Mechanisms of Abrupt Changes in the Starting Torque of Magnetic Fluid Seals Induced by the Microscopic Magnetic Field-Induced Chain Structure

A relatively successful application of magnetic fluids is their use in airborne electro-optical pods. Modern electro-optical pods increasingly demand miniaturization, lightweight design, and high precision. This necessitates magnetic fluid seals that maintain sealing integrity while demonstrating low starting torque under high shear rates (≥1000 s−1). Aiming at the engineering problem of sudden change in the start-up torque of magnetic fluid seals at high shear rates, this article, for the first time, through the coupled analysis of discrete element microscopic simulation and rotary seal experiments, reveals the regulation mechanism of the dynamic evolution of the chain structure induced by the magnetic field on the sudden change of torque. High volume fraction and high magnetization intensity can enhance the strength of the chain structure, resulting in higher yield stress, causing a sudden change in the starting torque of the magnetic fluid seal due to the structuring of microscopic magnetic particles. Moreover, experiments show that when the volume fraction exceeds the critical threshold of 6.5 vol% or the magnetization intensity is higher than 100 kA/m, the nonlinear jump of the yield stress (with a peak of 19.12 Pa) leads to a sudden increase of 149% in the starting torque (compared with the condition without magnetic fluid). The research results explain the sudden change in starting torque from the perspective of microstructure changes. The proposed chain density–torque model offers critical parameter guidance for the design of highly reliable seals.