Squeeze-strengthening of magnetorheological fluids (part 2): Simultaneous squeeze–shear at high speeds

The first part of this study demonstrated that magnetorheological fluid undergoes squeeze-strengthening in pure squeeze conditions that promote filtration. This behavior, however, is greatly reduced when shear deformation is superimposed onto the squeezing motion. In order to understand this phenomenon and achieve high yield stresses at high rotational velocities, the second part of this study conducts a thorough experimental characterization of magnetorheological fluid behavior under combined squeeze–shear. After demonstrating that a von Mises yield criterion is applicable to magnetorheological fluid, this criterion is included in the Péclet number, derived in the first part of this study, and used to predict filtration in magnetorheological fluid submitted to simultaneous squeeze–shear. Results show squeeze-strengthening is well predicted in squeeze-dominant flows but gradually delayed in shear-dominant flows. In such conditions, a better prediction is provided by a modified Péclet number, which also takes into account the evolution of the magnetorheological fluid microstructure through the squeeze-to-shear-rate ratio. This ratio is also found to dictate the linear relation between shear stress and compressive force when squeeze-strengthening is observed. Based on the provided understanding, high yield stresses (>1000 kPa) are obtained at high rotational velocities (200 r/min) by maximizing the filtration phenomenon in order to achieve squeeze-strengthening at high compression velocities (5 mm/s).