  Fluid-suspended microorganisms have evolved different swimming and feeding
strategies in order to cope with an environment dominated by viscous effects.
For instance ciliated organisms rely on the collective motion of flexible
appendices to move and feed. By performing a non-reciprocal motion, flexible
filaments can produce a net propulsive force, or pump fluid, in the absence of
inertia. Inspired by such fundamental concept, we propose a strategy to produce
macroscopic pumping and mixing in creeping flow. We measure experimentally the
net motion of a Newtonian viscous fluid induced by the reciprocal motion of a
flapper. When the flapper is rigid no net motion is induced. In contrast, when
the flapper is made of a flexible material, a net fluid pumping is measured. We
quantify the effectiveness of this pumping strategy and show that optimal
pumping is achieved when the length of the flapper is on the same order as the
elasto-hydrodynamic penetration length. We finally discuss the possible
applications of flexible impellers in mixing operations at low Reynolds
numbers.
