Executive Summary : | Active matter systems - active particles, such as microorganisms and autophoretic particles, in a fluid - are locally driven away from equilibrium. Active particles can create flow around them, even when stationary, due to nonequilibrium processes on their surface. Building the thermodynamics of active matter is a current major challenge in nonequilibrium statistical physics. In particular, the active pressure - the mechanical or osmotic pressure needed to confine active particles - has received attention from various research groups across the globe. This pressure is due to the extra momentum of such particles (for example, from the swimming of microorganisms). The current body of work in the literature on the pressure of active matter ignores the crucial role of fluid-mediated interactions between particles. Hydrodynamic interactions between active particles and their modification by boundaries in the fluid flow are critical in determining the collective dynamics of active particles. |