  It is shown that the addition of small amounts of microscopic rods in a
viscous fluid at low Reynolds number causes a significant increase of the flow
resistance. Numerical simulations of the dynamics of the solution reveal that
this phenomenon is associated to a transition from laminar to chaotic flow.
Polymer stresses give rise to flow instabilities which, in turn, perturb the
alignment of the rods. This coupled dynamics results in the activation of a
wide range of scales, which enhances the mixing efficiency of viscous flows.
