A core feature of many living systems is their ability to move, self-propel, and be active. From bird flocks to bacteria swarms, to even cytoskeletal networks, active matter systems exhibit collective and emergent dynamics owing to their constituents' ability to convert chemical fuel into mechanical activity. Active matter systems generate their own internal stress, which drives them far from equilibrium and thus frees them from conventional thermodynamic constraints, and by so doing they can control and direct their own behavior and that of their surrounding environment. This gives rise to fascinating behaviors such as spontaneous self-assembly and pattern formation, but also makes the theoretical understanding of their complex dynamical behaviors a challenging problem in the statistical physics of soft matter.
In this thesis, I present a new principle that all active matter systems display---namely, through their self-motion they generate an intrinsic `swim pressure' that impacts their dynamic and collective behavior. I combine experimental and computational methods to demonstrate how intrinsic activity imparts new behaviors to soft materials that explain a variety of complex phenomena, including the collective motion of self-propelled particles and the complete loss of shear viscosity in fluid suspensions. These nonequilibrium phenomena are driven fundamentally by the active constituent's tendency to diffuse, undergo a random walk, and exert a mechanical force or a pressure on a confining wall. The swim pressure theory is conceptually similar to the kinetic theory of gases, where molecular collisions with the container walls exert a pressure, or to the Brownian osmotic pressure exerted by molecular or colloidal solutes in solution. In contrast to thermodynamic quantities such as the chemical potential and free energy, the mechanical pressure (or stress) is valid out of equilibrium because it comes directly from the micromechanical equations of motion. I apply this swim pressure framework in a broad context to interpret living matter as a material and understand its complex behavior using tools of hydrodynamics, kinetic theory, and nonequilibrium statistical mechanics. The present theory is applied to active systems that are driven by self-propulsion and motility, but there are other types of nonequilibrium driving work that may fit into this general theoretical framework, like driven autocatalytic reactions in electrochemical and biochemical systems.</p
