  The elastic interactions between objects embedded in a nematic liquid crystal
are usually caused by the average distorsion-rather than by the fluctuations-of
the nematic orientational field. We argue that for sufficiently small
particles, the nematic-mediated interaction originates purely from the
fluctuations of the nematic director. This Casimir interaction decays as
d^(-6), d being the distance between the particles, and it dominates van der
Waals interactions close to the isotropic-to-nematic transition. Considering
the nematic as a polymer solvent, we show that the onset of this Casimir
interaction at the isotropic-to-nematic transition can discontinuously induce
the collapse of a flexible polymer chain from the swollen state to the globular
state, without crossing the Theta-point.
