Two-step yielding in novel CoNi nanoplatelet-based magnetic fluids under oscillatory rheology

• We have synthesized the novel CoNi hexagonal nanoplatelets in a CTAB-mediated hydrothermal reduction.
• The surfactant CTAB promotes the directive growth of plate-like morphology in the final nanostructures.
• The structure-dynamics correlation was investigated in oscillatory rheological studies of the magnetic fluids containing the CoNi platelets.
• A novel two-step yielding behavior was demonstrated by the magnetic fluids with different particulate concentrations.

The Core–shell CoNi hexagonal platelet-shaped nanocomposites with average diameter of 202 nm were synthesized by Cetyl-trimethylammoniumbromide (CTAB)-mediated hydrothermal reduction in aqueous precursor. The oscillatory rheology was employed to study the magnetomechanical response of magnetic fluids containing high aspect ratio CoNi nanoplatelets with particular emphasis to viscoelasticity and yielding properties. An interesting two-step yielding behavior in amplitude sweep test was observed for all the magnetic fluids with concentration in the range of 5–12 vol%. This was interpreted in terms of breaking of magnetically induced-microstructure aggregates below and above the cross-over strain. The strain-hardening of loss modulus above the flow point was more prominent in higher concentrated fluids owing to dominance of short-range interaction.

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