Ari Lukkarinen and Kimmo Kaski
Electrorheological (ER) fluids are suspensions consisting of dielectric particles of size 0.1-100 and dielectric base fluid. Since the dielectric constant of suspensions particles is larger than the dielectric constant of the base fluid, external electric field polarizes particles. Polarized particles interact and form chain like or even lattice like organized structures. Simultaneously rheological properties of the suspension change. ER-suspensions have also magnetic analog consisting of ferromagnetic particles and the base liquid. As the viscosity of the electrorheological liquid can be controlled with the electric field strength, the viscosity of magnetorheological (MR) fluid is sensitive to the magnetic field.
These fluids react very rapidly to the applied field. The response time of electrorheological fluids is of the order of 1-10 ms, which in principle enables the use of these liquids in such an applications as electrically controlled clutches, valves and active damping devices. Perhaps the most striking application utilizing electrorheological fluids is an artificial muscle made of polymer suspension particles in a polymer gel.
So far most of the suggested applications of the ER/MR-fluids operate in shear mode, i.e. direction of loading is perpendicular to the electric/magnetic field. However, instead of shearing, these fluids can also be stressed parallel to electric/magnetic field. Experimental measurements and our simulations  have shown that the difference in yield stresses between sheared and compressed system is of the order of one decade. However, in the light of our most recent studies, it seems that as the parallel loading increases the yield stress, it also decreases the applicable frequency range of oscillatory mechanical loading. As a continuation to our previous studies we will concentrate on the properties of electro-/magnetorheological fluids consisting of nonhomogeneous suspension particles