Conventionally, a microscopic particle that performs a reciprocal stroke
cannot move through its environment. This is because at small scales, the
response of simple Newtonian fluids is purely viscous and flows are
time-reversible. We show that by contrast, fluid elasticity enables propulsion
by reciprocal forcing that is otherwise impossible. We present experiments on
rigid objects actuated reciprocally in viscous fluids, demonstrating for the
first time a purely elastic propulsion set by the object's shape and boundary
conditions. We describe two different artificial "swimmers" that experimentally
realize this principle.Comment: 5 pages, 4 figure
