How to unloop a self-adherent sheet

TL;DR

This paper investigates the nonlinear mechanics of loop removal in self-adherent sheets, revealing a new interaction that increases peeling force and impacts applications like blister removal and graphene exfoliation.

Contribution

It uncovers a novel interaction between peeling front and loop in self-adherent sheets, extending beyond classical elastica theory and quantifying the force increase needed for loop removal.

Findings

Loop shrinks and resists opening due to a contact zone

Interaction causes nonlinear increase in peeling force

Critical loop size triggers rupture of contact zone

Abstract

The mechanics of adherent sheets is central to applications ranging from patching a band aid, coating technology, to the breakthrough discovery of peeling graphene flakes using sticky tape. These processes are often hindered by the formation of blisters and loops, which are notoriously difficult to remove. Here we describe and explain a remarkable phenomenon that arises when one attempts to remove a loop in a self-adherent sheet that is formed by, e.g., folding two adhesive sides of a tape together. One would expect the loop to simply unloop when pulling on its free ends. Surprisingly, however, 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 loop-shrinkage results from an interaction between the peeling front and the loop, across the contact zone. This new type of interaction falls outside the realm of the classical elastica theory and is responsible for a highly nonlinear increase in the peeling force. Our results reveal and quantify the increased force required to remove loops in self-adherent media, which is of importance for blister removal and exfoliation of graphene sheets.