Magnetic nanofluids (MNFs) are well-known colloidal stable systems of nanosized sub-domain magnetic particles dispersed in a non-magnetic carrier liquid. The preparation of stable MNFs in our laboratory consists of two consecutive steps:

For the synthesis of magnetic nanoparticles (spinel ferrite type), the bottom-up chemical method is often applied. Ferrite particles are obtained from the co-precipitation technique of aqueous solutions of trivalent and bivalent iron salts in a 2 : 1 molar ratio. The process of nucleation and growth of the spherical nanoparticles takes place at 80–82 °C in the presence of a concentrated NH₄OH solution (25%) and C₁₇H₃₃COOH (90%) used as precipitating and stabilizing agents, respectively. The temperature and rate of the addition of an excess of the precipitating agent are chosen to ensure the formation of magnetite nanoparticles predominantly. The surface of the magnetic nanomaterial is sterically functionalized via the chemisorption of chemically pure oleic acid to prevent both agglomeration and/or oxidation of the nanoparticles and ensure the hydrophobicity of their surface. The resultant black precipitate (containing a monolayer of surfactant covered nanoparticles) was separated using a permanent magnet and then the nanoparticles were washed with deionized water and acetone (flocculation removed the free oleic acid). The stabilized magnetic nanoparticles with hydrophobic character were homogenously dispersed (∼ 120 °C) in a carrier liquid of non-polar character (GTL-based transformer oil) to obtain a stable MNF without any phase separation and sedimentation.^2–4