Colloidal stability and rheological properties of bio-ferrofluids of polymer-coated single-core and multi-core nanoparticles

The polymethacrylic acid (PMAA)-coated magnetite nanoparticles with single and multi-cores were prepared by the emulsifier-free emulsion polymerization of methacrylic acid (MAA) monomers. X-ray diffraction analysis (XRD), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM) were used to identify the nanoparticles and the polymer layer. The coated particles have spherical shapes. The single core diameter is 6.6 nm, and the multi-core diameter is 33.8 nm, with the number of cores in a particle of 131. The PMAA layer thickness is around 4 nm for both coated particles. This polymer layer enhances nanoparticles` dispersion ability while retaining their magnetic properties much higher than in other studies. The coated multi-core particles show excellent dispersion stability. The aggregation rate of these particles is hundreds of times lower than that of single-core particles. All the nanofluids exhibit shear-thinning behavior without and with an external magnetic field. The field-dependent viscosity agrees with the theory regarding the interaction parameter λ. However, the magneto-viscous of the magnetic fluids here are much smaller than comparable bio-ferrofluids.