Deformation and Diffusion Compared
The suggestion in view is that when volume is lost by diffusive mass transfer, the consequent shortening rate along some direction n is controlled by ∇2σnn regardless of the spatial variations in other stress components. The nature of the argument advanced is comparable with the one on which the theory of relativity is based: “At two separate points in a universe, it is not reasonable to suppose that the fundamental laws of behavior will be different at one point from the other.” If it is only in respect to some reference frame set up by an observer that point P differs from point Q, one should not expect behavior at P to differ from behavior at Q. It is convenient to use anthropomorphic phrasing: “If there is nothing intrinsic about point P to tell the material there to behave differently, the material at P will behave in the same way as the material at Q.” The theme of this chapter is that the material process for diffusive mass transfer is almost indistinguishable from the process for volume-conserving viscous change of shape at a point. In fact it will be argued that the two processes are so similar that it is not reasonable to suppose that behavior will be governed by different laws in the two modes: only an observer can distinguish one process from the other. Again anthropomorphically, “The moving material itself has no means of knowing which process it is involved in. Hence, if it is direction-dependent quantities such as σnn that control behavior in change of shape at a point, it must also be direction-dependent quantities such as σnn that control diffusive mass transfer.” In presenting the argument, it is convenient to imagine an atomic material for purposes of example, and for the sake of concreteness; but it is emphasized at the outset that the atoms are of minimal significance—the objective is a theory for a continuum. We wish to treat a continuum in which diffusion occurs, and even a continuum with only one component in which self-diffusion occurs, and most people find that this requires imagining division of the continuum into particles on some scale: but we need this division only in the most abstract sense, just enough to permit the idea that the continuum is self-diffusive.