Electrorheological Fluids

Abstract An electrorheological (ER) fluid is a colloidal system in which one kind of particulate material is dispersed in a nonconductive liquid base. When an electric field of several kilovolts per millimeter is applied, ER fluids reversibly change from a liquid state to a solid crystallized state within a millisecond. The purpose of this article is to draw a logical relationship between the dielectric properties of the materials used for making an ER fluid and the ER mechanism. To achieve this goal, the dielectric investigation that was done to characterize an ER fluid and the related ER models that were developed to describe ER phenomena are reviewed. Attention is paid to the ER models proposed on the basis of the dielectric studies; other ER models are only briefly described. This article attempts to provide a clear physical picture of the ER response mechanism through dielectric analysis of ER phenomena. It begins with a general description of the dielectric properties of heterogeneous systems. Experimental evidence of the way dielectric parameters change the ER effect are discussed, and an empirical dielectric criterion for designing high‐performance ER fluids is proposed. A theoretical treatment on the dielectric criterion is described and an ER mechanism is proposed. A yield stress equation based on the ER mechanism is theoretically derived and comparisons with currently available experimental results are also made. Final conclusions are discussed.