  We study the appearance of a boundary condition along an interface between
two regions, one with constant diffusivity $1$ and the other with diffusivity
$\eps>0$, when $\eps\to0$. In particular, we take Fick's diffusion law in a
context of reaction-diffusion equation with bistable nonlinearity and show that
the limit of the reaction-diffusion equation satisfies the homogeneous Neumann
boundary condition along the interface. This problem is developed as an
application of heterogeneous diffusion laws to study the geometry effect of
domain.
