Talk:Mass diffusivity

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"diffusion coefficient" may be a misleading name[edit]

The article states that "diffusion coefficient" is another way to say "diffusivity". Diffusivity has units of (meter*meter/second). In the CFD books i looked up "diffusion coefficient" has the units of (kilogram/(meter*second)).

In other words, the "diffusion coefficient" described there is the dynamic viscosity, and the "diffusivity" is the kinematic viscosity. What i want to say with this is that "diffusion coefficient" is not another name for "diffusivity" but another constant 94.66.56.254 (talk) 18:11, 17 November 2020 (UTC)[reply]

Gas formula[edit]

The formula for D in a gas is somewhat unclear. Looks like a single molecule of helium in 100% CO2 has the same D as a single molecule of CO2 in 100% He? Seems unlikely. Therefore, what are M_1 and M_2 really? 2600:1700:8786:DA0:41EF:DE84:6BA4:5087 (talk) 21:09, 12 July 2022 (UTC)[reply]

Mobility[edit]

I believe that mobility aka the diffusive mobility is defined as

M={\frac {RT}{c}}D

where c is the concentration of a solute, and D is the diffusion coefficient. It's this, or something very close to this. But I can find no discussion on WP about this; only electron mobility which is an important special case. The concentration c shows up because of entropic forces and/or enthalpic forces when there are concentration gradients. (It plays a role analogous to the electric field for electron mobility; it provides a pressure.) One place where it almost shows up is in the diffusion equation but the treatment of diffusion there is incomplete. One place where it definitely shows up is in spinodal decomposition, which explains how concentration gradients arise naturally from entropic forces. Can someone please add text explaining mobility? 67.198.37.16 (talk) 20:21, 18 May 2024 (UTC)[reply]

A discussion & derivation of diffusive mobility can be found in an old version of Spinodal decomposition. It seems flawed and not entirely correct. I don't have the energy to get to the bottom of that. 67.198.37.16 (talk) 01:08, 19 May 2024 (UTC)[reply]