SPONTANEOUS FORMATION OF VORTEX IN A SYSTEM OF SELF MOTORIZED PARTICLES

We study a system of disks with inelastic collisions. Energy is given to the system via a force of constant modulus, applied on each particl e in the direction of its velocity. This system shows a dynamical phas e transition from a disordered system towards a system organized into a vortex when the dissipation is increased. The state equation of the disordered system is obtained by using kinetic theory arguments. We al so show that the transition is similar to the kinetic-cluster transiti on observed in the cooling of a gas of inelastic particles.