Preparation and Characterization of Glycerol-Based Fe3O4 Magnetic Fluids

To optimize the preparation protocol and tribological performance of glycerol-based Fe^3O^4 magnetic fluids, three hybrid agitation systems—(i) chemical co-precipitation coupled with magnetic stirring (H+C), (ii) chemical co-precipitation coupled with mechanical stirring (H+J), and (iii) chemical co-precipitation integrated with mechanical plus ultrasonic agitation (H+J+C)—were systematically investigated with respect to their influence on the physicochemical characteristics and lubricating behaviour of the resulting magnetic nanoparticles. Relative to the H+C and H+J protocols, the H+J+C protocol effectively suppressed intermediate agglomeration, yielding a 15 % increase in Fe^3O^4 productivity and a 15 % reduction in the full-width at half-maximum of the particle-size distribution. A binary surfactant system composed of oleic acid and citric acid achieved complete surface passivation, producing nanoparticles with a saturation magnetization of 59.2 emu g^–1. Under a magnetic flux density of 0.0341 T, tribometric evaluation revealed that the friction coefficient of the fluid prepared via the H+J+C route decreased to 0.041, corresponding to reductions of 7.1 % and 22 % relative to the H+J and H+C counterparts, respectively, thereby demonstrating superior tribological performance. The present work furnishes an experimental foundation for the rational design of high-performance magnetically responsive lubricants and the optimization of magnetic-fluid synthesis protocols.