Spider dragline silk’s surprising twist

Among its many remarkable mechanical properties, spider silk is most often touted for tensile strength that rivals steel but at a fraction of the density. Its torsional strength—as evidenced by how spiders like the golden orb-weaver in the photo manage to hang from draglines without spinning out of control—has more recently caught researchers’ attention. To test how spider silk responds to torsional strain, Yuming He of Huazhong University of Science and Technology in Wuhan, China, and his collaborators mounted threads drawn from golden orb-weavers in a torsion pendulum. When Kevlar thread, metal wires, and other conventional fibers are given a twist and released, they undergo damped oscillations around the initial resting point. In contrast, the spider-silk threads oscillated around an angular position that’s displaced from the original resting point. That indicates that some type of plastic deformation dissipates much of the energy of the twist and reduces the subsequent oscillation amplitude. A spider-silk thread is composed of a bundle of fibrils, and each fibril contains proteins strung in a combination of amorphous chains and crystalline sheets. The researchers speculate that the amorphous chains, which are loosely linked by hydrogen bonds, can easily separate and deform. That deformation, together with friction between fibrils, can quickly dissipate applied energy. Meanwhile, the crystalline sheets act to maintain the shape of the silk. (D. Liu et al., Appl. Phys. Lett. 111, 013701, 2017, doi:10.1063/1.4990676 ; photo by C. Frank Starmer.)