Exponential change of composition and stress

Turning to stress state, it is recalled that the main ideas of Chapters 14 and 15 are embodied in the four terms in eqn. (15.2) these four terms show how the stress fluctuation s and the composition fluctuation c interact. In fact the equation identifies a steady state—a relation between s and c that 

The result, corresponding to eqn. (15.2b), is that when = 0, for a composition profile and a stress profile to steepen together while remaining 

The physical interpretation of the separate terms is as before (see, for example, Chapter 15 Summary) but the negative sign in the denominator requires immediate comment. The equation suggests that for certain magnitudes of the parameters the denominator will be zero, and that a small composition variation ce will be accompanied by an infinitely large stress variation. What physical processes lie behind this improbable suggestion  

The right-hand-side shows the rate at which the z-dimension of a material element tends to increase because of composition-driven inflow of the additive a the left-hand-side first term shows the rate at which the z-dimension tends to decrease because of exceeding —the constriction 

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