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Many important biological functions depend on microorganisms' ability to move in viscoelastic fluids such as mucus, wet soil, and tissues. Yet the effects of fluid elasticity on microorganism motility are still poorly understood, partly because, in experiments the swimmer strokes (or gait) depend on the properties of the fluid medium, which obfuscates the mechanisms responsible for observed behavioral changes. In this study, we use experimental data on the gaits of Chlamydomonas reinhardtti swimming in Newtonian and viscoelastic fluids as inputs to numerical simulations that decouple the swimmer gait and fluid type. In viscoelastic fluids, cells employing Newtonian gaits swim faster, but cells with viscoelastic gaits are more efficient. Cells with Newtonian gaits generate larger fluid stresses and require more power to swim in a viscoelastic fluid, suggesting that the viscoelastic gait is an adaptation to power limitations. Generally, elastic stress accumulates at distal tips of flagella, which provides both forward and backward speed enhancements from an effective ``elastic inertia''. These findings can be explained by examining the orientation of the flagellar tip relative to the direction of motion, and we demonstrate that long thin objects in viscoelastic fluids accumulate more elastic stresses at tips when moving tangent to the swimming direction, compared to the normal direction. This stress/orientation relationship is reversed from viscous fluid theory. The importance of tip effects and orientation on the development of elastic stresses and the role of elastic inertia reshapes our understanding of how flagellated swimmers and cilia interact with viscoelastic fluid environments.

This is a PhD exit seminar. There will be a reception following the talk.