Tearing a floating polymer film

The fracture of bulk solids has been studied extensively to understand how materials fail. A drastically different set of behaviors is observed when tearing a thin sheet, such as paper or plastic wrap. Here, out-of-plane bending or buckling modifies the deformations and stresses throughout the sheet, which affects the path of the evolving crack. Adhering a film to a liquid surface significantly affects its bending and buckling behaviors, leading to complex wrinkling and folding behaviors. How does a floating film tear apart? Here we quasi-statically tear ultrathin polystyrene films (t~100 nm) floating on water, by pulling at their corners with steel needles. To elucidate the role of geometry in this system, we varied the size and planar shape of the films and the notch length. We observe one of two generic different outcomes: (1) for square sheets with small pre-cracks, the fracture path appears to be stochastic and sensitive to initial conditions; (2) for rectangular sheets or large pre-cracks, we observe large amplitude wrinkles that turn into a fold, which then guides the crack along a remarkably straight path. We collect our findings into a phase diagram and we explore possible geometric models to rationalize our results.