Electrorheological Fluid - ER Fluid Composition and Theory

ER Fluid Composition and Theory

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, and the electrostatic theory. The water bridge theory assumes a three phase system, the particles contain the third phase which is another liquid (e.g. water) immiscible with the main phase liquid (e.g. oil). With no applied electric field the third phase is strongly attracted to and held within the particles. This means the ER fluid is a suspension of particles, which behaves as a liquid. When an electric field is applied the third phase is driven to one side of the particles by electro osmosis and binds adjacent particles together to form chains. This chain structure means the ER fluid has become a solid. The electrostatic theory assumes just a two phase system, with dielectric particles forming chains aligned with an electric field in an analogous way to how magnetorheological fluid (MR) fluids work. An ER fluid has been constructed with the solid phase made from a conductor coated in an insulator. This ER fluid clearly cannot work by the water bridge model. However, although demonstrating that some ER fluids work by the electrostatic effect, it does not prove that all ER fluids do so. The advantage of having an ER fluid which operates on the electrostatic effect is the elimination of leakage current, i.e. potentially there is no DC current. Of course, since ER devices behave electrically as capacitors, and the main advantage of the ER effect is the speed of response, an AC current is to be expected.

The particles are electrically active. They can be ferroelectric or, as mentioned above, made from a conducting material coated with an insulator, or electro-osmotically active particles. In the case of ferroelectric or conducting material, the particles would have a high dielectric constant. There may be some confusion here as to the dielectric constant of a conductor, but "if a material with a high dielectric constant is placed in an electric field, the magnitude of that field will be measurably reduced within the volume of the dielectric" (see main page: Dielectric constant), and since the electric field is zero in an ideal conductor, then in this context the dielectric constant of a conductor is infinite.

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 grooved electrodes doubled the ER effect. 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.

The giant electrorheological (GER) fluid was discovered in 2003, and is able to sustain higher yield strengths than many other ER fluids. The GER fluid consists of Urea coated nanoparticles of Barium Titanium Oxalate suspended in silicone oil. The high yield strength is due to the high dielectric constant of the particles, the small size of the particles and the Urea coating. Another advantage of the GER is that the relationship between the electrical field strength and the yield strength is linear after the electric field reaches 1 kV/mm. The GER is a high yield strength, but low electrical field strength and low current density fluid compared to many other ER fluids. The procedure for preparation of the suspension is given in. The major concern is the use of oxalic acid for the preparation of the particles as it is a strong organic acid.