Rheological properties and stabilization of magnetorheological fluids in awater-in-oil emulsion

The rheological properties and dispersion stability of magnetorheological ( MR) fluids consisting of hydrophilic treated carbonyl iron particles disper sed in a water-in-oil emulsion were studied for the first time by the use o f a stress-controlled rheometer and sedimentation test. In order to improve the stability of the MR fluids, carbonyl iron magnetic particles were chem isorbed by TWeen 80 and a water-in-oil emulsion was employed as a continuou s phase for MR fluids. Attraction between hydrophilic-treated carbonyl iron and water emulsion in continuous phase plays a critical role in greatly im proving stability of dense carbonyl iron particles against sedimentation wi thout restricting rheological properties. On application of magnetic fields , the suspensions show a striking increase in viscosity. Since constant str ess is generated within the limit of zero shear rate, the plateau in the fl ow curve corresponds to the Bingham yield stress. Under a low external magn etic field, the yield stress varied as B-3/2, indicating that local magneti zation saturation occurs between the neighboring magnetized particles. The yield stress has an approximately linear relation to the particle volume fr action. (C) 2001 Academic Press.