Magnetorheology and sedimentation characteristics of carbonyl iron suspension in polyalphaolefin with a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> </mml:math> -Fe₂O₃ hard

Abstract The use of micron-sized soft magnetic carbonyl iron (CI) particles in magnetorheological (MR) fluids is often limited by the sedimentation that occurs due to the density difference between these particles and the carrier fluid. To alleviate this problem, in this study hard magnetic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> </mml:math> -Fe 2 O 3 nanoparticles (2 wt%) were introduced as an additive and polyalphaolefin (PAO) as a carrier fluid to prepare a CI (80 wt%) based MR fluid. Under a magnetic field of 343 kA m −1 , the dynamic yield stress for the MR fluid containing γ -Fe 2 O 3 was approximately 62.2 kPa, while the MR fluid containing pure CI was lower at 54.2 kPa. In addition, the MR fluid containing both <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> </mml:math> -Fe 2 O 3 and PAO-5 showed 3% less sedimentation than the sample without <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> </mml:math> -Fe 2 O 3 . The sedimentation observed for the MR fluid containing PAO-5 was approximately 24% less than that of the MR fluid using 100 cSt silicone oil as the carrier, and explained based on the interaction energy <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mtext> </mml:mtext> </mml:mrow> </mml:mrow> </mml:math> between the particles and the carrier fluid. Our finding indicates that the γ -Fe 2 O 3 additive system with PAO as the carrier fluid significantly improved both the MR characteristics with higher yield stress and the dispersion stability of the CI-based MR fluid.