Non-equilibrium phase transitions in suspensions of oppositely driven inertial particles

The structural evolution of a two-specie suspension of inertial particles under opposite driving forces is investigated by molecular dynamics simulations. The effects of the driving force (F) and the total number density of the particles (ρ) on the final structure are explored. When F is increased under a high ρ, the system starts with a frozen phase, passes through an ordered phase characterized by two demixed lanes moving in opposite directions, and finally returns to a disorder phase. When ρ is increased under a low F, a novel re-entrant phase transition is found: more than two lanes parallel to the driving forces are observed first, followed by a disordered phase with different kinds of particles blocking each other, and then an ordered state with all particles separating into two demixed lanes. We comment on the possible mechanisms underlying these phase transitions in terms of the compromise between the directional driven motion and random thermal fluctuation.