How efficient is towing a cargo by a micro-swimmer?

Abstract

We study the properties of arbitrary micro-swimmers towing a passive load through a viscous liquid. The simple close-form expression for the dragging efficiency of a general micro-swimmer dragging a distant load is found, and the leading order approximation for finite mutual separation is derived. We show that, while swimmer can be arbitrarily efficient, dragging efficiency is always bounded from above. It is also demonstrated, that opposite to Purcell's assumption, the hydrodynamic coupling can ''help" the swimmer to drag the load. We support our conclusions by rigorous numerical calculations for the "necklace-shaped" swimmer, towing a spherical cargo positioned at a finite distance.