Sorting of Chiral Microswimmers

Abstract

Microscopic swimmers, e.g., chemotactic bacteria and cells, are capable of directed motion by exerting a force on their environment. For asymmetric microswimmers, e.g., bacteria, spermatozoa and many artificial active colloidal particles, a torque is also present leading in two dimensions to circular motion and in three dimensions to helicoidal motion with a well-defined chirality. Here, we demonstrate with numerical simulations in two dimensions how the chirality of circular motion couples to chiral features present in the microswimmer environment. Levogyre and dextrogyre microswimmers as small as $50\,\mathrm{nm}$ can be separated and selectively trapped in \emph{chiral flowers} of ellipses. Patterned microchannels can be used as \emph{funnels} to rectify the microswimmer motion, as \emph{sorters} to separate microswimmers based on their linear and angular velocities, and as \emph{sieves} to trap microswimmers with specific parameters. We also demonstrate that these results can be extended to helicoidal motion in three dimensions.