Solved riddles of loosening laces

Only combination of occurrence and leg swinging loosen the knot

Why do knots in shoelaces often loosen on their own when they run - and how exactly does that happen? That's what researchers have found out now. © Jenmdlc / pixabay
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Physics of an everyday problem: Why does the loop on the shoe so often go by itself and what exactly happens? This question has now been explored by researchers in an experiment with high-speed cameras. Therefore, the knot fails in a two-step process - and only by the combination of the appearance and swinging of the leg while running. Interesting too: no matter which knot you make, the mechanism of failure is the same.

Scientists have been researching for some time how knots spontaneously form and why straight hair knots faster than curly hair. But hardly anything is known about the spontaneous failure of knots. "Although most people have seen their laces disengage on their own, little is known and less documented about the physical mechanisms behind them, " said Christopher Daily-Diamond of the University of California at Berkeley and his colleagues.

Shoelace knot in the test

The researchers decided to get to the bottom of this annoying evil. In their first observations, they found that the loops on the shoe did not loosen when the leg was just swung back and forth without appearing. Likewise, the knot remains firm when tamped several times with the foot. "This suggests that the failure of the nodule may involve an interaction between swinging and walking, " said the researchers.

To find out more, the researchers filmed the laces of a subject with a high-speed camera while it ran on a treadmill. The laces were closed either with a cross knot or with an old woman's knot. While the former is symmetrical and the ends of each lace are right next to its beginning, the old female knot is crooked and therefore considered to be less stable by nature.

The high-speed footage reveals how the knot loosens while running © UCB Dynamics Lab

First loose, then failure

The recordings revealed that node failure occurs in two phases. It begins to creep, then very suddenly lead to the release of the knot. "For many steps, we could barely see any changes to the knot until the dissolution began, " reports Daily-Diamond and his colleagues. "Then, however, the pace of failure was remarkable often in less than two steps." Ad

Further analysis revealed how the two-step process works: When the foot hits the ground, the knot tightens and relaxes the knuckle. This leads to a first loosening of the node. If the leg then swings forward and back, this will set the free ends of the lace in motion: inertia pulls on them and then quickly lead to the release of the already pre-loosened knot.

The swinging of the leg causes inertia forces to act on the ends, the impact causes the loosening of the knot base. -Daily-Diamond et al./ Royal Society

Node type and weight of the ends

These results finally clear the riddle of the loosing laces and why this does not always happen quite often. Because, as the researchers discovered, the knot type and the weight of the free ends and loops also play an important role: although old-woman and cross-knots can be solved in the same way, it takes a lot longer for the knot to grow the knot has loosened enough by running.

The heavier the ends of the laces are, the more impacting forces can be exerted on them during leg swinging and thus increase the loosening movement of the ends. The material can also promote node failure. "But even though some lace is more appropriate than others, the fundamental mechanics behind knot failure are the same for everyone, " says co-author Christine Gregg of the University of California.

Exciting also for material research

However, the new findings are not only interesting for the everyday problem of open lace, but also for other structures and materials in which knots play a role. "You begin to understand lace and then transfer it to many other things, including the DNA or microstructures that fail under dynamic loading, " explains Daily-Diamond. (Proceedings of the Royal Society A: Mathematical and Physical Sciences, 2017; doi: 10.1098 / rspa.2016.0770)

(University of California, Berkeley, 12.04.2017 - NPO)