Electrorheological fluid

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Electrorheological (ER) fluids are suspensions of extremely fine non-conducting but electrically active 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.^[1] The effect is sometimes called the Winslow effect after its discoverer, the American inventor Willis Winslow, who obtained a US patent on the effect in 1947^[2] and wrote an article published in 1949.^[3]

The ER effect

The change in apparent viscosity is dependent on the applied

ER fluids are a type of smart fluid. A simple ER fluid can be made by mixing cornflour in a light vegetable oil or (better) silicone oil.

There are two main theories to explain the effect: the interfacial tension or 'water bridge' theory,

Another factor that influences the ER effect is the geometry of the electrodes. The introduction of parallel grooved electrodes showed slight increase in the ER effect but perpendicular^{[clarification needed]} grooved electrodes doubled the ER effect.^[6] A much larger increase in ER effect can be obtained by coating the electrodes with electrically polarisable materials. This turns the usual disadvantage of dielectrophoresis into a useful effect. It also has the effect of reducing leakage currents in the ER fluid.^[7]

The giant electrorheological (GER) fluid was discovered in 2003,

There are many novel uses for these fluids. Potential uses are in accurate abrasive polishing[12] and as haptic controllers and tactile displays.^[13]

ER fluid has also been proposed to have potential applications in flexible electronics, with the fluid incorporated in elements such as rollable screens and keypads, in which the viscosity-changing qualities of the fluid allowing the rollable elements to become rigid for use, and flexible to roll and retract for storing when not in use. Motorola filed a patent application for mobile device applications in 2006.^[14]

Problems and advantages

A major problem is that ER fluids are suspensions, hence in time they tend to settle out, so advanced ER fluids tackle this problem by means such as matching the densities of the solid and liquid components, or by using nanoparticles, which brings ER fluids into line with the development of magnetorheological fluids. Another problem is that the breakdown voltage of air is ~ 3 kV/mm, which is near the electric field needed for ER devices to operate.

An advantage is that an ER device can control considerably more mechanical power than the electrical power used to control the effect, i.e. it can act as a power amplifier. But the main advantage is the speed of response. There are few other effects able to control such large amounts of mechanical or hydraulic power so rapidly.

Unfortunately, the increase in apparent viscosity experienced by most Electrorheological fluids used in shear or flow modes is relatively limited. The ER fluid changes from a Newtonian liquid to a partially crystalline "semi-hard slush". However, an almost complete liquid to solid phase change can be obtained when the electrorheological fluid additionally experiences compressive stress.^[15] This effect has been used to provide electrorheological Braille displays^[16] and very effective clutches.^[17]

See also

• Continuum mechanics
• Debye–Falkenhagen effect
• Electroactive polymers
• Electroadhesion
• Electroviscous effects
• Ferrofluid
• Fluid mechanics
• Magnetorheological fluid
• Electrowetting
• Smart fluid

References