Colloquium: Inclusions, boundaries, and disorder in scalar active matter

Active systems are driven out of equilibrium by exchanging energy and momentum with their environment. This endows them with anomalous mechanical properties that are reviewed in this Colloquium. The case of dry scalar active matter is considered, which encompasses systems whose large-scale behaviors are entirely captured by their density—a scalar field. Arguably the simplest of active-matter systems, they have attracted considerable attention due to their unusual properties when put in contact with boundaries, inclusions, tracers, or disordered potentials. Indeed, studies of the mechanical pressure of active fluids and of the dynamics of passive tracers have shown that active systems impact their environment in nontrivial ways, for example, by propelling and rotating anisotropic inclusions. Conversely, the long-range density and current modulations induced by localized obstacles show how the environment can have a far-reaching impact on active fluids. This is best exemplified by the propensity of bulk and boundary disorder to destroy bulk phase separation in active matter, thereby showing active systems to be much more sensitive to their surroundings than passive ones. This Colloquium aims to provide a unifying perspective on the rich interplay between active systems and their environments.