Self-propulsion of a freely-suspended, rotationally-symmetric swimmer enabled by viscoelastic normal stresses, Part I: Theory and simulation

Recently there has been interest in developing novel propulsion mechanisms that leverage the rheology of the surrounding fluid. Such artificial microswimmers could be used in a variety of applications, e.g. to infer the properties of the fluid they are immersed in or as a model for studying motility in complex fluids. To this end, we've designed a simple force- and torque-free swimmer consisting of two counter-rotating bodies of revolution(e.g. unequal sized spheres). Using a combination of microhydrodynamic theory and numerical simulations, we demonstrate that such a swimmer displays zero net translation in a Newtonian fluid at zero Reynolds numbers but translates in the direction of the larger sphere when placed in a viscoelastic fluid. We find that the swimming speed is nearly linear in the Deborah number (De), the appropriate measure of the elasticity of the fluid, and is nearly linear in the concentration of polymer in the fluid at low De. By considering a variety of relative sizes and shapes for the two halves of the swimmer, we can determine the specific geometry that maximizes the swimmer's speed. Part II of this talk will focus on recent experimental work we've conducted to realize this model swimmer in the lab, with comparison to theoretical and numerical predictions.