| Abstract: | Active systems are a class of nonequilibrium systems where energy is converted into systematic motion on a microscopic scale. They are ubiquitous in biological systems, and exhibit many interesting physical phenomena with potential use in self-assembly and microscopic engineering applications. We study a simple model of active systems and their interactions with boundaries and passive inclusions. We show that the force by active systems on a rigid boundary may satisfy an equilibrium-like equation of state, despite pressure inhomogeneities. In addition we show how active systems lead to a modulational instability of flexible boundaries and to the spontaneous motion of short filaments. Finally, we demonstrate how certain bodies in active systems may experience a negative mobility, leading to enhanced diffusion and spontaneous motility, and we show that generic long range interactions are present between two objects in an active fluid. We characterize them and discuss their consequences. |