The swimming speed of a confined rotating helix in creeping flow

Recent theoretical and numerical studies have shown that the swimming speed of a rotating helix confined in a tube or between walls is higher that the unconfined case, for the same helix properties (helix geometry and rotation speed). We conduct experiments using a magnetic self-propelled force-free robot placed in between two walls or inside a cylinder. We vary the degree of confinement and measure the translation speed for different helix geometries and rotation speeds. We do find an increase of the swimming speeds, which is in good agreement with the predictions of a wall-corrected resistive-force theory. However, since the torque also increases as a result of confinement, the experiments are restricted by the available magnetic torque. Therefore, the increase in swimming speed is only observed for low confinement levels.