In this work, the effects of oleic acid and dimer acid on the shear viscosity, thixotropic behavior and temperature dependence
of MRF were carried out by using a parallel plate rotational rheometer. 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.