THE PHENOMENON OF MR FLUID

Once activated, a magneto-rheological fluid can go from liquid to solid within a few milliseconds. This attribute means magnetic fluid can be used in anything from vehicles and earthquake-proof buildings to exercise bikes and washing machines. Text: Anders Lindgren

magneto-rheological fluid (also known as MR fluid or smart fluid) is a fluid with attributes that can be controlled by a magnetic field, usually generated by an electromagnet. Depending on the force of the magnetic field, the fluid can change from behaving like a free-flowing oil to a solid, rubbery material. The viscosity can be controlled very precisely and the transitions between different states take just a few milliseconds.

THIS EFFECT IS POSSIBLE THANKS TO the fluid containing small magnetic particles, usually iron, 0.1–10 nanometres in diameter. Without a magnetic field, the particles are randomly dispersed in the oil, which makes the fluid free-flowing. However, when subjected to the current in a magnetic field, the particles align with the flux of the magnetic field and the fluid becomes rigid.

MR FLUID CAN be used in three different ways, depending on the application. In shock absorbers, the fluid flows between two plates that do not move relative to each other. In couplings and brakes, the fluid regulates shear resistance between two or more places that move relative to each other. And, in some applications, the fluid is between two layers that are pressed together or pulled, especially in millimetre-sized movements involving major forces.

ALTHOUGH MAGNETIC fluids have been around since the 1940s and have been developed extensively from the 1960s onwards, there were few applications for MR fluids until around 1990. More than anything, this was because of a lack of reliable ways to control the fluid. Today, however, using signal processors and inexpensive computers, the magnetic field can be controlled with a high degree of precision. This has led to a virtual explosion in

the number of applications, above all in the form of various dampers.

For instance, its super-fast damping effect has won a place for MR fluid as a shock-absorber fluid in many car models. General Motors was first, and developed MagneRide, with the benefit that the viscosity in the shock-absorber fluid can be varied depending on loaded weight and driver preferences. However, it can also provide stability under a variety of road conditions. The readings and adjustments take just one millisecond.

THE SYSTEM HAS BEEN IN USE SINCE early 2002 in the Cadillac and, one year later, also in Chevrolet and Corvette-marque passenger cars. Since then, Audi and Ferrari have also purchased licences to be able to offer MagneRide in several of their models, while Honda and Porsche, for example, have introduced their own MR technology in their cars.

In construction machinery, the fluid is used as

both cab and seat shock absorbers. Another application under development is for helicopter seats, to reduce spinal injuries in the event of a crash.

As the fluid can absorb strong shockwaves, it is also used in seismic shock absorbers in buildings to make them more secure against earthquakes. And a number of military organisations and universities around the world are researching the possibility of creating MR-fluid vests, or liquid armour, with a degree of absorption that could withstand bullets.

A FEW MORE surprise applications for MR fluid are that it can increase the magnetic flux between loudspeaker coils and magnets in high-output loudspeakers, boosting the impact. And it has proved very precise as a polishing method, making MR fluid useful for polishing the corrective lens of the Hubble telescope, for example.