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Microorganisms often navigate complex environments, with significant consequences for evolution and human health. Mucus, for instance, is both viscoelastic and anisotropic, which affects the self-propulsion of mammalian spermatozoa. Complex fluid phenomena can enhance or retard a microorganism's swimming speed, swimming gait, and can even change the direction of swimming, depending on the body geometry and the properties of the fluid. Investigations of swimming in model viscoelastic and liquid-crystalline fluids will be discussed, emphasizing the critical and often dominant influence of nearby boundaries. We will then unify a broad spectrum of systems, from active suspensions in Newtonian fluids to individual active particles in confined or bulk complex flows, using three dimensionless parameters: the Deborah number, which compares the timescales of particle activity and environmental relaxation; a comparison of length scales which we term the Benes number; and the active particle volume fraction. As a venture into an underexplored territory in this framework, we will describe a mean-field theory describing the dynamics of active suspensions in bulk viscoelastic and anisotropic environments, predicting arrested states, traveling waves, and more dramatic thrashing modes.

Also on zoom https://ucdavis.zoom.us/j/98969645841