Researchers develop first detailed model for a 3-D strand of curly hair

February 13, 2014

By Denise Brehm
Department of Civil & Environmental Engineering

The heroes and villains in animated films tend to be on opposite ends of the moral spectrum. But they’re often similar in their hair, which is usually extremely rigid or — if it moves at all — is straight and swings to and fro. It’s rare to see an animated character with bouncy, curly hair, since computer animators don’t have a simple mathematical means for describing it.

Experimental configurations for two naturally curved rods with moderate and high natural curvatures, respectively: 2-D planar configuration (left) and 3-D non-planar (helical) configuration (right). Video / James Miller and Pedro M. Reis

However, change may be coming soon to a theater near you: In a paper appearing in the Feb. 13 issue of Physical Review Letters, researchers at MIT and the Université Pierre et Marie Curie in Paris provide the first detailed model for the 3-D shape of a strand of curly hair.

This work could have applications in the computer animation film industry, but it also could be used by engineers to predict the curve that long steel pipes, tubing, and cable develop after being coiled around a spool for transport. In the field, these materials often act like a stubborn garden hose whose intrinsic curves make it behave in unpredictable ways. In engineering terminology, these items — and hair — are all examples of a slender, flexible rod.

Co-authors on the paper are Pedro Reis, an assistant professor in MIT’s Department of Civil and Environmental Engineering and Department of Mechanical Engineering; Basile Audoly and Arnaud Lazarus, of the Université Pierre et Marie Curie; and former MIT graduate student James Miller, who is now a research associate at Schlumberger-Doll Research. Miller worked on this project as part of his doctoral thesis research and is lead author of the paper.

“Our work doesn’t deal with the collisions of all the hairs on a head, which is a very important effect for animators to control a hairstyle,” says Reis, who was recently promoted to associate professor effective July 1. “But it characterizes all the different degrees of curliness of a hair and describes mathematically how the properties of the curl change along the arc length of a hair.”

When Reis set out to investigate the natural curvature in flexible rods, he wasn’t thinking of hair. But as he studied several small flexible, curved segments of tubing suspended from a structure in his lab, he realized they weren’t so different from strands of curly hair hanging on a head. That’s when he contacted Audoly, who had previously developed a theory to explain the 2-D shape of human hair.

Comparison between experiments and simulations of a naturally curved rod as its total length is increased. Video / James Miller, Arnaud Lazarus and Pedro M. Reis

Using lab experimentation, computer simulation, and theory — “the perfect triangle of science,” Reis says — the team identified the main parameters for curly hair and simplified them into two dimensionless parameters for curvature (relating to the ratio of curvature and length) and weight (relating to the ratio of weight and stiffness). Given curvature, length, weight, and stiffness, their model will predict the shape of a hair, steel pipe, or Internet cable suspended under its own weight.

As a strand of hair curls up from the bottom, its 2-D hook grows larger until it reaches a point where it becomes unstable under its own weight and falls out of plane to become a 3-D helix. Reis and co-