  The realization of artificial microscopic swimmers able to propel in viscous
fluids is an emergent research field of fundamental interest and vast
technological applications. For certain functionalities, the efficiency of the
microswimmer in converting the input power provided through an external
actuation into propulsive power output can be critical. Here we use a
microswimmer composed by a self-assembled ferromagnetic rod and a paramagnetic
sphere and directly determine its swimming efficiency when it is actuated by a
swinging magnetic field. Using fast video recording and numerical simulations
we fully characterize the dynamics of the propeller and identify the two
independent degrees of freedom which allow its propulsion. We then obtain
experimentally the Lighthill's energetic efficiency of the swimmer by measuring
the power consumed during propulsion and the energy required to translate the
propeller at the same speed. Finally, we discuss how the efficiency of our
microswimmer could be increased upon suitable tuning of the different
experimental parameters.

Soft Condensed Matter