  Collective motion in nonequilibrium steady state suspensions of
self-propelled Janus motors driven by chemical reactions can arise due to
interactions coming from direct intermolecular forces, hydrodynamic flow
effects, or chemotactic effects mediated by chemical gradients. The relative
importance of these interactions depends on the reactive characteristics of the
motors, the way in which the system is maintained in a steady state, and
properties of the suspension, such as the volume fraction. From simulations of
a microscopic hard collision model for the interaction of fluid particles with
the Janus motor we show that dynamic cluster states exist and determine the
interaction mechanisms that are responsible for their formation. The relative
importance of chemotactic and hydrodynamic effects is identified by considering
a microscopic model in which chemotactic effects are turned off while the full
hydrodynamic interactions are retained. The system is maintained in a steady
state by means of a bulk reaction in which product particles are reconverted
into fuel particles. The influence of the bulk reaction rate on the collective
dynamics is also studied.
