A basic swimmer at low Reynolds number

Swimming and pumping at low Reynolds numbers are subject to the "Scallop theorem", which states that there is no net fluid flow for time-reversible motions. Microscale 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 theoretical swimmer proposed by Najafi and Golestanian (A. Najafi and R. Golestanian, Phys. Rev. E, 2004, 69, 062901), 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.