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As defined in Wikipedia; diffusion is the net movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential) as a result of random motion of the molecules or atoms. Whereas advection is the transport of a substance by bulk motion;that is the movement of fluids down a pressure or temperature gradient.

My confusion is that: when there is a high concentration of some quantity, doesn't that mean also a high pressure of this quantity ? This thought makes me see diffusion and advection the same..

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  • $\begingroup$ I think you should change the title. $\endgroup$
    – Zoltan Zimboras
    Commented Apr 1, 2018 at 15:07
  • $\begingroup$ @ZoltanZimboras I had to put it this way because its been like 7hours since I've posted and still I've got no answer plus the number of people viewing it is soooo small. But now its increasing much fast after I changed it :D $\endgroup$
    – user65035
    Commented Apr 1, 2018 at 15:10
  • $\begingroup$ mmm now I'm getting more downvotes :/ $\endgroup$
    – user65035
    Commented Apr 1, 2018 at 15:11
  • $\begingroup$ I'm guessing that the downvotes are actually a consequence of the title. See the an excellent guidelines on titles here: meta.stackexchange.com/questions/10647/… $\endgroup$
    – Zoltan Zimboras
    Commented Apr 1, 2018 at 15:13
  • $\begingroup$ I think the title is now good, thanks for changing it. $\endgroup$
    – Zoltan Zimboras
    Commented Apr 1, 2018 at 15:17

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No, advection is not the same as diffusion. Convection is the two processes combined. Diffusion is a statistical effect that smooths out local density variations without changing the large-scale mean distribution. Advection is the part of convection due to a net force acting on all the particles of one substance, causing them to drift as if they're a point mass. (By "particles" of a "substance" I could also mean quasiparticles, such as phonons of heat, which convect in a mathematically similar way; a container with only one fluid material in it will allow heat to convect up from the base.) In short, convection occurs partly because early randomness is unstable due to how collisions induce a negative feedback (diffusion), but also partly because there is a net bias towards movement in a particular direction (advection).

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  • $\begingroup$ Thank you. But even in diffusion when a local density changes, what could be happening inside is that particles would be colliding and so exerting a pressure on each others.. so it would a pressure gradient within the local density, which means a bulk motion, and so advection. Thats why I'm seeing them the same. $\endgroup$
    – user65035
    Commented Apr 3, 2018 at 8:40
  • $\begingroup$ @ZahraaKhalife I think you're taking a much too microscopic definition of pressure. Diffusion would occur even if particles were literal point masses; but in that case one couldn't compute what "pressure" one particle exerts on another when they collide. Advection is a consequence of a net macroscopic pressure gradient; it happens at the level of the particle population. $\endgroup$
    – J.G.
    Commented Apr 3, 2018 at 8:50
  • $\begingroup$ In an example of diffusion; when ink is dropped in water , it then diffuses in water. But if the pressure exerted by water on ink, before it diffuses, was bigger than pressure exerted by the ink on water , then diffusion wouldn't have occured. So again there should be a pressure gradient in order for the diffusion to occur. And so advection can't be separated from diffusion. What do u think about it? $\endgroup$
    – user65035
    Commented Apr 3, 2018 at 10:22
  • $\begingroup$ @ZahraaKhalife I think you should read these: en.wikipedia.org/wiki/Advection#Mathematics_of_advection en.wikipedia.org/wiki/Diffusion#Diffusion_vs._bulk_flow en.wikipedia.org/wiki/Diffusion#Basic_models_of_diffusion I'm not sure whether what will make it "click" is words or maths. $\endgroup$
    – J.G.
    Commented Apr 3, 2018 at 11:04

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