“Life at Low Reynolds Number”, 1977 ():
[Physics discussion by Edward Mills Purcell prompted by Victor Weisskopf, on the topic of fluid mechanics & viscous liquids with low Reynolds number, particularly for microorganisms.
At the small scale, things become strange and any motion is difficult: inertia is irrelevant, as it is proportional to a man swimming in a pool of molasses who can move 1 centimeter a minute.
Worse, only certain kinds of motion will result in any progress: a symmetrical or “reciprocal motion” such as a scallop opening & close will merely move in place! A scallop would need 2 hinges to go anywhere. To move requires stranger approaches, like a “flexible oar” which can bend one way and then another, or a corkscrew motion.
How do bacteria like E. coli move? With a flagellum or a cilium: a flagellum does not wiggle like everyone thought, but literally rotates on a gear (like a little machine), and so it can move in the goop that is small-scale water.
The motion is slow, and extremely inefficient, but that doesn’t matter. A bigger problem is diffusion: in a viscous medium, stirring things up around you does nothing, because the fluid won’t move much compared to normal diffusion. Your are still surrounded by your waste products & limited to what you can eat around you. Moving yourself is too slow to help either: diffusion is much faster.
So why move at all? To find greener pastures where diffusion will bring you more stuff than where you were before. And if you are going to move at all, you might as well move far enough to outrun diffusion and get a meaningful difference. This explains how and why bacteria move—it makes little sense if your intuitions are formed on the large-scale, but makes sense down there in the micro-scale of low Reynolds number fluids.]
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