Any type of fluid that can be controlled from the outside, for example by a magnetic field, presents a challenge to scientists interested in basic fluid mechanics/dynamics as well as engineers. Consider basic research: the ability to introduce an external but controllable artificial force into the basic equations extends into a fascinating field of potential new phenomena.
The fact that magnetic fields can be varied quite well and accurately, both in direction/orientation and field strength, makes them very attractive for adding such external forces. Unfortunately, natural liquids (most normals) do not offer these features. However, artificially generated suspensions of magnetic nanoparticles in appropriate transport liquids, i.e. ferrofluids, do. Although several different effects have been discussed so far, by far the most famous field-induced property of magnetic fluids is the change in their viscosity (McTague 1969).
In general, a magnetic field can be used effectively as a control or bifurcation parameter of the system, the change of which can lead to characteristically distinct types of hydrodynamic behavior. In this regard, turbulence and turbulence transition in magnetohydrodynamic (MHD) flows play an important role in many astrophysical and geophysical problems, e.g., magnetic field generation in celestial bodies, planets, and (sometimes) large-scale industrial plants. For example, Gellert et al. 2011 studied instabilities driven by helical field currents.