Normal stresses in a shear flow of magnetorheological suspensions: Viscoelastic versus Maxwell stresses

This work reports an experimental and theoretical study on the normal force developed by suspensions of magnetic microparticles subjected to magnetic fields. Experimental values of the normal force were obtained using a rotational rheometer, for a broad range of particle concentration in the suspensions. Applied magnetic fields up to 343 kA/m were generated in the plate-plate measuring geometry. It was found that the normal force exhibited a high-value plateau at low shear, followed by a decrease as the suspensions started to flow and a final low-value plateau at high shear. These three regions in the normal force vs shear rate curve were well correlated with the microscopic regimes in the suspensions: field-aligned structures filling the gap, inclined structures still filling the gap, and structures non-filling the gap. The theoretical model developed is based on the equilibrium between hydrodynamic and magnetostatic torques and forces in a field-induced aggregate of particles subjected to shear. The stress tensor was obtained and the normal force calculated as the integral of the stress over the total surface of the rotational plate. A good correspondence among the theoretical and experimental values was obtained. (c) 2010 The Society of Rheology [DOI: 10.1122/1.3479043]