Researchers from China and the UK have explained why dragline silk doesn't twist and spiral out of control when spiders use it to lower themselves from a height and are able to maintain control. Based on the study of two species of golden orb weaver spiders, they showed how spider silk has the ability to resist twisting forces by slightly yielding when twisted, helping to quickly dissipate the energy that would otherwise send the spider spinning uncontrollably on the end of its dragline.
The work on the “torsion dynamical response” of spider silk, which has appeared Applied Physics Letters [Liu et al. Appl. Phys. Lett. (2017) DOI: 10.1063/1.4990676], describes how dragline silk – which is used to create the outside edges and spokes of webs, and acts as a safety line when spiders drop to the ground – does not behave in the same way as more conventional materials such as human hair, metal wires or synthetic fibers as it hardly twists at all due to its torsional properties. A better understanding of how dragline silk could help in the development of biomimetic fibers that mimic such properties, with potential uses in improved synthetic ropes, helicopter rescue ladders, parachute cords and even violin strings.
"This spider silk is displaying a property that we simply don't know how to recreate ourselves, and that is fascinating"David Dunstan
The team used a torsion pendulum technique to assess the properties of the silk. This involved gathering strands from captive spiders before hanging them inside a cylinder with two washers placed at the end to mimic a spider. The cylinder acted to isolate the silk from being disturbed and maintained the silk at constant humidity since water can cause the fibers to contract. A rotating turntable then twisted the silk, which was a recorded using a high-speed camera as the silk oscillated over hundreds of cycles. The silk was shown to partly deform when twisted, discharging more than 75% of its potential energy, with the oscillations quickly slowing down. After twisting, the silk also somewhat snaps back into place.
As yet, the researchers are unsure if this behavior is due to the complex physical structure of the silk, which is made up of a core of multiple fibrils inside a skin, with each fibril containing segments of amino acids in organized sheets and others in unstructured looping chains. They think that torsion could result in the sheets stretching like elastic, and warping the hydrogen bonds that connect the chains, which deform in the same way as plastic. It is the sheets that recover their original shape while the chains stay somewhat deformed.
The team will now further explore how spider silk reacts to twisting and is able to maintain its stiffness during torsion, as well the effect of humidity and the extent to which air helps dissipate the energy.