A Brownian cyclic engine operating in a viscoelastic active suspension

Abstract

We investigate a model for a Stirling-like engine consisting of a passive Brownian particle confined by a harmonic potential and interacting with a suspension of active Brownian particles that self-propel in a viscous solvent, which cyclically operates under isothermal conditions by means of temporal variations of the trap stiffness and the self-propulsion speed of the active particles. We derive an effective stochastic equation of motion of the trapped Brownian particle, which includes a friction memory kernel as well as thermal and active fluctuating forces due to its coupling with the active suspension, from which we analytically compute the efficiency of the engine in the quasi-static limit. We find that, on average, the engine is capable to produce mechanical work with an efficiency that depends on the interplay between the different time scales of the system, where the general effect of the ensuing viscoelasticity of the active suspension is to reduce the quasi-static efficiency of the Brownian engine, as compared to the case of a system with instantaneous friction. Nevertheless, there are regions in the parameters space of the system where such memory effects are negligible in the performance of the engine, thus effectively behaving as in contact with an inert viscous bath working at two different temperatures related to the propulsion speed of the active particles.