  Active matter systems may be characterised by the conversion of energy into
active motion, e.g. the self-propulsion of microorganisms. Artificial active
colloids form models which exhibit essential properties of more complex
biological systems but are amenable to laboratory experiments. While most
experimental models consist of spheres, such as Janus particles, active
particles of different shapes are less understood. In particular, interactions
between such active colloidal "molecules" are largely unexplored. Here, we
investigate the motion of active colloidal molecules and the interactions
between them. We focus on self-assembled dumbbells and trimers powered by an
external electric field. For dumbbells, we observe an activity-dependent
behaviour of spinning, circular and orbital motion. Moreover, collisions
between dumbbells lead to the hierarchical self-assembly of tetramers and
hexamers, both of which form rotational excited states. On the other hand,
trimers exhibit a novel type of flipping motion that leads to trajectories
reminiscent of a honeycomb lattice.
