Dynamics of a small neutrally buoyant sphere in a fluid and targeting in   Hamiltonian systems
  We show that, even in the most favorable case, the motion of a small spherical tracer suspended in a fluid of the same density may differ from the corresponding motion of an ideal passive particle. We demonstrate furthermore how its dynamics may be applied to target trajectories in Hamiltonian systems. 
Armando Babiano, Julyan H. E. Cartwright, Oreste Piro, and Antonello   Provenzale