Peeling with (almost) no torque: three-phase contact lines of highly bendable films

Peeling a thin sheet off an adhesive substrate gives rise to 3-phase contact line in whose vicinity stresses are typically large. Here we elucidate the universal nature of this “extreme mechanics” by studying two distinct geometries – a one-dimensional (1D) geometry, in which one edge of a thin polymer film floating on water is lifted up in the air, and an axial geometry, in which an air bubble is locked between the liquid bath and the floating film. We show that in both geometries the curvature is substantially enhanced at the contact line, but varies continuously across it. Furthermore, the curvature profile is characterized by two distinct “microscopic” scales – the familiar bendo-capillary length (~10-100 micrometres in our experiment), and another, “bendo-tensile” scale, which is affected by the in-plane stress tensor, normal to the contact line. The former determines a discontinuity in the gradient of the curvature at the contact line, and the latter (which may be substantially smaller than the former) determines a ``penetration length” over which the excess curvature decays. These observations underlie a generalization of the classical peeling condition for solid layers from a rigid substrate (Obreimoff 1930) to peeling from a liquid foundation.