Magnetically reactive fluids are offering smoother operation across a vast range of industries
At first glance, magnetorheological (MR) fluid isn’t that exciting. It’s grey, oily and about three times denser than water. Left to its own devices, it slowly spreads over a surface, forming a thick pool of liquid. Place this liquid close to a magnet, however, and a remarkable transformation takes place.
Within milliseconds of coming close to the magnet, the once free-flowing MR liquid hardens into a near-solid state. The particles suspended in the fluid line up in long chains along the lines of magnetic flux, restricting the movement of the liquid and increasing its viscosity. The process is reversible and when the magnetic field is removed, the solid once again becomes a fluid.
’It’s an amazing technology,’ said Olivier Raynauld, manager of controlled suspensions forward engineering at chassis company BWI Group. ’On the surface, the transformation sounds like a very simple process. But in fact many people who have attempted to use MR fluid have failed… A lot of money has been spent on trying to get it right.’
Most of this money has come from the automotive industry, where the variable nature of MR fluid has been put to use in shock absorbers, engine mounts and clutches. In a vehicle’s suspension, for example, replacing conventional oil in the shock absorbers with MR fluid allows the amount of damping on a vehicle to be adjusted 1,000 times a second according to driving style or the weight being carried by the vehicle.
Lord Corporation, a private company in North Carolina, was one of the early commercial developers of the technology. It began using MR liquid to dampen vibrations for drivers in large trucks and later, in collaboration with BWI, used it for car suspensions by developing a system known as MagneRide. The technology first appeared on Cadillac’s Seville models and is now used in a range of vehicles, including the Audi TT, Audi R8 and Ferrari 458 Italia.
In a car’s suspension, replacing conventional oil with MR fluid allows damping to be adjusted 1,000 times a second
’All the action is around the piston,’ explained Raynauld. ’You have a piston core with a coil and when you energise the coil it creates a magnetic field. The magnetic field changes the state of the fluid to provide variable damping when it’s needed. If you don’t feel it happening, you know it’s working because it will provide a smoother ride and give you better control.’
On a larger scale, MR liquid can be found in China’s Dong Ting Lake bridge to counteract vibrations caused by sudden gusts of wind. The same principle is also being used to stabilise buildings against earthquakes. When sensors on the buildings sense disturbance, they send signals to supply an electrical charge to the dampers. The MR fluid in these dampers changes state constantly in line with the movement of the building, acting as a huge shock absorber.
In applications such as these, the fluid has already proven to be valuable, but scientists believe it has the potential to do a great deal more.
There are many different uses for MR fluid,’ said Dr Holger Böse, a researcher at the Fraunhofer Material Research Institute.
Unfortunately, the microscopic mechanisms at work in the fluid are not at all clear. We’re still not sure what influences the fluid’s strength in the magnetic field.’
Until this can be found out, the commercial availability of the liquid is likely to be limited. One of the biggest issues is sedimentation within the fluid caused by the large density difference between the particles and their carrier liquid. In vehicle shock absorbers, the constant motion of the device is enough to remix any stratified fluid. But where the device is inactive for long periods, such as in seismic damage mitigation, sedimentation of particles can prove to be problematic to performance.
Sedimentation also causes issues for scientists trying to understand the underlying physics of MR fluid. NASA’s InSPACE (Investigating the Structure of Paramagnetic Aggregates from Colloidal Emulsions) programme has taken its MR fluid experiment on board the International Space Station to eliminate gravity. It hopes that, by doing this, it will remove the problem of sedimentation and gain a clearer idea of why particles in the fluid act as they do when exposed to rapidly alternating magnetic fields.
“We’re still not sure what influences the fluid’s strength in the magnetic field”
HOLGER BÖSE, FRAUNHOFER INSTITUTE
Back on Earth, the Fraunhofer Institute is formulating new compositions of MR fluids to tune to different applications. It has been exploring the use of carrier fluids, such as hydrocarbon oils and silicon oils, as well as chemical additives to stabilise the particles. ’The additives form a soft network in the liquid that can hold the particles,’ said Böse. ’If they settle, a gentle shake of the fluid will allow them to be redistributed and used again.’
As researchers step up their efforts to understand the inner workings of MR fluid, so too are the companies commercialising the technology. Lord is developing the use of MR tactile feedback devices in steer-by-wire electrical steering systems that offer more precise steering control. The MR tactile feedback systems will be used to measure how far a driver wants to turn the steering wheel and make them feel more connected to the vehicle when using electric steering.
Össur, the company behind paralympic champion Oscar Pistorius’s Cheetah carbon-fibre prosthetic legs, has commercialised the first artificially intelligent knee system that uses MR fluid to adapt to its user’s movements. The Rheo Knee incorporates sensors that provide input at 1,000 times per second a response similar to that of an actual nervous system. By changing magnetic forces around the MR fluid within the knee, different levels of resistance are activated, giving the user a more natural motion.
In the longer term, MR fluids could be used in the next generation of liquid body armour that stiffens when extra strength is needed. With further improvements, it may one day be used to move robots more naturally or improve haptic computer systems in training simulations. Even if the underlying physics of the fluid remains a mystery, its potential has sparked the imagination of engineers and scientists, and is now fast being realised.
in depth – size matters
Why MR fluid’s tiny particles make it so suitable for motion dampingThe main difference between magnetorheological fluids and ferrofluid another magnetically responsive liquid is the particle size. In MR fluid, particles are much smaller, in the micron-diameter range, while ferrofluid uses nanoparticles.
Vijay Patel, director of Liquids Research in Bangor, explains why this is so important for motion-damping applications. ’Unlike ferrofluid, MR fluid rheology is dramatically affected by the application of a magnetic field. The viscosity of such fluids can change by many orders of magnitude and the fluid rheology can recover in milliseconds once the applied field is removed. This property of dispersions lends them to various applications where vibration control is desired.’