Untangling the science of curly hair

Scientists have created a toolset to explain the phenomenon of how hair curls under its own weight. The answer could have implications beyond mere curiosity.-

They are the scientific discoveries that hide in plain sight: the mechanics behind ordinary phenomena such as the physics of bursting soap bubbles or how cats lap up milk. Now, a small team of researchers from the Massachusetts Institute of Technology and the University of Pierre and Marie Curie in Paris have unraveled the everyday mystery of how hair curls under its own weight.

In a new paper published in the journal Physical Review Letters, the researchers describe their solution to what they call “the deceivingly simple problem” of knowing what shape a dangling curved rod, such as a hair strand, will form. Given a strand with a certain natural curvature, they’ve created a toolset to predict whether it will dangle long and straight with a slight curl at the end, or as a tight 3-D helix.

Although such questions may seem trivial, they often have implications beyond mere curiosity. Understanding why curved rods form various geometrical shapes could help computer animators trying to create more realistic hairstyles for animated characters. It could also be useful in the telecommunications, medical, or oil and gas industries, in which long cables, tubes, and pipes are unspooled.

“We try to develop very simple experimental model systems that allow us to explore the behavior,” said Pedro Reis, an assistant professor of civil and environmental engineering at MIT, who calls himself a professional “question-asker.”

“Science is all about asking a question that hasn’t been asked,” Reis said.

The researchers were able to construct a diagram that would allow them to predict where a hair would fall on a curliness spectrum if they knew just a few key parameters, such as the relationship between the curvature of the hair and the length, and the stiffness of the hair.

As a scientist driven by curiosity, Reis concedes that the major irony of the study, called “Shapes of a Suspended Curly Hair,” is that he himself is bald.

And despite his close scrutiny of curved rods, he still doesn’t know exactly why the stubborn kinks show up in a garden hose. That question, he says, is still out there for scientific scrutiny.