Science Friction

 The story behind sticky rubber.

- Sam Lemonick

WHAT’S THE RIGHT shoe for balancing on miniscule granite nubs in Yosemite? Polished limestone in Rifle? Plastic at your local gym? Every year, Climbing’s gear testers spend hundreds of hours parsing the differences between each rock shoe to find the best kicks for each discipline. But long before our testers crammed their grubby climber toes into the newest shoes, chemists and engineers were developing the rubber compounds that make climbing shoes the one thing they need to be: sticky.

“Rubber” refers broadly to a class of tough, elastic materials so ubiquitous it is difficult to define. About 40 percent of the rubber we use is latex, made naturally by rubber trees. On an atomic level, latex and other rubbers are made up of long strands of carbon atoms decorated with hydrogen atoms. Each string is a single molecule that’s 40,000 to 50,000 carbons long.

Each chain is floppy on its own, but together they are strong along the axis, like a section of rope. They tangle and stick to one another to form flexible, sticky rubber that can bend with your foot and mold itself around sandstone grains, monzonite crystals, or the patterned plastic of gym holds. The more contact it makes with a surface, the more friction it generates, keeping your foot where it needs to stick.

Rubber has another trick, too: It’s called strain-induced crystallization. Put enough pressure on natural rubber—say shifting most of your weight onto a shallow toehold—and the rubber changes from a tangle of strands to a highly ordered crystal form. That gives rubber extra strength and durability, explains Matthew Yang, a professor at Ferris State University in Michigan, one of the last dedicated U.S. programs training rubber scientists.