What makes ribbons curl

A physical property called yield. The point where stress will deform a material permanently. And it turns out the way a ribbon stretches once you reach that yield can tell you exactly how to get that perfect curl. In March of this year a UK-based team of researchers published the first study analyzing the physics behind ribbon curling in the journal PNAS. That's right, they did an entire study just to figure this out.

The researchers attached one end of a ribbon made of PVC, a kind of plastic, to a winding cylinder and the other end to a weight. The cylinder pulled the ribbon across a blade while the weight provided a resistance. When a ribbon runs over a blade it's actually the outside of the ribbon that's being stretched and deformed.

While the inside basically stays the same. When the force being applied to the ribbon is stronger than the yield, the outside of the ribbon gets permanently stretched out, creating that festive spiral shape. Now, the force you need to reach the yield depends on the material. So not all ribbons will curl exactly the same way. But the team found that in general three main factors affect the shape of the curls.

First using a sharper blade forms tighter curls, because it bends each part of the ribbon more. Pulling the ribbon across the blade more slowly also forms tighter curls, because it gives the material more time to stretch and deform. Finally using more weight makes tighter curls, because a heavier weight forces the ribbon to stretch more, until the weight gets too heavy and then the ribbon breaks.

Although for sharper blades it only helps to increase the weight up to a certain point. After that, the inside of the ribbon starts to stretch out too, reducing the curls. As the ribbon was pulled across the blade, its outermost side would begin to stretch and deform, while the side touching the blade would remain the same.

This uneven stretching, scientists explain, is what caused the ribbon to curl. Sharper blades, the scientists found, increased the stretch in the ribbon, causing curls to form. He notes that whatever the microscopic details are, pulling slower allows the plastic to relax into a curly state, because it cannot easily snap back into its old, flat one.

Do not try the scissors method on satin strands, he says, because it will not work. The reason: stretching does not break the material down in the same way, because satin is woven and not a continuous sheet. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options.

Discover World-Changing Science. A little Valentine's Day tip: if you want to curl ribbons using a pair of scissors, researchers say the secret is to be firm but slow.