  The possibility of microscopic swimming by extraction of energy from an
external flow is discussed, focusing on the migration of a simple trimer across
a linear shear flow. The geometric properties of swimming, together with the
possible generalization to the case of a vesicle, are analyzed.The mechanism of
energy extraction from the flow appears to be the generalization to a discrete
swimmer of the tank-treading regime of a vesicle. The swimmer takes advantage
of the external flow by both extracting energy for swimming and "sailing"
through it. The migration velocity is found to scale linearly in the stroke
amplitude, and not quadratically as in a quiescent fluid. This effect turns out
to be connected with the non-applicability of the scallop theorem in the
presence of external flow fields.
