Swimming at Microscale

Tiny organisms live in a world dominated by viscosity. There’s no coasting or gliding. If a microorganism stops swimming, friction will bring it to a halt in less than the space of a hydrogen atom! To make matters worse, simply flapping an appendage forward and backward will get them nowhere. As we’ve seen before, these highly viscous laminar flows are reversible, meaning that a backward power stroke is simply undone by a mirrored forward recovery stroke. Instead, microorganisms like the paramecium swimming above are covered in tiny hairlike cilia which beat asymmetrically. They extend to their full length during the power stroke, but they stay bent during the forward recovery stroke. That asymmetry guarantees that they move more fluid backward than forward, thereby letting the paramecium make progress. (Image credit: C. Baroud, source)

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