A capacitive method and experiment for measurement of sedimentation of magnetorheological fluid

Sedimentation is one of the key issues in the practical engineering applications of magnetorheological fluids (MRFs). Testing and evaluation of the suspension stability of MRFs should be a prerequisite procedure before their applications. There are a few reported methods of measurement of sedimentation of MRFs. The opaque zone below the mudline, especially the sediment zone at the bottom in the MRF column, is the true key factor that affects the applications of MRF due to the possible irreversibility caused by the caking of the sediment zone. This study proposes a novel capacitive method on the basis of the change in dielectric constant resulting from the change in concentration of the magnetic metal particles of MRF upon sedimentation. First, an analytic model was established to present a positive correlation between the dielectric constant and volume fraction of particles throughout the entire MRF column and the theoretical expression of the dielectric constant of MRF in a test tube was obtained. Second, a capacitive sensor and the relevant experimental set-up were designed and fabricated. Especially, the effect of eccentricity error of the capacitor sensor structure on the measurement was analyzed. Then, a sedimentation experiment for prepared MRF samples was carried out, and the testing results were discussed and verified by a visual mudline observation of the supernatant zone, indicating the feasibility of testing and evaluating the suspension stability of MRF by the capacitive method.