The role of the dispersed-phase remnant magnetization on the redispersibility of magnetorheological fluids

The influence of the remnant magnetization of the soft magnetic particulates, used as a dispersed phase, on the redispersibility of magnetorheological (MR) fluids is discussed. Calculations of the magnetic interaction energy showed that for 33-vol% MR fluids based on particles of iron (similar to 6 mu m), manganese zinc ferrite (similar to 2.3 mu m), and nickel zinc ferrite (similar to 2.1 trm), the ratios of the magnetic interaction energy to the thermal energy were 161,000, 6400, and 3900, respectively. These calculations showed that even the seemingly small levels of remnant magnetization, associated with particulates employed in MR fluids, introduced significant dipole-dipole interparticle interactions. It is proposed that this interaction causes most MR fluids to show a tendency for "cake formation," which makes it difficult to redisperse these fluids. Our modeling presented here also suggests practical strategies to enhance the redispersibility of MR fluids.