  The advection of passive tracers in an oscillating vortex chain is
investigated. It is shown that by adding a suitable perturbation to the ideal
flow, the induced chaotic advection exhibits two remarkable properties compared
with a generic perturbation : Particles remain trapped within a specific domain
bounded by two oscillating barriers (suppression of chaotic transport along the
channel), and the stochastic sea seems to cover the whole domain (enhancement
of mixing within the rolls).
