abstract: A model for N spherical particles immersed in a viscous fluid is derived. We suppose that only a part of them, the so called leaders, is active, so that their velocity can be prescribed, and thus used as a control. First of all we show that an unconstrained active particle can steer a passive particle from any initial point to any final point using geometric arguments. Than the limit equation valid for the number of particles tending to infinity is derived using a Boltzmann approach as a kinetic limit, preserving the percentage of controlled spheres, ending up with a PDE. Finally the optimal control problem starting from the solution for the finite dimensional one is studied.