Giuseppe Gonnella — Università di Bari#
Diffusion, fluctuations and phase separation in a system of active particles. #
We will   review the results obtained from the study of   a 
two-dimensional system of active dumbbells, introduced as a paradigmatic example of a system of   non symmetrical brownian  particles with self-propulsion.
Each dumbbell is composed by two colloids permanently kept together, with an excluded volume interaction modeled through a Weeks-Chandler-Anderson (WCA) potential. They are  immersed in an implicit solvent modeled by the Langevin equation. The activity or self-propulsion  is represented  by a constant force acting on the principal direction of the dumbbell.

We find that activity  triggers a nonequilibrium phase separation between a gaseous phase and a phase with clusters.
We study the kinetics of the aggregates of dumbbells in the phase separated region. The clusters spontaneously break chiral symmetry and rotate; they also display a nematic ordering with spiral patterns.  We will also relate this phase separation to the transitions occurring in the   system without activity.

On the other hand, for the  phase without aggregation, we determine the translational and rotational diffusion properties.  Different regimes can be observed, depending on the combination of the random noise,  activity and density of the system.
Unusual increase with density of the  rotational diffusion coefficient  is found and explained as due to clustering.
Fluctuations have been also examined from the point of view of large deviation theory.
The rate function for the active work  on each particle has been determined showing  non-singular behaviour interpreted as a condensation   transition.