Nonequilibrium clustering of self-propelled rods

Motivated by aggregation phenomena in gliding bacteria, we study collective motion in a two-dimensional model of active, self-propelled rods interacting through volume exclusion. In simulations with individual particles, we find that particle clustering is facilitated by a sufficiently large packing fraction eta or length-to-width ratio kappa. The transition to clustering in simulations is well captured by a mean-field model for the cluster size distribution, which predicts that the transition values kappa(c) of the aspect ratio for a fixed packing fraction eta are given by kappa(c)=C/eta-1 where C is a constant.