Controllable fluids such as Electro-rheological (ER) and Magneto-rheological (MR) fluids belong to a class of ‘smart’ materials that dramatically alter their physical properties when an appropriate field (electric and magnetic fields respectively) is applied across them.
ER fluids are colloidal suspensions which exhibit dramatic reversible changes in properties when acted upon by an electric field. The phenomenon of electro-rheology was first observed by W M Winslow in 1947 and as a result is sometimes referred to as the Winslow effect. ER fluids exhibit significant increases in shear yield stress and viscosity when an external electric field is applied. The particles suspended in the fluid tend to fibrillate and form chain-like structures between the electrodes along the direction of the electric field. These structures inhibit the free flow of ER fluids. The behaviour of ER fluids under the influence of an electric field is complex. ER fluids behave like Newtonian fluids under zero field conditions. With the field applied, ER fluids behave similarly to Bingham plastics, exhibiting a finite yield stress with the shear stress dependent on the shear rate. Thus, for field strengths in the order of 3kV/mm, ER fluids `solidify’ with static and dynamic yield stresses as high as 10kPa and 5kPa respectively. This reversible liquid to solid transition has been shown to be useful in many engineering applications. Moreover, these changes could be reversed in time intervals in the order of a few milliseconds, thus, offering excellent control capabilities.
The capability of ER fluids to undergo a pseudo phase change on application of a voltage makes them an attractive choice for use in a wide range of engineering devices such as brakes, clutches, hydraulic valves, dampers and engine mounts.
MR fluids are very similar to ER fluids except that a magnetic field has to be applied in this case. These fluids are becoming more popular due to the significantly reduced field strengths needed to activate the fluids. Moreover, the yield stresses are of a higher order than those of ER fluids.
For those of you interested in finding out more information about rheonetic systems, a good place to start is the Lord Corporation’s Web page (http://www.mrfluid.com). Here there is a tutorial on the nature of the fluid itself as well as a description of a variety of applications.
The Motion Master controller updates information on the seat position some 500 times per second. In response to data from the position sensor, the controller determines the intensity of magnetism to be applied to the fluid in the magnetic damper