Diffusion Within a Plane Surface

On a plane, all diffusing substances enter the film of hemolymph through the faces of the film, and very little enters through the edges. If the thickness of the sheet and the diffusion constants do not change, a steady state is reached in which the concentration of our solute molecules becomes uniform throughout the sheet, so that any difference in concentration with distance is zero. Now again, imagine two locations separated by a distance with a gradient of concentration for our substance existing between them. Our once three-dimensional volume is now sandwiched into a film. As before, the difference in concentration determines how fast transfer occurs between our two points. (Simple experimental arrangements for measuring diffusion coefficients in planar situations are in Newns, A. C. (1950). J.Tex. Inst. 41: T269.)

If we imagine our sheet of hemolymph to be a thicker sandwich of superimposed layered films, then the fall in concentration through the sheet is the sum of the falls through each layer, and the resistance to diffusion through the sheet is the sum of the resistances of its separate layers. We must assume, of course, we have no diffusion barriers between the layers.

0 0

Post a comment