Why the reciprocal two-sphere swimmer moves in a viscoelastic environment

Abstract

In Newtonian fluids, microswimmers need to perform a non-reciprocal shape change to move forward. However, this is no longer required in biological fluids with their viscoelastic properties. In this work, we investigate an oscillating two-sphere swimmer in a weakly viscoelastic fluid and show that the swimmer moves toward the smaller sphere. We use the flow fields generated by the individual spheres. Since they contain a viscoelastic contribution quadratic in the sphere velocities, the forces needed to expand and contract the swimmer differ from each other. This causes a non-zero net displacement during one cycle. We also find that the mean flow field generated by the two-sphere swimmer is the one of a contractile force dipole.