Sedimentation Stability of Magnetorheological Fluids: The State of the Art and Challenging Issues

Among the many factors causing particle sedimentation, three principal ingredients are heavily involved: magnetic particles, a carrier liquid (base oil), and additives (surfactant). Therefore, many works have been carried out to improve the sedimentation stability of magnetorheological fluids (MRFs) by adopting the three methods. In the particle modification stage, the weight concentration, size distribution, particle shape, coated materials, and combinations of different sizes of the particles have been proposed, while for the modification of the carrier liquid, several works on the density increment, wettability control, and the use of natural oils, lubricant oil, grease, and ethyl- and butyl-acetate oils have been undertaken. Recently, in certain recipes to improve sedimentation stability, some additives such as aluminum stearate were used to increase the redispersibility of the aggregated particles. In addition, several works using more than two recipes modifying both the particles and base oils are being actively carried out to achieve higher sedimentation stability. This review article comprehensively introduces and discuses the recipes to improve sedimentation stability from the aspects of the three ingredients. A few conceptual methodologies to prevent the sedimentation occurring via a bottle's storage on the shelves of the application systems are also presented, since, to the author's knowledge, there has not been a report on this issue. These are challenging works to be explored and developed for successful application systems' MRFs.