  Individual self-propelled colloidal particles, like active Brownian particles
(ABP) or run-and-tumble swimmers (RT), 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) actuated by AC electric fields and suspended
and cruising through silica particles organized in rafts by mutual phoretic
attraction. A typical island contains dozens of particles. The JP travels
straight in obstacle-free regions and reorients systematically upon approaching
an island. As an underlying mechanism, we tentatively propose a hydrodynamic
torque exerted by the solvent flow towards the islands on the JP local flow
field, leading to an alignment of respective solvent flow directions. This
systematic behavior is in contrast with the reorientation observed for free
active Brownian particles and run-and-tumble microswimmers.
