A three-dimensional study of nonlinear particle magnetization in an idealized magnetorheological fluid.

摘要

As with many technological breakthroughs, the usefulness of the magnetorheological effect as a smart material was identified nearly fifty years ago, yet few commercial devices have emerged. Magnetorheological (MR) fluids consist of micron-sized, magnetic particles in a typically nonmagnetic carrier fluid. The interest in these fluids, which are Newtonian in an off-state, is the solid behavior attained in an on-state (under the influence of a magnetic field). The complexity involved in formulating a viable MR fluid only increases the difficulty in understanding the mechanism of the MR effect. In this work, I investigate one aspect thought to have a direct influence on the yield strength of magnetorheological fluids—particle saturation. Previous research indicates that nonlinear particle magnetization reduces the dependence of the shear yield stress to subquadratic with applied magnetic field. Using a three-dimensional finite element method, the local field variables (magnetic field, magnetic induction, and magnetization) are found and analyzed. The inclusion of a three-dimensional MR fluid structure is also explicitly modeled. A micromechanical approach, directly using these local field variables, is used and compared to a macromechanical (continuum) approach. The results of this study show that particles begin to saturate at applied magnetic fields as low as μ₀H_appl = 0.05 T and are completely saturated by μ₀H_appl = 0.5 T. This study provides visualization of the particle magnetization as a function of applied field, shear angle, volume fraction, and intra-chain particle distance. This study reinforces the subquadratic dependence of shear yield stress on applied magnetic field, concluding τ_y = τ_y(H_appl3/2 ) for 0.01 T ≤ μ₀H_appl ≤ 0.3 T.