  In this paper we study an archetypical scenario in which repulsive
Casimir-Polder forces between an atom or molecule and two macroscopic bodies
can be achieved. This is an extension of previous studies of the interaction
between a polarizable atom and a wedge, in which repulsion occurs if the atom
is sufficiently anisotropic and close enough to the symmetry plane of the
wedge. A similar repulsion occurs if such an atom passes a thin cylinder or a
wire. An obvious extension is to compute the interaction between such an atom
and two facing wedges, which includes as a special case the interaction of an
atom with a conducting screen possessing a slit, or between two parallel wires.
To this end we further extend the electromagnetic multiple-scattering formalism
for three-body interactions. To test this machinery we reinvestigate the
interaction of a polarizable atom between two parallel conducting plates. In
that case, three-body effects are shown to be small, and are dominated by
three- and four-scattering terms. The atom-wedge calculation is illustrated by
an analogous scalar situation, described in the Appendix. The wedge-wedge-atom
geometry is difficult to analyze because this is a scale-free problem. But it
is not so hard to investigate the three-body corrections to the interaction
between an anisotropic atom or nanoparticle and a pair of parallel conducting
cylinders, and show that the three-body effects are very small and do not
affect the Casimir-Polder repulsion at large distances between the cylinders.
Finally, we consider whether such highly anisotropic atoms needed for repulsion
are practically realizable. Since this appears rather difficult to accomplish,
it may be more feasible to observe such effects with highly anisotropic nano
particles.
