When shop assistants are gift-wrapping goods, they make the decorative ribbon curl by skimming it with the blade of a pair of scissors. How does this work?
鈥� The silk or, more commonly, synthetic fibres from which ribbons are woven can withstand a certain amount of stretching or bending without being permanently deformed. This is because their long polymers can switch between being curled up and being straight, a bit like the way in which spring can bounce back after being stretched.
However, just like the spring, if the fibres are deformed too much, the polymer molecules cannot be straightened again, so the shape of the fibres is permanently changed.
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To curl a ribbon with a pair of scissors, you pull the ribbon tightly across the sharp side of the blade 鈥� or the blunt side if you are safety-conscious. If the latter, a finger is pressed against this scissor edge, creating a right angle around which the ribbon has to bend quite sharply. This bending takes the fabric beyond its elastic limit, which leaves it permanently bent.
鈥淛ust like a spring, if the fibres in the ribbon are deformed too much, their polymer molecules cannot be straightened again and are permanently changed鈥�
The net effect of bending the ribbon along its whole length like this is to make it curl. By changing the angle that the ribbon must bend around, it is possible to alter how much the ribbon corkscrews.
Russell Goodall, Research Assistant, Ecole Polytechnique F茅d茅rale de Lausanne, Switzerland