  The viscosity of ferrofluid has an important role in liquid sealing of the
hard disk drives, biomedical applications as drug delivery, hyperthermia, and
magnetic resonance imaging. In the absence of a magnetic field, the viscosity
of ferrofluid depends on the volume concentration of magnetic nanoparticles
including surfactant layers. However, under the influence of a stationary
magnetic field, the viscosity of ferrofluid depends on the angle between the
applied magnetic field and vorticity in the flow. If this angle is 90o, then
there is a maximum increase in the viscosity. If the magnetic field and the
vorticity in the flow are parallel to each other, then there is no change in
the viscosity since the applied magnetic field does not change the speed of the
rotation of magnetic nanoparticles in the fluid. The viscosity of ferrofluid in
the presence of an alternating magnetic field demonstrates interesting
behavior. When field frequency matches with the relaxation time, known as
resonance condition, then there is no impact of an alternating magnetic field
in the viscosity of ferrofluid. If the frequency of an alternating magnetic
field is less than resonance frequency, then an alternating magnetic field
increases the viscosity of ferrofluid. Using higher frequency than resonance
condition reduces the viscosity of ferrofluid and researchers reported this
incident as the negative viscosity effect. If the frequency of an alternating
magnetic field tends to infinite, then ferrofluid ceases to feel a magnetic
field. In this case, there is no impact of an alternating magnetic field on the
viscosity of ferrofluid.
