How to unloop a sticky tape

By folding two adhesive sides of a tape together a loop is formed, which one expects to unloop by pulling on its free ends. Surprisingly, the loop does not immediately open up but shrinks in size, held together by a tenuous contact region that propagates along the tape. This adhesive contact region only ruptures once the loop is reduced to a critical size. We experimentally show that the shrinkage of the loop results from an interaction between the peeling front and the loop across the contact zone, accompanied by a highly nonlinear increase of the peeling force. Loop opening only occurs once the curvatures on both sides of the contact zone are equal. The interactions across the contact zone call for a description beyond the classical elastica theory. We propose a mechanical model that captures the experimental observations such as the scaling law for the critical loop size. Our results reveal and quantify the increased force required to remove loops in self-adherent media, which is of importance in applications ranging from blister removal to exfoliation of graphene sheets.