Modeling, Simulation and Optimization of Electrorheological Fluids

Electrorheological fluids are concentrated suspensions of electrically polarizable particles of small size in the range of micrometers in non-conducting or semi-conducting liquids such as silicone oils. Under the influence of an outer electric field, the particles form chains along the field lines followed by a coalescence of the chains into columns in the plane orthogonal to the field due to short-ranged potentials arising from charge-density fluctuations. The formation of the chains is a process which happens in milliseconds, whereas the aggregation to columns occurs on a time scale that is larger by an order of magnitude. On a macroscopic scale, the chainlike and columnar structures have a significant impact on the rheological properties of the suspensions. In particular, the viscosity increases rapidly with increasing electric field strength in the direction perpendicular to the field. The fluid experiences a phase transition to a viscoplastic state, and the flow shows a pronounced anisotropic behavior. Under the influence of large stresses, the columns break into continuously fragmenting and aggregating volatile structures which tilt away from strict field alignment. As a result, the viscosity decreases and the fluid flow behaves less anisotropic. The electrorheological effect is reversible, i.e., the viscosity decreases for decreasing electric field strength such that for vanishing field strength the fluid behaves again like a Newtonian one. The fast response to an outer electric field and the reversibility of the effect make electrorheological fluids particularly attractive for all technical applications which require a controllable power transmission