Many microorganisms and cells function in complex (non-Newtonian) fluids, which are mixtures of different materials and exhibit both viscous and elastic stresses. For example, mammalian sperm swim through cervical mucus on their journey through the female reproductive tract, and they must penetrate the viscoelastic gel outside the ovum to fertilize. In micro-scale swimming the dynamics emerge from the coupled interactions between the complex rheology of the surrounding media and the passive and active body dynamics of the swimmer. We use computational models of swimmers in viscoelastic fluids to investigate and provide mechanistic explanations for emergent swimming behaviors.  I will discuss how flexible filaments (such as flagella) can store energy from a viscoelastic fluid to gain stroke boosts from fluid elasticity. I will also describe a 3D simulation of the model organism C. Reinhardtii that we use to separate naturally coupled stroke and fluid effects and explore why strokes that are adapted to Newtonian fluid environments might not do well in viscoelastic environments.

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