  The theoretical investigations on electrorheological (ER) fluids are usually
concentrated on monodisperse systems. Real ER fluids must be polydisperse in
nature, i.e., the suspended particles can have various sizes and/or different
dielectric constants. An initial approach for these studies would be the
point-dipole (PD) approximation, which is known to err considerably when the
particles approach and finally touch due to multipolar interactions. In a
recent work, we proposed a dipole-induced-dipole (DID) model for computer
simulation of ER fluids, which was shown to be both more accurate than the PD
model and easy to use. The DID model was applied to simulate the athermal
aggregation of particles in ER fluids and the aggregation time was found to be
significantly reduced as compared to the PD model. In this work, we will report
results for the case when the dielectric contrasts of some particles can be
negative. In which case, the direction of the force is reversed. Moreover, the
inclusion of DID force further complicates the results because the symmetry
between positive and negative contrasts will be broken by the presence of
dipole-induced interactions.
