  We consider a porous media equation with balanced bistable reactions,
equipped with some general nonlinear boundary condition. When the coefficient
of the reaction term is much larger than that of the diffusion term, we see
that, besides the possible free boundary, sharp interfaces appear between two
stable steady states. By using the method of matched asymptotic expansions, we
derive the motion law of each interface, which is a mean curvature flow (may
depends on normal direction of the interface). In addition, the original
boundary condition reduces to Robin ones at the points where the interface
contacts the domain boundary.
