Polarization forces and conductivity effects in electrorheological fluids

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Abstract
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.