(398g) Self-Propulsion of a Freely Suspended Swimmer By a Swirling Tail in a Viscoelastic Fluid

Recently there has been a great deal of interest in developing methods of self-propulsion that specifically work in complex fluids like biological mucus but otherwise fail in Newtonian fluids like water. Such synthetic swimmers propel themselves by leveraging non-Newtonian fluid behavior, such as fluid elasticity. These artificial swimmers can be designed for a number of applications; for example, they can serve as simple models for understanding how rheology impacts microorganism motility. One interesting application is to use the swimmer to infer the properties of the surrounding fluid, i.e. to use it as rheometer. We present a simple class of force- and torque-free swimmers consisting of two spheres (or, more generally, bodies of revolution) of unequal size that rotate in opposite directions. Using a combination of analytical theory and numerical simulations, we show that this class of model swimmers undergoes zero translation in a Newtonian fluid under Stokes flow but propels itself when placed in a viscoelastic fluid. Specifically, we find that the swimming speed is nearly linear in the Deborah number (De), the appropriate measure of the fluid's elasticity, and for small De the speed is also nearly linear in the concentration of polymer in the surrounding fluid. This speed shows a non-monotonic dependence on the relative size of the two spheres, with the maximum speed obtained when the ratio of the small to large sphere's radius is 0.75. Through an analysis of the surrounding flow field and the tractions exerted on the swimmer, we show that propulsion is driven by a thrust due to pressure that is created by flow being advected inward via hoop stresses toward the faster-spinning smaller sphere.