  Swimming and pumping at low Reynolds numbers are subject to the "Scallop
theorem", which states that there will be no net fluid flow for time reversible
motions. Living organisms such as bacteria and cells are subject to this
constraint, and so are existing and future artificial "nano-bots" or
microfluidic pumps. We study a very simple mechanism to induce fluid pumping,
based on the forced motion of three colloidal beads through a cycle that breaks
time-reversal symmetry. Optical tweezers are used to vary the inter-bead
distance. This model is inspired by a strut-based theoretical swimmer proposed
by Najafi and Golestanian [Phys.Rev. E, 69, 062901, 2004], but in this work the
relative softness of the optical trapping potential introduces a new control
parameter. We show that this system is able to generate flow in a controlled
fashion, characterizing the model experimentally and numerically.
