Energy Recuperation of Driven Colloids in Non-Equilibrium Baths

When objects move within a classical fluid, this leads to viscous friction, which is irreversibly converted into heat. In particular, at microscopic length scales, such energy loss is detrimental to applications making use of externally and self-propelled colloidal particles such as microscopic heat engines and microrobots. Here, using a colloidal particle in a viscoelastic fluid, we experimentally demonstrate energy recuperation (ER), where up to 30\% of the energy coupled to the surrounding can be recovered in useful work. This effect is due to the time-delayed structural response of viscoelastic fluids to external forces, which prevents an immediate relaxation back to equilibrium. As a consequence this allows the temporary storage and bidirectional energy exchange between the such a non-equilibrium bath and the particle. Our results are in excellent agreement with a micro-mechanical generic model which only makes use of the time-delayed response of the surrounding to a driven particle. Therefore, we expect our results to equally apply for critical fluids or active baths.