I really enjoyed reading this, and felt excited when the author promised to explain viscosity at a particle level. But there was just a short presentation about two colliding molecules and I didn't understand the connection to viscosity. It's like a section is missing or something..?
viscosity has very interesting units - stress (force / area) divided by rate (1 / time). viscosity is measured (a field known as rheology) by, in some way, moving a thing through a fluid at increasingly fast accelerations, or equivalently, at increasingly high frequencies. that is, imagine moving your hand back and forth in a fluid - the faster you do so (the number of back and forth motions per second), the more resistance you will feel from the fluid. for newtonian fluids, the resistance you feel (measured in force / area, ie the area of your hand), is proportional to the frequency of your hand moving back and forth in the liquid, so, the graph is a line. non newtonion fluids do not have a linear relationship between shear stress and shear rate. air is also a fluid - all gasses are, and thus possess rheological properties. air, however, at stp, is essentially an ideal gas, that is, it is non-interactive, and thus, has 0 viscosity. the point here, is that viscosity is a consequence of the interactions of particles. as gases become denser, their viscosity increases. liquids, for comparison, is ~1000x as dense as air. the details of how molecular interactions lead to viscosity is actually quite complicated.
Thank you. I seem to have trouble using rate as a concept, especially dividing by it :)
But I think I get it when I add a virtual distance into what you are saying.
You are saying (force / area) / (1 / time). I add two distances that cancel out: (distance * force / area) / (distance * 1 / time) and get (energy / area) / speed, which is energy used per area and speed. I can feel that, and it seems to be what you are saying, right?
Not really a full answer for you, but one thing that this page clarified for me:
I had generally previously thought of viscosity as "how slow" a fluid is. High viscosity means high "thickness," which means it flows slowly (like molasses vs. water).
But as presented on this page, viscosity is actually a measure of "how fast" — how fast the effects on one molecule can spread out from there to neighboring molecules. Perhaps you could think of sounds waves moving through a substance — a "thick" substance like solid metal propagates those waves quickly (on a molecular level), while with a "thin" substance like air it's much slower. In the more precise language from the article: "viscosity controls the diffusion of momentum..."
So, because this diffusion happens quickly in a high-viscosity situation, little whorls of turbulence are inhibited, because the forces governing those whorls get spread out/diffused quickly.
Perhaps you missed the part of the article talking about diffusion, or did not see the connection? The link between that and viscosity was not immediately apparent to me, either.
Though I don't think I missed a part of the article, I feel more like the author did ;)
What I still don't get is what the difference between high and low viscosity looks like on a particle level. I don't understand why he introduced the collision between two molecules and then never explained that.. :)
How does viscosity work?