Measured viscosity characteristics of Fe₃O₄ ferrofluid in magnetic and thermal fields

The rheological mechanisms governing the viscosity characteristics of nano-ferrofluids are very complicated; there is no universal theoretical treatment that explains the dependence of the ferrofluid viscosity on the flow, magnetic, and temperature fields. Thus, determining the viscosity characteristics of ferrofluids in various physical fields is of great theoretical and practical significance. This study explores experimentally the relationship between the ferrofluid viscosity and temperature, magnetic-field strength, and magnetic-field inclination. A special experimental bench on which the magnetic field and temperature can be precisely controlled is designed and constructed. It is found that the ferrofluid viscosity is negatively correlated with temperature. Increasing the percentage of the magnetic particles in the ferrofluid increases the viscosity at any given temperature. Ferrofluids are shown to exhibit the magnetic–viscosity phenomenon: under the action of a magnetic field, the viscosity increases until a magnetic viscosity saturation value is reached. Increasing the magnetic field inclination can aggravate the magnetic–viscosity phenomenon but does not change the saturation value. Contrary to the naïve Hall theory but in agreement with earlier phenomenological studies, the magneto-viscous effect is greater with horizontal than with vertical magnetic fields. Simultaneous exposure to temperature and magnetic fields is investigated; the two fields appear to act independently on the viscosity. The magnetic viscosity saturation value is not affected by temperatures in the range of 30–60 °C.