QQs: Why do ribbons curl?

So you're at the store watching a clerk wrap a gift that you just bought. The clerk drags their scissors across the ribbon and it magically curls, right before your eyes. But there's a good reason why a ribbon curls when you run a blade down one side of it. 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.

So if you're looking for the tightest curl the next time you're wrapping a present, use a sharp blade, pull the ribbon over it slowly, and pull hard. But not too hard.

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