Electrorheological and magnetorheological fluids
Electrorheological and magnetorheological fluids are two component suspensions which undergo a phase change from the liquid to the solid state on the application of an external electrical or magnetic field respectively.
In the case of electrorheological fluids (ERF), the carrier fluid is usually mineral oil and the suspensions comprise electrically polarizable polymer particles. An electrical field strength of several thousand volts per mm are required but being essentially capacitive current flow is extremely low (microamp range).
During the HASASEM research project a 256 Element tactile using ERF was developed. This design of tactile array has several advantages over other techniques such as pneumatic, hydraulic, electromagnetic etc.
1. Due to the small size of the elements a good spatial resolution (a few mm) is possible.
2. Very low power - under 50 mW/element compared with several Watts with many other methods.
3. The control array area is not much larger than the tactile array area.
The latter advantage results from the use of direct band gap semiconductors, such as GaAs, capable of controlling high voltages optically. As a consequence, the electrical resistance of the GaAs elements, and in turn controls the viscosity of the ERF, may be controlled by projecting an image from a DLP-beamer directly onto the GaAs array. This has the additional advantage of obviating the need for additional opto-isolators.
Similarly, magnetorheological fluids (MRF) usually consist of mineral oil carrier fluid but with carbonyl iron particles (typically around 5 micrometer diameter). A magnetic flux density of between 0.1 and 0.5 Tesla is usually adequate. This is usually generated from a permanent magnet or electromagnet. In the latter case the voltage is much lower than with ERF but the current flow leads to a greater power consumption. A related topic is magnetoactive elastomers (MAE), sometimes known as magnetoactive polymers (MAP).