Inertial effects on crystallization of active particles

The phase behavior of inertial active particles in two dimensions is investigated numerically by using structural and dynamical freezing criteria, and melting criterion with a modified Lindemann parameter. It is found that inertial active particles can crystallize at sufficiently high densities. Comparing to the overdamped active particles, the inertia hinders crystallization of inertial active particles. For a given damping coefficient, structural and dynamical freezing criteria are different, and do not coincide for any self-propelling force due to the competition of dissipation and the inertial effect. Because the increasing inertia suppresses the effect of self-propulsion force, the transition region between liquid and solid becomes wider and the position of this region shifts downward to small coupling strength with damping coefficient. There is no structural difference between two suspensions with different inertia in the liquid phase because diffusion dominates the dynamics. Whereas the suspension for large damping coefficient is more structured on the whole but there exist well-separated liquid-like bubbles in the transition region. In addition, it is very difficult to crystallize into the perfect hexagonal crystal for small damping coefficient, which needs very large coupling strength due to the inertial effect.