Reciprocal microswimmers in a viscoelastic fluid

We suggest several reciprocal swimming mechanisms that lead to locomotion only in viscoelastic fluids. In the first situation, we consider a three-sphere microswimmer with a difference in oscillation amplitudes for the two arms. In the second situation, we consider a three-sphere microswimmer in which one of the frequencies of the arm motion is twice as large as the other one. In the third situation, we consider a two-sphere microswimmer with a difference in size for the two spheres. In all these three cases, the average velocity is proportional to the imaginary part of the complex shear viscosity of a surrounding viscoelastic medium. We show that it is essential for a micromachine to break its structural symmetry in order to swim in a viscoelastic fluid by performing reciprocal body motions.