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.

Principle of operation of the nanoclutch: red spheres rotate clockwise, and an opposing torque is applied to a central axle.

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.