Performance of bidisperse magnetorheological fluids utilizing superparamagnetic maghemite nanoparticles

Magnetorheological (MR) fluids consisted of micron-size particles generally often faces the instability problem due to the large density mismatch between the large particulate matter and continuous oil medium. Recently, researches have been conducted on the advantages of bidisperse MR fluids that is a mixture of micron and nano-sized magnetic particles. In this work, laboratory prepared maghemite nanoparticles (γ-Fe2O3) with average size of 9 nm were incorporated to the MR fluids comprising carbonyl iron (CI) to reduce the sedimentation rate of the MR fluids. Three different fluids with the same solid concentration of 80.98% have been prepared to contain 5 and 10% substitution of γ-Fe2O3 nanoparticles to the CI particles and the monodisperse CI particles for comparison purpose. The sedimentation rate was measured by optical tracking of the interface between the carrier liquid and particle suspension that formed in the fluids over time. The rheological properties of the bidisperse MR fluids were investigated using the rheometer with a parallel-plate measuring cell under the different applied magnetic fields and shear rates. The results indicated that the sedimentation rate was reduced considerably. Moreover, by replacing only 5% of CI particles with γ-Fe2O3 nanoparticles, the yield stress was increased. The formation of heterogeneous aggregates was considered to improve the stability of the fluids and easier building of well-arranged field-induced structures. Therefore, the MR fluids performances were improved in general.