Swimming speed enhancement of a helix in viscoelastic fluids

We report on the viscoelastic effects on the swimming speed of a model helix. In this study, we experimentally measure the thrust and drag forces on a rotating and translating helix in viscous fluids using a novel technique. The swimming speed of the helix is then inferred from  the balance between the measured thrust and drag forces on the helix. Three helices of different pitch angles are tested in both Newtonian fluids and viscoelastic Boger fluids. Compared with the results from Newtonian fluids, we found that the swimming speed can be larger in viscoelastic fluids, depending on the Deborah number (De) and the helix geometry (pitch angle). When the Deborah number is close to 1 (De ~ 1), the swimming speed enhancement reaches its maximum. When the Deborah number is De < 1 or De > 1, the swimming speed enhancement decreases and can actually fall below the Newtonian swimming speed. In addition, the viscoelastic swimming speed enhancement also depends strongly on the helix pitch angle . Particle Image Velocimetry measurements are also performed to understand this swimming speed enhancement via the fluid pumping induced by the helix.