Microorganisms swimming in Giesekus viscoelastic fluids at finite Reynolds numbers

Using the immersed boundary method, we numerically investigate the locomotion of microorganisms in Giesekus fluids at finite Reynolds numbers, focusing on swimmers with asymmetric strokes and varying stiffness. The kicker stroke, based on the Caenorhabditis elegans, has larger undulations at the tail, while the burrower stroke has larger undulations at the head. Our results show that soft swimmers can swim faster in viscoelastic fluids than in Newtonian fluids at small Reynolds numbers. At finite Reynolds numbers, however, both stiff and soft swimmers experience lower speeds in viscoelastic fluids, with fluid inertia further reducing their speed and efficiency. We find that it is associated with hydrodynamic forces: the pressure force propels the swimmer, while the viscous and polymer forces resist motion. At small Reynolds numbers, increased pressure in viscoelastic fluids enhances speed, but at finite Reynolds numbers, pressure decreases with increasing Weissenberg number, causing deceleration. Additionally, it is observed that fluid shear-thinning properties have minimal impact on the swimming behavior of both kicker and burrower in this complex environment.