The sedimentation of dilute and concentrated MR suspensions was also investigated. It can be said that the modification of iron particles by surfactants with proper functional groups, gives rise to the changes of surface hydrophobicity,which is essential in determining the dispersed state of suspensions, consequently,the stability and rheological characteristics of the disperse systems.In the oil medium,particles polar interactions can be either attractive( hydrophilic particles) or repulsive (hydrophobic particles),so a compromise should be considered between steric effect and the polar attraction provided by the surfactant in the MRF.Our results suggest that optimization of viscosity and sedimentation properties of MRF is possible by carefully
making an adjustment of the hydrophobicity of iron particles, which may find practical applications such as dynamic seals,selfassembly and sensors devices,etc.