Phase transitions in dense active suspensions

We study dense suspensions of active particles embedded in a Newtonian fluid medium using discrete element computations. The particles are modeled as soft spheres capable of generating a thrust oriented in the direction of its instantaneous velocity. The embedding fluid provides viscous drag in the Stokes regime. The dynamics of the active suspension are investigated in a square cavity. Simulations of dilute suspensions show classical clustering and collective motion. In dense suspensions, the ratio of the thrust to drag force (denoted by $\lambda )$ is found to be an important dimensionless parameter governing the system dynamics. Phase transitions in this material are investigated in this parameter space. It was observed that for low values of $\lambda $, the material arranges itself an oscillatory modes. At intermediate values of $\lambda $, the oscillatory modes transition to a single steady vortex. At higher $\lambda $, multiple vortices are observed in the computational domain. At very high $\lambda $, diffusive effects dominate and a gas-like phase is observed. All the transitions occur over small changes in $\lambda $ indicating sharp transitions between the phases. This model system shows multiple phase transitions driven by a single parameter.