| In suspensions of polarizable particles the addition of polar molecules can dramatically increase the yield stress under an applied electric field, leading to a giant electrorheological (GER) effect. We report experiments on dense suspensions of strontium titanyl oxalate in silicon oil, where we find a yield stress of up to 200kPa at 5kV/mm. The magnitude of this yield stress directly correlates with the water content in the particles. In the dynamic response we observe behavior not previously reported for GER fluids and similar to sheared granular materials, including a direct proportionality between shear and normal stresses and the creation of a shear band a few particles in width. An important consequence is that the dynamic response can be varied dramatically by changing the confinement of the suspension or by imposing a normal stress. Magnetorheological (MR) fluids are colloidal suspensions of magnetizable particles that exhibit an increase in the yield stress and in the apparent viscosity when a magnetic field is applied. It has been shown previously that MR fluids can also be used for field-controlled static adhesion to non-magnetic surfaces. Here we demonstrate the important role surface tension plays in this adhesion and that the adhesive property is not related to the shear resistance of the field-dependent yield stress, as previously proposed. |
