Smart fluids had a renaissance of sorts in the 1990’s when the interest in their research got renewed although their properties had been known for over sixty years. Finally the research got to the peak and culminating with the use of smart fluid on the suspension of the 2002 model of the Cadillac Seville STS automobile and more recently, on the suspension of the second-generation Audi TT. After that the interest has been going down again as some drawbacks in their functionality are yet to be overcome. One of these includes their nonlinear behaviour. A central issue is the hysteresis that is observed in the force versus velocity response. A recent article in the Journal of Sound and Vibration attributed this hysteretic behaviour to the compressibility of the operating fluid , and proposed a hydraulic model of this behaviour.
The main use of smart fluids is as dampers. There are basically two types of smart fluids Magnetorheological (MR) fluids and electrorheological (ER) fluids. It was a MR fluid that was used in the suspension of Cadillac Seville STS automobile and Audi TT. The most developed of these are the ones whose viscosity increases when a magnetic field is applied. For this small magnetic dipoles are suspended in a non-magnetic field and this increases their viscosity as the small magnets are made to line up and form strings. When used as suspensions their viscosity is changed according to different road conditions. Their other proposed uses include reducing vibrations in washing machines, air conditioners’ compressors, rockets and satellites etc.
The ER fluids are different from the MR fluids in the sense that their yield stress point is altered rather than their viscosity when an electric field is applied. Their proposed uses include clutches, brakes, shock absorbers and even as bullet proof vests.
Although MR and ER fluids remain the major types, there is one more category which changes its surface tension when an electric field is applied.
A team from the University of Sheffield has developed an approach to the study of smart fluids.