Experimental Characterization of Magnetorheological Fluids Using a Custom Searle Magnetorheometer: Influence of the Rotor Shape

In this paper, we investigated the behavior of magnetorheological fluids (MRFs) by means of a custom Searle magnetorheometer. The Searle measuring system is composed of an outer stationary cylinder and an inner cylinder driven by a motor. The MRF is placed between the two concentric cylinders. While the bob is rotating, the fluid transmits a torque to the outer cylinder, which is related to the velocity profile through the gap and the magnetic field applied. In particular, we wanted to assess the influence of different bob shapes on the shear stress-shear rate curves. Indeed, a non-circular cross section causes cyclic variations on the gap thickness and, consequently, can squeeze the active MRFs, causing the formation of thicker columns and higher values of shear stresses. This phenomenon known as a squeeze-strengthen effect can be exploited to achieve higher energy absorption on magnetorheological devices like brakes and clutches. Two different cross sections were taken into account and then compared with the cylindrical one: 1) elliptical and 2) quadrilobate. In both cases, the gap size varies from 0.25 to 0.75 mm. The experimental results showed that with the new bob shapes, the squeeze-strengthen effect does not occur. However, using the elliptical and quadrilobate bobs, changes in the slope of the flow curves have been measured. This may suggest that the post-yield viscosity can be influenced by more complex factors like non-uniform magnetic field or hydrodynamic phenomena. Further studies, such as 3-D magnetic simulation and CFD analysis, will be taken into account to explain the new phenomena brought up in this paper.