Polar Active Liquid Crystals : microscopics, hydrodynamics and rheology
Colonies of swimming bacteria, mixtures of cytoskeletal protein filaments and motor proteins, and vibrated granular rods are examples of active systems composed of interacting units that consume energy and collectively generate motion and mechanical stresses. Due to their elongated shape, active particles can exhibit orientational order at high concentration and have been likened to ``living liquid crystals". Their rich collective behavior includes nonequilibrium phase transitions and pattern formation on mesoscopic scales. I will describe and summarise recent theoretical results characterising the behaviour of such soft active systems.