When driving a car, the clutch mechanically carries the torque produced by the engine to the chassis of the vehicle – a coupling that has long been tested and optimized in such macroscopic machines, giving us highly efficient engines. At microscopic length scales, different physics need to be considered. A model microscopic system consists of a ring of colloidal particles localized in optical tweezers and automatically translated on a circular path, transferring a rotational motion to an assembly of identical colloids confined to the interior region.
Through optical manipulation, the particle ring can be squeezed at will, altering the coupling between the driven and loaded parts of the assembly, and providing a clutch-like operation mode. Torque coupling at the nanoscale enables the measurement of nanomachine efficiency, which is small, but can be optimized through careful control of the system parameters.