NA = JA* + CA*(NA+NB)/C for a binary mixture. NA is the flux of the fluid relative to a stationary point. JA* is Fick's law, the flux relative to the bulk motion of the fluid? (NA+NB)/C is the bulk velocity of the fluid. But I don't see why this is the case. I don't see why NB should affect NA at all. For example, for an ideal gas, the molecules of component B should have no impact on the motion of the molecules of component A. I (think I) understand the need for accounting for a moving reference frame, but it doesn't make sense to me that if we had a hypothetical situation where A is concentrated initially at a small volume, and somehow we were able to make all of the "B" molecules move in some direction, that this would affect NA. An explanation or clarification would be appreciated!