Original: https://doi.org/10.1051/e3sconf/202345808003
Magnetic fluids have special physical properties that make them useful in modern devices such as bearings, seals, and sliding guides. However, their use is limited because they can lose stability and form rigid structures when exposed to strong magnetic fields.
One way to improve stability is to reduce the size of the magnetic particles in the fluid. However, this approach is limited because the particles must remain ferromagnetic, which is restricted by fundamental physical laws.
From a practical point of view, it is important to find ways to reduce internal friction in devices that use magnetic fluids with complex, thixotropic nanostructures.
In this study, researchers examined the flow behavior of a magnetic fluid using a device that simulates a magnetohydrostatic bearing. The tested fluid was a colloidal system containing 10% magnetite nanoparticles dispersed in a silicone-based fluid (PESV-2). At 20°C, the fluid had a dynamic viscosity of about 0.05 Pa·s.
The results showed that the formation of internal structures in a magnetic field can take hundreds of hours. This process mainly depends on the viscosity of the base fluid and the concentration of magnetite particles.
It was also found that a moving cylinder inside the fluid starts to move only when the applied shear stress exceeds a certain minimum value. Once this happens, the cylinder moves at a constant speed. When the shear stress becomes too high, the internal structure of the fluid begins to break down.
To compare the strength of different magnetic fluids, a value called critical shear stress was defined. This value was measured with an accuracy of about 50 Pa by analyzing how the sliding speed changed over time.
Finally, the study showed that the critical shear stress decreases very rapidly as temperature increases, following an almost exponential trend.