  Molecular dynamics simulations were employed to investigate the phase
separation process of a two-dimensional active Brownian dumbbell model. We
evaluated the time dependence of the typical size of the dense component using
the scaling properties of the structure factor, along with the averaged number
of clusters and their radii of gyration. The growth observed is faster than in
active particle (disk) models, and this effect is further enhanced under
stronger activity. Next, we focused on studying the hexatic order of the
clusters. The length associated to the orientational order grows algebraically
and faster than for active Brownian particles. Under weak active forces, most
clusters exhibit a uniform internal orientational order. However, under strong
forcing, large clusters consist of domains with different orientational orders.
We demonstrated that the latter configurations are not stable, and given
sufficient time to evolve, they eventually reach homogeneous configurations as
well. No gas bubbles are formed within the clusters, even when there are
patches of different hexatic order. Finally, attention was directed towards the
geometry and motion of the clusters themselves. By employing a tracking
algorithm, we showed that clusters smaller than the typical size at the
observation time exhibit regular shapes, while larger ones display fractal
characteristics. In between collisions or break-ups, the clusters behave as
solid bodies. Their centers of mass undergo circular motion, with radii
increasing with the cluster size. The center of mass angular velocity equals
that of the constituents with respect to their center of mass. These
observations were rationalised with a simple mechanical model.
