Electrorheological fluids
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#1
16-10-2009, 12:39 PM


Electrorheological fluids are suspensions of extremely fine non-conducting particles (up to 50 micrometres diameter) in an electrically insulating fluid. The apparent viscosity of these fluids changes reversibly by an order of up to 100,000 in response to an electric field. For example, a typical ER fluid can go from the consistency of a liquid to that of a gel, and back, with response times on the order of milliseconds. The effect is sometimes called the Winslow effect

The ER effect

The change in apparent viscosity is dependent on the applied electric field, i.e. the potential divided by the distance between the plates. The change is not a simple change in viscosity, hence these fluids are now known as ER fluids, rather than by the older term Electro Viscous fluids. The effect is better described as an electric field dependent shear yield stress. When activated an ER fluid behaves as a Bingham plastic (a type of viscoelastic material), with a yield point which is determined by the electric field strength. After the yield point is reached, the fluid shears as a fluid, i.e. the incremental shear stress is proportional to the rate of shear (in a Newtonian fluid there is no yield point and stress is directly proportional to shear). Hence the resistance to motion of the fluid can be controlled by adjusting the applied electric field

Applications

The normal application of ER fluids is in fast acting hydraulic valves [3] and clutches, with the separation between plates being in the order of 1 mm and the applied potential being in the order of 1 kV. In simple terms, when the electric field is applied, an ER hydraulic valve is shut or the plates of an ER clutch are locked together, when the electric field is removed the ER hydraulic valve is open or the clutch plates are disengaged. Other common applications are in ER brakes [4] (think of a brake as a clutch with one side fixed) and shock absorbers [5] (which can be thought of as closed hydraulic systems where the shock is used to try and pump fluid through a valve).

There are many novel uses for these fluids, including use in the US army's planned future force warrior project and implimentation. They plan to create bulletproof vests using an ER fluid because the ability to soak the fluid into cloth creates the potential for a very light vest that can change from a normal cloth into a hard covering almost instantaneously. Other potential uses are in accurate abrasive polishing and as hap tic controllers and tactile displays

ER fluid has also been proposed to have potential applications in flexible electronics, with the fluid incorporated in elements such as roll able screens and keypads, in which the viscosity-changing qualities of the fluid allowing the roll able elements to become rigid for use, and flexible to roll and retract for storing when not in use
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