text: Studies published beginning in the late 2000s which explore the effect of varying the aspect ratio of the ferromagnetic particles have shown several improvements over conventional MR fluids. Nanowire-based fluids show no sedimentation after qualitative observation over a period of three months. This observation has been attributed to a lower close-packing density due to decreased symmetry of the wires compared to spheres, as well as the structurally supportive nature of a nanowire lattice held together by remnant magnetization. Further, they show a different range of loading of particles (typically measured in either volume or weight fraction) than conventional sphere- or ellipsoid-based fluids. Conventional commercial fluids exhibit a typical loading of 30 to 90 wt%, while nanowire-based fluids show a percolation threshold of ~0.5 wt% (depending on the aspect ratio). They also show a maximum loading of ~35 wt%, since high aspect ratio particles exhibit a larger per particle excluded volume as well as inter-particle tangling as they attempt to rotate end-over-end, resulting in a limit imposed by high off-state apparent viscosity of the fluids. This range of loadings suggest a new set of applications are possible which may have not been possible with conventional sphere-based fluids.
  Newer studies have focused on dimorphic magnetorheological fluids, which are conventional sphere-based fluids in which a fraction of the spheres, typically 2 to 8 wt%, are replaced with nanowires. These fluids exhibit a much lower sedimentation rate than conventional fluids, yet exhibit a similar range of loading as conventional commercial fluids, making them also useful in existing high-force applications such as damping. Moreover, they also exhibit an improvement in apparent yield stress of 10% across those amounts of particle substitution.
  Another way to increase the performance of magnetorheological fluids is to apply a pressure to them. In particular the properties in term of yield strength can be increased up to ten times in shear mode and up five times in flow mode. The motivation of this behaviour is the increase in the ferromagnetic particles friction, as described by the semiempirical magneto-tribological model by Zhang et al. Even though applying a pressure strongly improves the magnetorheological fluids behaviour, particular attention must be paid in terms of mechanical resistance and chemical compatibility of the sealing system used.