We investigated the effect of surface treatment for iron particles in the magnetorheological fluids (MRFs) on rheological properties. Silane coupling treatment with an excess amount of dodecyltrimethoxysilane (DTMS) was conducted for iron particles. MRFs were prepared by dispersing iron particles with and without the silane coupling treatment. MRFs with iron particles washed with n-hexane after the silane coupling treatment was also prepared. The rheological properties of MRFs were measured in the presence and absence of a magnetic field. Without a magnetic field, MRFs with the silane-coupled particles exhibited higher stress than the untreated particles. Under a magnetic field, MRFs with the silane-coupled particles showed lower stress than the untreated analogue. These results imply that the silane coupling treatment enhances the aggregation of particles without a magnetic field but suppresses the cluster formation under the magnetic field. MRFs with the washed iron particles exhibited almost the same rheological properties as that with the untreated particles. Thus we suggest that DTMS is mainly physically absorbed on the surface of iron particles, without forming chemical bonds.
