Researchers: Ari Lukkarinen and Kimmo Kaski
Electrorheological (ER) fluids are suspensions consisting of dielectric particles of size 0.1-100 m and dielectric base fluid. Since the dielectric constant of suspensions particles differs from the dielectric constant of the base fluid, external electric field polarizes particles. These polarized particles interact and form chain-like or even lattice-like organized structures. Simultaneously the rheological properties of the suspension change effectively, e.g. the effective viscosity increases dramatically. ER-suspensions have also a 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.
|Figure 32: Snapshots of the grown structure (on the left). The colour of the particle denotes its dielectric constant. On the right a system consisting of homogeneous particles is sheared in the direction that is almost normal to the surface of the paper. Particles form well-organized hexagonal shear-induced structures.|
These fluids react 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 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.
Recently we have studied structure formation and properties of mechnically stressed systems. For this study we deviced a model taking long range forces and many-particle interactions into account at the dipolar level. This model was also utilized in the study of a new kind of electrorheological fluid consisting of the base fluid and two kinds of suspension particles -- particles with larger and particles with smaller dielectric constant than that of the base fluid. It could be expected that some physical properties of such systems might differ from those observed in the systems where only one kind of particles were present. However, contrary to the static finite element studies, dynamic studies suggested that ER/MR systems get easily trapped to states of local energy minima corresponding to some kind of a polycrystalline structure. Moreover, there were some evidence that the ground state structure might be dependent on the volume fraction of the suspension particles such that only small volume fractions would favour the formation of isolated particle columns having a clear lattice symmetry.