Bacteria Questions

June 28, 2004 2:27 PM

I've occasionally heard it said that "For a bacteria, moving through water is like a person trying to wade through liquid concrete" (or jello, or mud, or whatever...) What is a good layman's explanation for this scaling of viscosity? Is it simply that at the scale of a bacterium, the strength of the intermolecular forces in a fluid are more comparable to the strength that a bacterium has to push itself through the fluid, or is there something deeper to it?
posted by badstone to Science & Nature (3 answers total)

Viscosity, in and of itself, doesn't change with scale. The statement in the linked article that "as things get smaller, water gets more viscous (thick)" is more or less nonsensical: pure water at 20 deg C has a viscosity of 1 centipoise, regardless of the amount or "scale" of water.

That said, fluid properties do depend strongly on scale: at small scales, the viscous properties of a fluid begin to dominate over what are known as the "inertial" properties of a fluid. Fluidic inertia, as you might guess, has to do with the tendancy of a flowing fluid to continue flowing at a constant velocity. Inertial flows tend to display a lot of turbulance and chaotic characteristics, whereas viscous, or "laminar", flows tend to be more predictable. All other things being equal, a relatively viscous fluid will flow in the viscous regime at larger scales than a less viscous fluid. In this sense, concrete or mud (jello, being a hydrogel, is a tricky case) will display viscous flow at a human scale, whereas such a flow regime will only be seen in water at an appoximately bacterial scale.

For more information, read about the Reynolds Number!
posted by mr_roboto at 3:14 PM on June 28, 2004

Response by poster: So, rather than viscosity, what property of fluids is it that characterizes the greater resistance to motion felt by smaller objects? Is there an individual characterstic (some function of density and perhaps the shape of the molecules), or is this effect entirely a result of the kind(s) of fluid flow?
posted by badstone at 3:28 PM on June 28, 2004

All I can think of is that a bacterium has much more surface area per unit mass than a human, so it has a much larger viscosity-to-inertia ratio (viscosity being a surface effect while inertia is a mass effect). This means that the inertia of a bacterium is dissipated much more quickly by viscous forces than the inertia of a human moving with the same momentum. So that might be what they mean by that. There's more viscous dissipation per unit mass for a small object.
posted by mr_roboto at 3:44 PM on June 28, 2004