Balance in a 2D zone

We reason in the same way as previously, except that in the exchange term there is no diffusion flux across the direction normal to the surface at x + dx. If a chemical species passes through the interface, it is no longer the same component that is on the other side the interface s traversal is actually a reaction of disappearance of the component from the inside the slice, which is a source term. Therefore, instead of [6.18], we will obtain  

In terms of volume concentration, if a = 8x denotes a length of the order of magnitude of the mesh parameter, by using [6.19] we will obtain  

By making the flux expression explicit through the Pick s first law [3.32] and by introducing the areal speeds (i.e. per unit of area), we obtain  

We can also reason in terms of surface concentration defined by  

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