Micro-capsule swimmer controlled by flow oscillation

Recently, many kinds of micro-swimmer have been proposed for medical and engineering applications. The flow field around a micro-swimmer can be regarded as Stokes flow, in which the swimmer cannot migrate by reciprocal motion. We propose a novel micro-capsule model that propel itself by nonreciprocal deformation under flow oscillation conditions in Stokes flow regime. To the best of our knowledge, this is the first study on the propulsion principle using an elastic capsule, and one of only a few studies using flow oscillation. The micro-capsule consists of a thin elastic membrane containing fluid and a rigid sphere. A micro-capsule in liquid is oscillated by a vertical excitation force. The flow oscillation generates opposing forces on a capsule and a rigid sphere, because the densities of the internal fluid and sphere are different. The opposing forces induce nonreciprocal body deformation. Thus, our micro-capsule locomotes upward overcoming gravity. Using numerical simulations, we found that the micro-swimmer propel itself by effective and recovery strokes. We discuss the feasibility of the proposed micro-swimmer and show that the most efficient swimmer can migrate tens of micro-meters per second. These new findings pave the way for future artificial micro-swimmer designs.