grow at the cost of the other. Examples are phase separation by sp- odal decomposition or nucleation and subsequent growth of the nucleus in the nourishing phase [139]. Another example which has often been discussed as a paradigmatic problem is that of dendritic solidi?cation [29, 64, 79, 199]. The phenomenological description of these phenomena involves the de?- tion of a precisely located interfacial surface on which boundary conditions are imposed. One of those boundary conditions typically yields a normal - locity at which the interface is moving. This is the so-calledsharp interface approach, adopted both in analytical and numerical studies for a variety of contexts involving a moving boundary. The origin of such a description is - ten transparent, being obtained by symmetry arguments and common sense.

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