We observed the collective motion of colloidal particles moving along a circular path in water as a model system of
artificial active matter. The particles were driven by optical vortex using holographic optical tweezer. They exhibit
rhythmic motion with spontaneous formation of clusters and their dissociation by hydrodynamic interaction. The
hydrodynamic interaction in spatially confined system alter their rhythmic motion dramatically. For example, we found
that the relative magnitude of the angular velocity for a doublet to a singlet reversed in free space and in strongly
confined system. The transition of rhythmic motions was observed by varying spatial confinement.