MASSACHUSETTS — 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 MIT 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. It’s the newest finding in a field of science that unravels conundrums that can seem so plain and ordinary that the fact no one knows why they occur can be surprising.
Although such questions may seem trivial, they often have implications that ripple far 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.
It was only after Reis and colleagues had started pondering the question of how rods with different physical properties and different curvatures would look when they hung under their own weight that they realized hair was a great way to conceptualize the question.
The researchers weren’t motivated by questions about frizz. In fact, the team completely ignored many of the questions that shampoo companies care about most, such as how, at the molecular level, the proteins in hair form a strand with a particular natural curvature. Instead, they were interested in what shape hair strands of various natural curvatures formed when suspended.
The researchers used a combination of experiments and simulations to construct a diagram that would allow them to predict where a hair would fall. By knowing the factors that mattered, which included the relationship between the curvature of the hair and the length, and the stiffness of the hair, they could predict where it would fall on a continuum of straight hair, to two-dimensional wave, to a 3-dimensional curl.
By coupling this work with computer programs that model the interaction of many hair strands, Reis thinks that the people who create animated characters could increase the diversity of their cast. Generally, he says, straight hair has dominated because it’s just so much easier to model a long, straight mane.
He also sees applications for this in industries that wind things up and lay them down. For example, he said, if you twist a cable a lot, it can develop a “plectoneme”—a mechanical instability like the tight twisted helixes that can cause kinks in a landline phone cord.
There are cases, Reis said, where the natural curvature of a rod could be used to delay the form of such instabilities. His insights might help scientists coming up with new materials optimized for particular applications in oil pipelines or medical tubing.
As a scientist driven by curiosity, Reis admits 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.