You know you tied your shoelaces properly. You know you did. Yet halfway down the block, you feel your sneaker slipping away from your foot. How could a knot untie itself? Scientists using slow-motion cameras may have figured it out. They published their report in the journal Proceedings of the Royal Academy A.
There are two classic shoelace knots. There’s the “rabbit comes out of the hole, runs around the tree, and returns to his hole” loop-and-pull, and then there’s the crossed-loop bunny ears approach. (Rabbits have apparently cornered the market on shoelace knots.) Both knots are prone to unraveling under strain, although bunny ears are generally considered the more secure option.
Mechanical engineers at Berkeley wondered what was producing the two knots’ different success rates. Their interest, said co-author Christopher Daily-Diamond, goes well beyond untied cross-trainers. “If you can start to understand the shoelace,” he said in a statement, “then you can apply it to other things, like DNA or microstructures, that fail under dynamic forces. This is the first step toward understanding why certain knots are better than others, which no one has really done."
The first step was to catch the knots in the act of coming undone. The researchers trained a high-speed camera on a treadmill, then co-author and runner Christine Gregg put on her sneakers and got moving.
By slowing down the camera and analyzing the physics of Gregg’s footfalls, the research team was able to spot the conspirators tugging away at those poor, unsuspecting knots. The first was the violence of each footfall, as Gregg’s shoes smacked against the treadmill with seven times the force of gravity. This impact stretched and pushed the knot. And while it was weakening, the whipping motion of the bow’s loops and ends simultaneously pulled the strands apart. These two forces combined untied Gregg’s well-formed knots.
"You really need both the impulsive force at the base of the knot and you need the pulling forces of the free ends and the loops," Daily-Diamond said. "You can't seem to get knot failure without both."
The researchers noted that knot failure is not inevitable; often it takes one specific jolt to the laces to start the unraveling clock. “[Your] laces can be fine for a really long time,” Gregg said, “and it's not until you get one little bit of motion to cause loosening that starts this avalanche effect leading to knot failure.”