  The Casimir force between macroscopic bodies is well understood, but not the
Casimir stress inside bodies. Suppose empty space or a uniform medium meets a
soft wall where the refractive index is continuous but its derivative jumps.
For this situation we predict a characteristic power law for the stress inside
the soft wall and close to its edges. Our result shows that such edges are not
tolerated in the aggregation of liquids at surfaces, regardless whether the
liquid is attracted or repelled.
