Preface to the special topic on active matter dynamics

Active matter refers to systems composed of individual units that consume locally stored energy to generate mechanical motion.Examples of active matter include biological entities such as schools of fish, flocks of birds, bacterial colonies, and synthetic systems such as self-propelled colloidal particles and engineered nanobots.The study of active matter seeks to provide understanding for complex behaviors and emergent phenomena arising from interactions of these energy-consuming units, with implications for fields ranging from physics and biology to materials science and robotics.Here we organize a special topic on "active matter", which includes three review articles and four research articles.These contributions cover a wide range of active matter systems, including active colloids, quorum sensing particles, chiral active matter and macroscopic artificial systems.Traditional active matter research focuses primarily on linearly moving particles which have a symmetric body and self-propel along one of the symmetry axes.However, the building blocks of active systems, such as cytoskeleton filaments and molecular motors, often exhibit chiral asymmetry.The left-right symmetry breaking can give rise to chiral motility.Through particle interactions, this individual chiral motility can lead to many fascinating phenomena such as odd viscosity, odd diffusivity, dislocation dynamics in odd elasticity crystals, and hyperuniform states.In a review, Mecke et al. 1 summarized recent experimental and theoretical advances in chiral active matter system, such as the emergence of anti-symmetric odd stresses and topologically protected edge modes.In addition to the discussions on the fundamental mechanisms, the authors also provided insights into the potential of chiral active matter for various applications.Newton's third law establishes that the fundamental microscopic interactions between particles are reciprocal.However, this action-reaction symmetry can be broken when interactions are mediated through nonequilibrium environments.Such nonreciprocal interactions are prevalent in active matter systems where thermal equilibrium is broken at the individual level.In a research article, Zhou et al. 2 investigated the effect of the nonreciprocal interactions in a numerical model with a quorum sensing mechanism.In this model, individual particles change their diffusivity from high to low values, as the local concentration of their