  It is known that obstacles can hydrodynamically trap bacteria and synthetic
microswimmers in orbits, where the trapping time heavily depends on the swimmer
flow field and noise is needed to escape the trap. Here, we use experiments and
simulations to investigate the trapping of microrollers by obstacles.
Microrollers are rotating particles close to a bottom surface, which have a
prescribed propulsion direction imposed by an external rotating magnetic field.
The flow field that drives their motion is quite different from previously
studied swimmers. We found that the trapping time can be controlled by
modifying the obstacle size or the colloid-obstacle repulsive potential. We
detail the mechanisms of the trapping and find two remarkable features: The
microroller is confined in the wake of the obstacle, and it can only enter the
trap with Brownian motion. While noise is usually needed to escape traps in
dynamical systems, here, we show that it is the only means to reach the
hydrodynamic attractor.
