A magnetorheological fluid (MR fluid) is a type of smart fluid pored in a fluid, usually a type of oil. When subjected to a magnetic field the fluid greatly increases its viscosity, till it become a viscoelastic solid .The yield stress of the fluid when in its active state can be controlled very accurately by varying the magnetic field intensity. The upshot of this is that the fluid's ability to transmit force be controlled with an electromagnet.
How it works:
The magnetic particles are suspended within the carrier oil are distributed randomly and in suspension under normal circumstances as in picture below
Smart_fluid_off_state.jpg

When a magnetic field is applied, the microscopic particles align themselves along the lines of magnetic flux, in picture below. When the fluid is contained between two poles ,the resulting chains of particles restrict the movement of the fluid, perpendicular to the direction of flux, effectively increasing its viscosity
Smart_fluid_on_state.jpg

Applications

The application set for MR fluids is vast, and it expands with each advance in the dynamics of the fluid.

Mechanical engineering

MR dampers of various applications have been and continue to be developed. These dampers are mainly used in heavy industry with applications such as heavy motor damping, operator seat/cab damping in construction vehicles, and more.

liquid-body-armor-6.jpg

Limitations

Although smart fluids are rightly seen as having many potential applications, they are limited in commercial feasibility for the following reasons:
  • High density, due to presence of iron, makes them heavy. However, operating volumes are small, so while this is a problem, it is not insurmountable.
  • High-quality fluids are expensive.
  • Fluids are subject to thickening after prolonged use and need replacing.
  • Settling of ferro-particles can be a problem for some applications



Video:
http://www.youtube.com/watch?NR=1&feature=endscreen&v=toPsSqwxOG

References:
http://en.wikipedia.org/wiki/Magnetorheological_fluid