Study of the laws of internal friction in magnetic fluids with a strongly developed thixotropic nanostructure

Abstract

Due to their unique physical properties, magnetic fluids are promising for use in bearings, seals, sliding guides, and other devices of modern technology. Some restrictions on their use are imposed by the tendency of magnetic fluids to lose colloidal stability and structure formation in strong magnetic fields. Increasing the stability of a colloid by reducing the size of the dispersed particles of the magnetic fluid is limited by the Heisenberg uncertainty relation, on the condition of maintaining their ferromagnetic state. The search for ways to reduce internal friction in technical devices with magnetic fluids having a highly developed thixotropic nanostructure is important from a practical point of view. Using a device, simulating the operation of a magnetohydrostatic bearing, the rheological characteristics of a fluid nanostructured by a magnetic field, which is a colloidal system with a dispersed phase of magnetite particles (10 vol.%) and a dispersion medium of silicon organic fluid PESV-2, were studied. The dynamic viscosity of the magnetic fluid was about 0.05 Pa.s at 20°C. It has been established that the process of structuring a magnetic fluid in an external field can last hundreds of hours and depends mainly on the viscosity of the dispersion medium and the concentration of magnetite. It has been revealed that the motion of a cylinder with a terminal velocity begins only at shear stresses exceeding the limiting static stress and proceeds at a constant velocity. The breakdown of the structure begins after the shear stress exceeds the critical value. The critical stress is introduced to compare the strength of the structure of different fluids. The value of the critical stress was determined with an accuracy of up to 50 Pa by analyzing the curves of the change in the sliding speed with time. It has been established that the temperature dependence of the critical shear stress is very sharp and close to exponential.