Like most microswimmers, these Synura uvella algae use cilia to swim. Cilia are tiny, hair-like appendages that flap to produce thrust. Even under a microscope, the cilia are hard to see because they are so thin and move quickly in and out of the microscope’s narrow focus. A cilia’s stroke is always asymmetric — no simple back-and-forth motions for them — because, at the algae’s scale, symmetric motion won’t move you anywhere. This is a peculiar feature of small swimmers in viscous fluids. At the human scale, we can mimic the same physics by mixing and unmixing fluids like corn syrup. (Video and image credit: L. Cesteros; via Nikon Small World in Motion)
Celebrating the physics of all that flows