TITLE: Deformable inclusion jamming for improved magnetorheological performance

Abstract: Magnetorheological fluids (MRFs) are simple systems of magnetic particles dispersed in a non-magnetic carrier fluid.  When a magnetic field is applied to the MRF, magnetic particles form chains and the fluid becomes more solid-like and able to resist shear forces.  This liquid to solid transition, which is fast and reversible, has led to the study of MRFs for energy absorption and damping applications in civil infrastructure, braking systems, and prosthetics.  However, although the MRF system is simple, it has many challenges that have hampered widespread application of MRFs.  One challenge is the power required to achieve the liquid to solid transition, called the MR effect, as many of the applications stated above either need to be low power for safety (devices worn on the body for example) or need to be low power in case of emergency (a damper for an earthquake needs to be useful even if power plants are offline).  While there are many potential solutions to this challenge, adding non-magnetic particles to the MRF system has been one commonly studied way to improve the MR effect with a lower magnetic field (and thus less power).  Many have studied these composites, and while there are many hypothesized theoretical frameworks for why non-magnetic particles increase the ratio of liquid to solid viscosity (or the yield stress of the solid system), there is as of yet no agreed upon mechanism or single descriptive system of equations. Imaging these systems is difficult so there is also no mechanistic understanding of the exact structure of mixed MRFs.  In the Koh Lab, we have taken this challenge a step further.  Previous work has looked at mixing rigid non-magnetizable particles with magnetic materials to improve the MR effect.  Instead, the Koh Lab has introduced emulsion droplets, i.e., liquid, deformable, spherical particles, to MRFs and also seen a significant improvement in MR effect.  The improvement has been seen to be a function of emulsion concentration and droplet size.  None of the rationales currently published about mixed MRF systems, which generally discuss some kind of stress transfer from non-magnetic to magnetic particles, can account for liquid droplets improving the viscosity of the solid-like state of the MRF.  We hypothesize that there is a jamming-type transition occurring with a local increase in particle/droplet concentration that is increasing viscosity and yield stress.  However, in order to fully understand this system, as well as optimize the many parameters that impact the forces at work on the system, a theoretical framework that describes the MRF is absolutely necessary in addition to significant experimental exploration.