Experiments using a viscoelastic fluid to beat the Scallop Theorem

At vanishingly-small Reynolds number, the Scallop Theorem states that a time-reversible or ``reciprocal'' motion in a Newtonian fluid produces no net force. In principal, a viscoelastic fluid, combined with a reciprocal motion driven at an appropriate frequency---e.g. one that is comparable to the inverse of the intrinsic time scale of the fluid---could break symmetry in the flow and generate propulsion. Here, we present experimental evidence of net flow driven by a low Reynolds number, two-link (one-degree-of-freedom) flapper in a viscoelastic fluid. Particle image velocimetry (PIV) experiments reveal that propulsion has a strong dependence on the Deborah numbers of the flow.