Bacteria Swimming in finitely extensible viscoelastic fluids

Locomotion in viscoelastic fluids is critical for many biological systems. Depending on the shape and kinematics of microswimmer, swimming performance in viscoelastic fluids can be enhanced or reduced compared to that of Newtonian fluids. To date, most of the theoretical and computational studies assume infinitely stretched polymer assumptions in the formulation, while realistic polymeric fluids have finitely extensible polymers. In this work, we provide a systematic numerical and experimental investigation on the effects of polymer extensibility on swimming dynamics of the helical swimmer shape. Adaptive mesh refinement is used to model the swimmer body and helix and FENE-P model is assumed to calculate the viscoelastic stress tensor. We will discuss the relationship between the pitch angle and thickness of the helix and the viscoelastic properties of the flow for optimal swimming performance. The changes of the far-field front-back flow symmetry of the flow are correlated to the formation of a strong negative wake in the rear of the swimmer in viscoelastic fluid.  Our analysis indicates that finite extensibility of the fluid affects the near helix strain-rate tensor and modifies the contribution of the viscoelastic stress tensor in the free-force locomotion. This illustrates the importance of elasticity and the presence of a feedback loop between the near body flow and swimmer characteristics in finitely extensible polymeric flows.