An integral equation method is used to calculate particle-particle forces in electrorheological fluids. The method focuses on the gap region between particles where large electric-field concentrations occur. Effects due to time-dependent excitation and nonlinear (field-dependent) fluid conductivity are analyzed. It is found that the response to step-function changes in applied field closely follows a simple form that can be derived from the dipole approximation. Qualitatively different stress-vs-time curves are obtained for large dielectric mismatch (e.g., barium titanate/dodecane) relative to large conductivity mismatch (zeolite/silicone oil). In fluids where the conductivity is strongly field dependent, it is found that particle-particle forces scale linearly with applied field E(0) at large fields. Likewise, the shear yield stress scales as E(0)(3/2). (C) 1997 American Institute of Physics.
