Helical Swimming in a Boger Fluid: A Systematic Study on the Effect of Swimmer Shape

Locomotion in viscoelastic fluids is critical for biological functions. Recent theoretical studies by multiple research groups have suggested that depending on the shape and flexibility of the swimmer, swimming in elastic fluids can be enhanced or reduced compared to that of the Newtonian fluids. In this work, we provide a systematic experimental investigation on the effects of the helical swimmer shape (i.e., the pitch angle and tail thickness) on swimming dynamics in a Boger fluid using a combination of particle tracking, particle image velocimetry and rheology. The 3D printed helical swimmer is actuated in a magnetic field using a custom-built Helmholtz coil. Our results indicate that at low pitch angles (i.e., π/11), the swimming speed is slower than the Newtonian fluid. However, increasing the pitch angle beyond a critical threshold (>2π/9) leads to swimming speed enhancement in the Boger fluid. In addition, increasing the tail thickness gives rise to enhancement in the swimming speed, in apparent disagreement with the existing simulations. To explain these results, we further perform particle image velocimetry and analyze the flow field around the swimmers.