  We reveal that slow-light enhanced optical forces between side-coupled
photonic-crystal nanowire waveguides can be flexibly controlled by introducing
a relative longitudinal shift. We predict that close to the photonic band-edge,
where the group velocity is reduced, the transverse force can be tuned from
repulse to attractive, and the force is suppressed for a particular shift
value. Additionally the shift leads to symmetry breaking that can facilitate
longitudinal forces acting on the waveguides, in contrast to unshifted
structures where such forces vanish.
