Island hopping of active colloids

Abstract

Individual self-propelled colloidal particles, like active Brownian particles (ABP) or run-and-tumble (RT) swimmers, exhibit characteristic and well-known motion patterns. However, their interaction with obstacles remains an open and important problem. We here investigate the two-dimensional motion of silica–gold Janus particles (JP) suspended in a bath of smaller silica passive particles. Actuated by AC electric fields, the JP cruise through passive colloids organized in ‘islands’ due to attractive electrohydrodynamic (EHD) flows. A typical island contains dozens of particles. The JP travels straight in obstacle-free regions and reorients abruptly upon collision with an island. As an underlying mechanism, we propose that the scattering events are caused by the interplay of EHD flows, self-propulsion and local torques. The combination of directed motion and sudden reorientations leads to active trajectories resembling the RT behavior of biological microswimmers.