The structural phase diagram of electrorheological (ER) fluids as a function of dimensionless parameters λ (the ratio of the dipolar force to the Brownian force) and Pe (the ratio of the shear force to the Brownian force) is constructed. In the phase diagram, there exist three single phases: a layerlike or chainlike crystalline phase (C), a shear-string phase (S), and a liquid phase (L). Between these single phases two phase coexistence zones of C+S and C+L are located. The transitions between the phases are driven by the competition between the dipolar, shear, and Brownian forces. In the different phase zones, the viscosity of the ER fluid has different dependencies on Pe and λ. The observed structures and corresponding rheological properties are discussed. © 1996 The American Physical Society.

• Received 27 October 1995

DOI:https://doi.org/10.1103/PhysRevE.53.3823