Steady shear rheological response of ferrofluids containing hydrophilic fumed silica under magnetic fields

This study investigates the steady shear rheological behavior of water-based ferrofluids composited with hydrophilic fumed silica under different magnetic field strengths, with particular attention to avoiding gelation that reduces fluidity. Seven composite ferrofluid samples were prepared and characterized. By adjusting silica particle size and volume fraction, their effects on viscosity and yield stress were explored. As a result, pronounced shear-thinning behavior is observed in this dispersion, with their flow curves under different magnetic field strengths effectively scaled by the Mason number. Higher silica concentration or larger particle size increases the critical Mason number, showing that field-induced structures become more stable. In contrast, only high silica concentrations significantly enhance shear-thinning, as reflected by a larger flow index, whereas particle size has little influence. Yield stress analysis further shows that macroscopic models capture normalized Bingham yield stress, while microscopic models better predict normalized static yield stress. Overall, this work demonstrates that hydrophilic fumed silica offers a simple and effective route to tune the magnetorheology of water-based ferrofluids without inducing gelation, ensuring controllable rheology and good fluidity.