A simple catch: thermal fluctuations enable hydrodynamic trapping of microrollers by obstacles

In order to leverage colloidal swimmers in microfluidic applications, it is crucial to understand their interaction with surface features such as obstacles. Previous studies have shown that this interaction can result in the hydrodynamic trapping, where the trapping strength heavily depends on the flow field of the swimmer, and that (thermal) noise is needed to escape the trap. Here, we use both experiments and simulations to investigate the interaction of driven microrollers with an obstacle. Microrollers have a prescribed propulsion direction and the flow field that drives their motion is quite different from previously-studied, bacteria-like or phoretic swimmers. We observe hydrodynamic trapping in our system, and find that the strength of the trap is controlled by obstacle curvature and repulsive potential. We detail the mechanisms of the trapping and find two remarkable features: it only happens in the wake of the obstacle and, more importantly, it can only occur with Brownian motion. While noise is usually needed to escape traps in dynamical systems, here we show it is the only means to reach a hydrodynamic attractor. Our findings indicate that while hydrodynamic trapping of microswimmers by obstacles is generic, thermal fluctuations can play an unexpected role.