  Surface interactions provide a class of mechanisms which can be employed for
propulsion of micro- and nanometer sized particles. We investigate the related
efficiency of externally and self-propelled swimmers. A general scaling
relation is derived showing that only swimmers whose size is comparable to, or
smaller than, the interaction range can have appreciable efficiency. An upper
bound for efficiency at maximum power is 1/2. Numerical calculations for the
case of diffusiophoresis are found to be in good agreement with analytical
expressions for the efficiency.

Soft Condensed Matter;Statistical Mechanics;Fluid Dynamics