  We present a theoretical study of dielectrophoretic (DEP) crossover spectrum
of two polarizable particles under the action of a nonuniform AC electric
field. For two approaching particles, the mutual polarization interaction
yields a change in their respective dipole moments, and hence, in the DEP
crossover spectrum. The induced polarization effects are captured by the
multiple image method. Using spectral representation theory, an analytic
expression for the DEP force is derived. We find that the mutual polarization
effects can change the crossover frequency at which the DEP force changes sign.
The results are found to be in agreement with recent experimental observation
and as they go beyond the standard theory, they help to clarify the important
question of the underlying polarization mechanisms.
