Structure-enhanced yield stress of magnetorheological fluids

The yield stress of magnetorheological (MR) fluids depends on the induced s olid structure. Since thick columns have a yield stress much higher than a single-chain structure, we improve the yield stress of MR fluids by changin g the fluid microstructure. Immediately after a magnetic field is applied, we compress the MR fluid along the field direction. Scanning electron micro scopy images show that particle chains are pushed together to form thick co lumns. The shear force measured after the compression shows that the struct ure-enhanced static yield stress can reach as high as 800 kPa under a moder ate magnetic field, while the same MR fluid has a yield stress of 80 kPa wi thout compression. This improved yield stress increases with the magnetic f ield and compression pressure and has an upper limit well above 800 kPa. Th e method may also be useful for electrorheological fluids. (C) 2000 America n Institute of Physics. [S0021-8979(00)02405-1].