Microrheology of Active Systems 

Microrheology has emerged as an important experimental tool for probing the mechanical properties of soft matter. There are two common techniques: active and passive microrheology. For equilibrium systems, the fluctuation-dissipation theorem ensures that the two methods yield the same information. Recently, a new class of nonequilibrium soft matter has emerged, termed active systems. They differ from conventional soft matter in that they contain microscopic components that continuously consume and dissipate energy to their surroundings, creating a state that is far from equilibrium.  Active systems arise primarily from biology, e.g., the cytoskeleton of living cells, active gels (polymer-network with molecular motors), active membranes (cell membranes with ion pumps), and self-propelled microorganisms.  To extend microrheological techniques to study active systems requires a generalization of theoretical framework for microrheology to nonequilibrium systems.  In this talk, we will discuss such a theoretical framework which can be used for interpreting microrheological data of active systems such as living cells and bacterial baths.