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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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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).

J.G.
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