consequence, by an increase in the restoring torque, acting on them. The dissipation effects in the suspension under oscillating flow are rather produced by contact friction between the particles in the dense structures.