Crack patterns on soft substrates containing a wrinkled layer inside

Abstract

We study crack patterns in thin brittle films coated on soft tri-layer substrates that contain a wrinkled layer in the middle. Such cracks on complex multi-layer substrates can be found in the epidermis layer of the skin and laminated fabrics. Due to the geometry and residual stresses formed during the wrinkling process of a wrinkled layer, cracks occur in a striped pattern because of uneven stress distribution as the soft substrates are stretched. We first fabricate the wrinkled surface by pre-stretching a thick and soft polymer plate, spin-coating a thin and rigid film on top of it, and then releasing it. Then, we coat the soft polymer on top of the wrinkled 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 a crack pattern. We experimentally investigate the crack patterns by measuring the crack density. Furthermore, we conduct finite element method (FEM) simulations to analyze uneven stress distribution on the surface of soft tri-layer substrates under uniaxial tension. We focus on the effect of a wrinkled layer on the stress field of the surface by conducting FEM simulations on three cases: without a wrinkled layer, with a wavy layer (without residual stress), and with a wrinkled layer. The residual stress in the wrinkled layer is relieved as the specimen is stretched, and as a consequence, the most dramatic stress field appears on the surface of the soft substrate containing a wrinkled layer. In addition, we vary the push coating thickness and find a critical thickness that creates the most extreme stress variation. We also find lower limit thickness under which another stress concentration phenomenon appears. In conclusion, we can concentrate cracks in certain regions by maximizing stress variation on the surface by tailoring the underneath structure. Our results can be used to develop stretchable soft sensors and design stable film coating on wrinkled surfaces.