The Physics of a Valentine Treat: Slower Is Better

Take a closer look at those curled ribbons
Next time someone hands you a Valentine bouquet — or next time you buy flowers for that special someone — take a second look at the ribbon that’s holding the bouquet together. Unless your florist used a pre-made bow, chances are they curled up the ribbon’s ends by sliding them between a blade and their thumb. Have you ever wondered how those pretty curls came to be? Scientists have now unrolled the physics of ribbon curling. Their precise description of the laws of curling could help in the development of new manufacturing processes, including in nanotechnology.

Buddhapriya Chakrabarti started wondering about the physics of curling ribbons one day about two years ago, as he was watching a florist gift-wrap a bouquet for him. He went looking up popular science books and engineering textbooks, searching for an explanation. To his surprise, he said, there didn’t seem to be any existing research on what exactly makes a ribbon curl when you slide it between your thumb and the blade of your scissors. “Nobody really understood what’s going on.”

Soon Chakrabarti, who is a physicist at Harvard University, in Cambridge, Mass., started a research project with several colleagues, determined to find out more. “Initially, it started out as a game,” he said, recalling the curiosity his collaborator Vincenzo Vitelli arose in female passers-by while experimenting with ribbons while sitting in a Harvard hallway. “I guess it was unusual to see us tinkering with ribbons in that context — where you see people all the time writing advanced physics formulas on blackboards,” said Vitelli, now at the University of Pennsylvania, in Philadelphia.

With the help of Anna Klales, an undergraduate student at Haverford College, in Haverford, Pa., Chakrabarti’s team set up an instrument to precisely measure the physics of ribbon curling. The experiments revealed the three most important variables that determine if and how a ribbon will curl: the sharpness of the blade (the sharper, the better), the tension applied (the more, the better), and the speed of sliding.

About speed, one finding was surprising. “The common intuition is if you do it very fast you get tighter curls,” Chakrabarti said, an intuition that also seemed to be confirmed by Vitelli’s early informal tests. But the more-objective lab experiments revealed that, other variables being equal, the opposite is true: faster sliding means less curling. “When we were doing it by hand we were putting more pressure without realizing it,” Chakrabarti said, so they were observing more curling, not less.

When the ribbon slides between your thumb and the blade, Chakrabarti said, its molecules rearrange themselves into a new configuration. Layers of molecules on the thumb’s side become get stretched more compared with those on the blade’s side, due to the sharp bending around the blade. “The outer surface of the ribbon — the one in contact with your thumb — is being bent more than that of the inner surface, in contact with the blade.” This excess of elongation of one side, coupled with the fact that the layers tend to stick back together, is what causes ribbons to curl up.

This also explains why slower is better: Sliding the ribbon faster gives the ribbon’s layers less time to relax themselves into a new arrangement, and they just tend to slap back to their original state.

Curling or warping happens in nature whenever something becomes longer — or shorter — on one side than on the other. Doors may warp as they age; leaves curl up as they shrink; and Shrinky Dinks curl up in the oven. But curling of the ribbon kind, if one knows how to control it precisely, could be useful in industrial manufacturing. That’s where one needs quantitative predictions based on the mechanical properties of different materials, Chakrabarti said. “For example, we are able to say, ‘for this material, if you make the tension less than this critical value, it shouldn’t curl’ — and that’s what we see”

But his team’s finds could be especially useful in nanotechnology, he said, for example in producing springs only a few millions of a millimeter long and with very reliable characteristics.

(This was an article I wrote for Inside Science News Service, and is reproduced here courtesy of the American Institute of Physics.)

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