Transitions in swimming behavior at intermediate Reynolds numbers of a reciprocal two-sphere swimmer

We used the immersed boundary method to investigate an internally-powered reciprocal swimmer composed of two unequal sized spheres that oscillate with respect to each other. We investigated its swimming behavior over a range of Reynolds numbers ranging from Stokes flow (Re=0) to intermediate Reynolds numbers (Re=150). Because the swimmer has a reciprocal stroke, it does not swim in the Stokes regime but it will swim at intermediate Reynolds numbers. We were surprised to find that the direction of swimming depends on the Reynolds number. We identified two motility regimes: one where the swimmer moves in the direction of the large sphere and one where the swimmer moves in the direction of the small sphere. We also found that the average flow around the swimmer is different in the two regimes and qualitatively similar to pushers and pullers.