Rip to a point

Trying to peel wallpaper? Why doesn't it roll off in a straight line instead of tearing off in pointed shards?

Lifting tape from a stubborn roll? It will likely rip to a point, a frustrating triangle that leaves a sticky and unusable remnant. Removing a label? Once again, the label will verge into triangles.

Pedro Reis, an applied mathematics instructor at MIT, says, "This pattern, where two cracks propagate toward each other and meet at a point, is extremely robust. It applies not only to wallpaper but other adhesives such as tape, as well as nonadhesive plastic sheets such as the shrink-wrap that envelops compact discs. It even extends to fruit: The skin on a tomato or a grape typically forms a triangle when peeled off. This has happened to everyone. it's frustrating.”

The team found that those ubiquitous triangular tears arise from interactions between three inherent properties of adhesive materials: elasticity (stiffness), adhesive energy (how strongly the adhesive sticks to a surface) and fracture energy (how tough it is to rip).

[more at Science blog]