Creases and wrinkles on the surface of a swollen gel

Consider a layer of a gel attached to a rigid substrate, immersed in a solvent, and swelling in the thickness direction. The flat surface of the gel remains stable if the swelling ratio is small, but becomes unstable if the swelling ratio is large. While creases have been commonly observed, wrinkles have also been observed under certain conditions. We compare the critical conditions for the onset of creases and wrinkles by using a nonlinear field theory of gels. The critical swelling ratio for the onset of creases is calculated by using a finite element method, and that for wrinkles is calculated by using an analytical method. We find that the critical swelling ratio for the onset of creases is significantly lower than that for wrinkles.