A sticky issue

From New Orleans, at a meeting of the American Physical Society

Old decals die hard. Try removing one from a wall by pulling its side, and usually only a small wedge will come off. The same tends to happen with pieces of adhesive tape. Unless the glue is weak or the tape is strong and doesn’t break, the removal takes patience.

“Why is it so frustrating?” says Benoît Roman of the National Center for Scientific Research in Paris. Now Roman and his collaborators have found the answer.

The researchers tried pulling tape off surfaces using a machine that also measured the forces involved. Every time they began pulling a strip of tape off, the strip’s edges tended to close up toward each other, forming a wedge (and making it impossible to pull the tape off in one clean sweep). For a given kind of tape and a given rate of pulling, the wedge’s angle was always the same, regardless of the width of the initial strip.

Some hypothesizing helped the team understand why the wedge forms. When tape is pulled, the researchers reasoned, a sharp bend forms right at the edge where the tape is coming off the surface. Where the material bends or folds, it stores energy, much like a compressed spring. The wider the fold, the more energy it stores. During pulling, the bend will try to minimize its stored energy by becoming narrower. As it narrows, it releases energy, and that energy tears the material.

The team noticed that faster pulling or stronger glue led to sharper bend angles. The sharper the bend, the harder it was to pull the tape off—more energy went into tearing and smaller bits of tape came off. Slower pulling essentially softened the glue. “If you pull fast, the adhesion is higher,” Roman says.

So slower is better. Still, Roman adds: “There’s no way out. Tape will always be difficult to tear [off].”