Dynamic and rate-dependent yielding behavior of Co0.9Ni0.1 microcluster based magnetorheological fluids

• A typical polymer/surfactant free polyol reduction method was used to synthesize large-scale Co0.9Ni0.1 nanoclusters.
• Room temperature rate-dependent magnetorheology of CoNi-nanoclusters based MR fluids revealed viscoplastic behavior.
• Magnetic fields were replaced by powder particle magnetization (M) for better yield stress scaling.
• In addition to previously reported M2-dependence, higher order relations (~M3) were also noted for static yield stress.
• An interesting viscous relaxation phenomenon occurred at higher magnetic fields.

In this paper we performed steady shear and oscillatory magnetorheological (MR) studies in magnetic fluids containing CoNi sub-micron sized clusters of 450 nm in diameter. Such Co-rich nanoclusters were synthesized by conventional homogeneous nucleation without any external surfactant or reducing agent in liquid polyol at elevated temperature. The x-ray diffraction, energy dispersive x-ray analysis, scanning and transmission electron microscopy studies were done for analyzing the sample composition and morphology. Two variants of fluid samples were prepared by dispersing 15 vol% and 20 vol% of CoNi powders in castor oil. Room temperature steady magnetoshear studies indicate viscoplastic behavior with stronger dependence of static yield stress on magnetization than a dipolar coupling that was operational in the dynamic yield stress. Magnetosweep measurements at constant shear rate showed interesting viscous relaxation at high magnetic fields. We also explored dynamical elastic behavior through oscillatory magnetorheological studies under both strain sweep and frequency sweep modes, and showed glass transition like phenomenon occurring in them above critical shear amplitudes.