Forces on a Flexible Helix in Dilute Suspensions

To understand the effect of geometry and elasticity on the swimming dynamics of small-scale organisms, we study the behavior of deformable helices of varying pitch lengths in fluid suspensions of varying particle concentration. The helices are submerged in a tank that is rotated at a constant rotational speed. We measure thrust force and torque directly using the transducer on a TA ARES G2 rheometer to determine the normal and tangential force coefficient ratio. The tails are also placed on a swimmer with a cylindrical head that is actuated by a rotating magnetic field. The speed of the swimmers is compared for different pitch angles and fluids. We use neutrally buoyant fluid suspensions. Results for the flexible helices are compared with results for rigid helices. For rigid helices, the drag coefficient ratio—and consequently, the swimming speed—increases with particle concentration for most pitch angles. For flexible helices, the increase of drag coefficient ratio disappears. Possible explanations for these findings are discussed.