How to Capture Active Particles

In many applications, it is important to catch collections of autonomously navigating microbes and man-made microswimmers in a controlled way. Using computer simulation of a two-dimensional system of self-propelled rods we show that a static chevron-shaped wall represents an excellent trapping device for self-motile particles. Its catching efficiency can be controlled by varying the apex angle of the trap which defines the sharpness of the cusp. Upon decreasing the angle we find a sequence of three emergent states: no trapping at wide angles followed by a sharp transition towards complete trapping at medium angles and a crossover to partial trapping at small cusp angles. A generic trapping "phase diagram" maps out the conditions at which the capture of active particles at a given density is rendered optimal.