  We investigate the collective transport properties of microscopic magnetic
rollers that propel close to a surface due to a circularly polarized, rotating
magnetic field. The applied field exerts a torque to the particles, which
induces a net rolling motion close to a surface. The collective dynamics of the
particles result from the balance between magnetic dipolar interactions and
hydrodynamic ones. We show that, when hydrodynamics dominate, i.e. for high
particle spinning, the collective mean velocity linearly increases with the
particle density. In this regime we analyse the clustering kinetics, and find
that hydrodynamic interactions between the anisotropic, elongated particles,
induce preferential cluster growth along a direction perpendicular to the
driving one, leading to dynamic clusters that easily break and reform during
propulsion.
