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Secrets of ribbon curling revealed

What's the best way to curl a ribbon? Dr Karl applies his thumb - and some physics - to the perfect bow.

Curled ribbon

You can curl flower ribbon, but not a satin ribbon (Source: juliardi/iStockphoto)

If you have ever bought someone a bunch of flowers, you will have almost certainly seen the florist wrap the flowers into a bouquet — and then they'll grab some scissors and with some scissors/ribbon/thumb magic, curl up the ends of the ribbon that hold the bouquet together.

People have been doing this for years, but it took a physicist to understand what was going on — and to work out the best way to curl ribbons.

In general, an object 'curls' when it gets longer on one side than the other.

Back in 2005, Dr Buddhapriya Chakrabarti from Harvard University saw a florist wrap up a bouquet of flowers for him — and wondered what was really going on down at a molecular level. Both scientists and engineers are well aware of the very sensible old adage: "Don't reinvent the wheel". So he searched both science and engineering textbooks for an explanation — and was really surprised to not find one.

The next obvious step was to assemble a team to find out what really happens when you curl a ribbon. To do this, his team built a 'motorised curling device'. The machine had to simulate the action that you do so easily without thinking.

When you curl a ribbon, you place it on the edge of a single blade of a pair of scissors, press your thumb on it, and then pull the end of the ribbon. The fleshy part of your thumb is soft and it 'flows' around the scissor blade. So the ribbon has to go around a sharp corner when you pull on it.

Their quest to unravel the secrets of ribbon curling took them two years, but Dr Chakrabarti and his team made three major findings.

First, the sharper the blade the better the curls. That intuitively makes sense.

Second, they found that the more tension that you apply to the ribbon the better — but only up to a certain point. Once you get a good curl going, you don't get any benefit from any extra tension.

The third finding was surprising. Dr Chakrabarti said: "The common intuition is that if you do it very fast you get tight curls". His team found the exact opposite.

As the ribbon is pulled between the blade and your thumb, the molecules that make up the ribbon get rearranged into a new configuration. Because they have to go around a bigger curve, the molecules on the side of the ribbon closest to your thumb are stretched more than those next to the metal blade. That all makes sense. But, if you pull the ribbon through too rapidly, the molecules don't have time to settle into their new stretched-out configuration. Instead, they elastically slip back to their original configuration, and the amount of curl is less. So pull the ribbon through fairly slowly to get a better curl.

By the way, they found out why you can curl flower ribbon, but not a satin ribbon. Satin is not a continuous sheet like a flower ribbon, but instead, is a woven material made from individual threads. When you do the scissors/thumb thingie on a satin strand, the molecules in the satin do not break down, so you don't get the fabulous curling phenomenon.

Two years spent investigating the mechanics of curling ribbon might seem excessive, but, understanding other examples of curling has some important implications.

Not the least of these is human hair, which sometimes needs to be curled or straightened. But how about plants? What's going on at the micro level when the tendrils of some plants respond to being touched by curling? And doors curl (or warp) as they age — why?

With regard to human health, when malaria parasites erupt from the red blood cell they have infected, the membrane of that red blood cell curls outwards before it splits open. Understanding this specific curling might be a help in the treatment of malaria.

Have you heard of micromachines or nanomachines? One problem is powering them. Curling has been used as part of a high-speed actuation mechanism. Think of a tiny spring — only a few millionths of a millimetre long. When you heat up, or shine a light, on these springs, they curl, and so can provide power for some tiny machine.

So ribbon curling is not just a complicated story, it's a twisted tale.

Tags: physics

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Published 02 October 2012

© 2015 Karl S. Kruszelnicki Pty Ltd