Inclusions, Boundaries, and Disorder in Active Matter

Active systems are driven out of equilibrium by consuming energy and dissipating it into the environment. This endows them with anomalous mechanical properties, very different from equilibrium systems, that lead to rich physics when they interact with boundaries, bodies (inclusion), or disordered potentials. While on the one hand, active systems can propel, and rotate bodies placed in them, the bodies also have a non-trivial influence on active fluids. This is best exemplified by the propensity of bulk and boundary disorder to destroy phases in active matter, showing active systems to be much more sensitive to their surroundings than equilibrium ones. The colloquium aims at providing a unifying perspective on the rich interplay between active systems and their environments using the simplest class of active systems.