Chapter 3: Active Nematics: Mesoscale Turbulence and Self-propelled Topological Defects

This chapter describes the properties of dense active nematics. We start by summarising the continuum theory of active nematics, introducing the active stress and showing how it results in mesoscale turbulence and motile topological defects. Relevant experimental systems are suspensions of microtubules driven by motor proteins and crowded microswimmers, and we compare experimental and numerical results in bulk, in confinement, and in the presence of friction. We then consider the extent to which the behaviour of other biological systems, such as crawling bacteria or epithelial cells, can be interpreted in terms of active nematic physics.