Response of an active gas to shearing

We explore how introducing activity influences the response of a gas to external driving forces. We consider an active gas of self-propelled spheres in the presence of a moving wall, and study the ability of the active gas to transport momentum introduced by shearing at the boundary. We identify several new and surprising behaviors. For large enough activity, increasing density decreases the ability of the gas to transport momentum, while for small activity the opposite effect is observed. In addition, a vorticity is observed on the scale of the particle diameter, which for certain parameter values results in a flow reversal near the boundary. This phenomenon is not captured in a fluid dynamical description, but we can explain it in terms of a kinetic sorting mechanism unique to active systems. Our present results are intended to form the basis for future study of active rheology at a more fundamental level, in terms of a Chapman-Enskog type theory.