Stability of straight delamination blisters

We consider the buckle-driven delamination of biaxially compressed thin fil ms. Telephone-cord-like patterns observed in experiments are explained as a result of the buckling behavior of the film. We perform a stability analys is of a straight blister. A mechanism of instability causing undulations in an advancing finger of delamination is pointed out, which we claim to be t he basic phenomenon explaining the zigzag pattern. We predict a transition to a varicose (unobserved as yet) pattern at low Poisson ratios.