From high colloidal stability ferrofluids to magnetorheological fluids: tuning the flow behavior by magnetite nanoclusters

The flow behavior of practically structureless ferrofluids was tuned over a large range by adding well-defined amounts of magnetite nanoparticle clusters. Magnetite nanoparticle clusters with hydrophobic coating were dispersed in high colloidal stability transformer oil based ferrofluid samples, initially free of clusters. The resulting ferrofluid composition consists both of single (approx. 7 nm mean size) and clustered (280 nm average size) magnetite nanoparticles, each of them in a prescribed amount. This new approach allows a precise evaluation of the role of clusters on the ferrofluid flow behavior. The magnetite nanoparticle clusters obtained by a solvothermal method have a saturation magnetization of 75 emu g(-1) and a remnant magnetization of 0.2 emu g(-1) due to the large size (similar to 25 nm) of the magnetite nanoparticles in their composition. Long bundles of thin elongated needle like aggregates of magnetic nanocluster particles are formed in a magnetic field and produce a significant magnetorheological response of the bidisperse magnetic suspensions observed already for moderate applied magnetic field values. The dynamic yield stress and magnetoviscous effect are analyzed and compared with previous results concerning suspensions of micrometer size iron particles in a ferrofluid carrier. The viscoelastic properties and the interactions between the components of ferrofluid based magnetite nanoparticle cluster suspensions were evaluated by creep tests in zero and non-zero applied magnetic field. The bidisperse magnetic nanocomposite suspensions investigated provide a promising new formulation of MR fluids with improved kinetic stability to be used in seismic dampers and flow control devices.