Yield stress measurements of magnetorheological fluids in tubes

A pressure-driven apparatus was used to measure the yield stress of magneto rheological (MR) fluids as a function of the applied magnetic field, the ca rbonyl iron particle content, and the amount of surfactant used to stabiliz e the MR fluid. The yield stress was measured from the pressure difference necessary to initiate flow of a MR fluid in a straight tube present in a ma gnetic field. Yield stress measurements were made on MR fluids that contain ed up to 30 vol % carbonyl iron particles, up to 14 vol % surfactant oleic acid, and the remainder 100 cS silicone oil. In the absence of an applied m agnetic field, the MR fluids did not have a yield stress and behaved as New tonian fluids. In the presence of magnetic fields up to 2.2 kG, the MR flui ds had yield stresses up to 1.9 kPa. An effect of the tube size on the yiel d stress was observed-for MR fluids above 15 vol % carbonyl iron and magnet ic fields above 1.0 kG.