Hydrodynamics of soft active matter

Active matter consists of self-driven particles (fish, birds, and many biological systems such as bacteria or cytoskeletal extracts) and exhibits intriguing non-equilibrium behavior such as flocking, swarming, bizarre fluctuations, and pattern formation. This article reviews continuum models for such systems, presenting the hydrodynamics of soft active matter. This review summarizes theoretical progress in the field of active matter, placing it in the context of recent experiments. This approach offers a unified framework for the mechanical and statistical properties of living matter: bio-filaments and molecular motors in vitro or in vivo, collections of motile microorganisms, animal flocks, and chemical or mechanical imitations. A major goal of this review is to integrate several approaches proposed in the literature, from semi-microscopic to phenomenological. Last, the outlook for a quantitative understanding of active matter, through the interplay of detailed theory with controlled experiments on simplified systems, with living or artificial constituents, is summarized.