Efficiency of isothermal active matter engines: Strong driving beats weak driving

Abstract: We study microscopic engines that use a single active particle as their "working medium". Part of the energy required to drive the directed motion of the particle can be recovered as work, even at constant temperature. A wide class of synthetic active particles can be captured by schematically accounting for the chemical degrees of freedom that power the directed motion without having to resolve the exact microscopic mechanism. We derive analytical results for the quasi-static thermodynamic efficiency, i.e., the fraction of available chemical energy that can be recovered as mechanical work. While this efficiency is vanishingly small for colloidal particles, it increases as the dissipation is increased beyond the linear response regime and goes through a maximum at large propulsion speeds. Our results demonstrate that driving beyond the linear response regime has non-trivial consequences for the efficiency of active engines.