Active Matter: Liquid‐Crystal Hydrodynamics With a Difference

Coherently moving flocks of beasts, birds and bacteria are an example of polar nematic liquid‐crystalline order in the living world. The highly ordered local structures seen in the configurations of the biopolymeric filaments, energized by molecular motors, in the cytoskeleton of a living cell are another example; and chemically or mechanically agitated orientable particles such as catalytic colloidal rods or monolayers of macroscopic bits of wire are a third. There has been a great deal of progress in understanding the states, phase transitions, and fluctuations of these nonequilibrium systems, known broadly as Active Matter, and the methods used are a nice generalization of the hydrodynamic approach to liquid crystals. Among the interesting results that have emerged are some curious instabilities in bulk as well as thin‐film geometries; the peculiar kinetics of domain growth of active nematics; anomalies in the dynamics of a stiff filament in an active medium, and the twisted instabilities of chiral act...