Yielding silk keeps abseiling spiders from spinning out of control
Spiders don’t spin out of control when descending because their silk has an unusual ability to resist twisting forces.
In a new paper in Applied Physics Letters, researchers from QMUL, Huazhong University of Science and Technology, Hubei Key Laboratory of Engineering Structural Analysis and Safety Assessment, Hubei University and Beihang University showed that unlike human hair, metal wires or synthetic fibres, spider silk partially yields when twisted.
This property quickly dissipates the energy that would otherwise send an excited spider spinning on the end of its silk.
Greater understanding of how spider silk resists spinning could lead to biomimetic fibres that mimic these properties for use in violin strings, helicopter rescue ladders and parachute cords. “If we understood how spider silk achieves this, then maybe we could incorporate the properties into our own synthetic ropes,” said QMUL’s Professor David Dunstan.
Intriguing dragline silk
Spiders use dragline silk as the outer rim and spokes of their webs and as a lifeline when dropping to the ground. The material has intrigued scientists because of its incredible strength, stretchiness and ability to conduct heat, but little research has focused on its torsional properties – how it responds to twisting.
“Spider silk is very different from other, more conventional materials,” said Dabiao Liu of Huazhong University of Science and Technology. “We find that the dragline from the web hardly twists, so we want to know why.”
Researchers used a torsion pendulum - the same tool used by Henry Cavendish to weigh the Earth in the 1790s - to investigate dragline silk from two species of golden silk orb weavers.
They collected strands of silk from captive spiders and suspended the strands inside a cylinder with two washers at the end to mimic a spider. The cylinder prevented drafts and kept the silk at a constant humidity, because water can cause the fibres to contract. A rotating turntable gave the silk a twist and a high-speed camera recorded the silk’s back and forth oscillations over hundreds of cycles.
Unlike synthetic fibres and metals, spider silk deforms slightly when twisted, which releases more than 75 per cent of the potential energy, and rapidly slows the oscillations. After twisting, the silk partially snaps back.
Complex physical structure
The researchers suspect that this unusual behaviour is linked to the silk’s complex physical structure, consisting of a core of multiple fibrils inside a skin. Each fibril has segments of amino acids in organized sheets and others in unstructured looping chains. The researchers propose that torsion causes the sheets to stretch like elastic, and snaps the hydrogen bonds linking the chains, which deforms like plastic. The sheets can recover to their original shape, but the chains remain partially deformed. This change reduces the size of the oscillations and shifts the equilibrium point.
The group will continue to investigate how spider silk reacts to twisting in this way and is also looking into how it maintains its stiffness during torsion, the effects of humidity and to what degree air helps dissipate the energy.
“There is a lot of further work needed,” Professor Dunstan said. “This spider silk is displaying a property that we simply don’t know how to recreate ourselves, and that is fascinating.”
Research paper: "Peculiar torsion dynamical response of spider dragline silk," by Dabiao Liu, Longteng Yu, Yuming He, Kai Peng, Jie Liu, Juan Guan and DJ Dunstan. Applied Physics Letters.
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