Hydrodynamic interaction between microswimmers and a circular surface: Circular motion and flower-like paths

How motile microorganisms or self-propelled synthetic swimmers interact with a curved surface is crucial in determining their locomotion patterns in complex geometry. We used a self-propelled micrsoswimmer model (i.e., the squirmer) and performed two-dimensional study on the hydrodynamic interaction between the microswimmers and a circular obstacle. We revealed that both pullers and pushers, i.e., the two types of squirmers, may exhibit flower-like paths as they are circling around the obstacle at nonzero Reynolds numbers. Flowers with various shapes and numbers of petals were created by a microswimmer by varying the Reynolds number, squirmer-type parameter, or relative curvature of the obstacle. Moreover, pullers showed quite different dynamical features from their counterparts in terms of their motion direction, swimming speed, and shape of flower-like paths. The possible mechanisms were revealed in detail. In particular, pullers interacting with a large obstacle may attain an enhanced speed. The findings of this study display potential usefulness in micro/nanofluidic applications associated with a collection or separation of microorganisms and artificial mircroswimmer navigation.