  We investigate the Casimir-Polder force acting on a polarizable microparticle
in the geometry of a straight cosmic string. In order to develop this analysis
we evaluate the electromagnetic field Green tensor on the imaginary frequency
axis. The expression for the Casimir-Polder force is derived in the general
case of anisotropic polarizability. In dependence of the eigenvalues for the
polarizability tensor and of the orientation of its principal axes, the
Casimir-Polder force can be either repulsive or attractive. Moreover, there are
situations where the force changes the sign with separation. We show that for
an isotropic polarizability tensor the force is always repulsive. At large
separations between the microparticle and the string, the force varies
inversely as the fifth power of the distance. In the non-retarded regime,
corresponding to separations smaller than the relevant transition wavelengths,
the force decays as the inverse fourth power of the distance. In the case of
anisotropic polarizability, the dependence of the Casimir-Polder potential on
the orientation of the polarizability tensor principal axes also leads to the
moment of force acting on the particle.
