Periodic interactions and geometry-dependent trajectories of rotating micro-cylinders in complex confinement

From micro-organisms to artificial microswimmers, rotation is a fundamental form of locomotion. For example, flagellated bacteria such as E. coli move by rotating their semi-rigid flagella, while micro-rotors and micro-rollers are popular choices of artificial microswimmers that carry great potential in drug delivery, microsurgery, and mixing. These microswimmers operate in complex geometries that can induce nontrivial hydrodynamic effects. However, studies of interactions of microswimmers have mainly focused on ideal geometries such as unbounded space or half-space. Therefore, this study aims to extend these studies to more complex geometries. In this work, we employ a boundary-integral equation based numerical method to study the dynamics of active rotating microcylinders bounded by complex confining geometries. The active cylinders are driven by an internal torque and interact with each other via fluid drag. We show that the cylinders trace interesting periodic trajectories when the torques on each cylinder differ. Additionally, we show that having more than one active cylinder in complex geometry expands the accessible positions of the cylinders. Finally, we showcase an example in which a weak active cylinder can be delivered from one compartment to another by another active cylinder with a controllable torque.