  We investigate the effects of helical swimmer shape (i.e., helical pitch
angle and tail thickness) on swimming dynamics in a constant viscosity
viscoelastic (Boger) fluid via a combination of particle tracking velocimetry,
particle image velocimetry and 3D simulations of the FENE-P model. The 3D
printed helical swimmer is actuated in a magnetic field using a custom-built
rotating Helmholtz coil. Our results indicate that increasing the swimmer tail
thickness and pitch angle enhances the normalized swimming speed (i.e., ratio
of swimming speed in the Boger fluid to that of the Newtonian fluid).
Strikingly, unlike the Newtonian fluid, the viscoelastic flow around the
swimmer is characterized by formation of a front-back flow asymmetry that is
characterized by a strong negative wake downstream of the swimmer. Evidently,
the strength of the negative wake is inversely proportional to the normalized
swimming speed. Three-dimensional simulations of the swimmer with FENE-P model
with conditions that match those of experiments, confirm formation of a similar
front-back flow asymmetry around the swimmer. Finally, by developing an
approximate force balance in the streamwise direction, we show that the
contribution of polymer stresses in the interior region of the helix may
provide a mechanism for swimming enhancement or diminution in the viscoelastic
fluid.
