Crack patterns on soft substrates containing a wrinkled layer inside

We study crack patterns in thin films coated on tri-layer substrates that contain a wrinkled layer in the middle. Examples of such complex substrates can be found in the epidermis layer of the skin and laminated fabrics. Due to the geometry and residual stress formed during the wrinkling process, cracks occur in a unique way because of uneven stress distribution. We first fabricate the wrinkled surface by pre-stretching a thick soft polymer plate, spin-coating a thin rigid film on it, and then releasing it. Then, we coat another polymer as the top layer by a push-coating process which fills the valley of the wrinkled surface and becomes widespread. Finally, brittle material is coated on the tri-layer substrate and uniaxially stretched to acquire cracks. We combine the experimental results with finite element simulations and find uneven stress distribution appears on the surface inducing cracks at certain locations. Furthermore, we systematically vary the wrinkling shape and push coating thickness to understand the crack-inducing mechanism.