Polarization forces and conductivity effects in electrorheological fluids

Forces between particles aligned into chains by an applied electric field in an electrorheological (ER) fluid are calculated using finite-element techniques and, approximately, using a dipole approximation with local-field effects. Evaluation of the effective dielectric constant is emphasized and the shear modulus is derived from the shear dependence. For high-frequency (f≳0.1–1 kHz) applied electric fields, the forces and the modulus depend upon the dielectric constants of the suspending fluid and the dispersed particles. For low-frequency or dc electric fields, the conductivities of the components are dominant. These effects are treated within a Maxwell–Wagner approach. If the ratio of particle-to-fluid conductivities substantially exceeds the ratio of dielectric constants, a large enhancement of the modulus is found. Implications for the design of ER fluids are discussed briefly.