Tensional twist-folding and scrolling of sheets

The simple act of twisting a flat sheet has been used since ancient time in making surgical sutures, food packaging, redeployable fabric wearables, and has been proposed more recently as a strategy to make multifonctinal yarns. In this talk, we will discuss experiments where hyperelastic sheets are undergoing extremely large shape change as they are held under tension and twisted way above the initial wrinkling instability. Using 3D shape reconstruction based on x-ray Computed Tomography, we demonstrate that the wrinkles grow in amplitude and localize to form a star-shaped accordion folded spiral structure with self-contact at a half-turn. As the twist is increased further, a nestled helical structure forms at the waist, before a secondary instability occurs which leads to recursive folding and a scrolled yarn. We found that each of the major shape transformations causes the rate of change of applied torque to change sign, leading to a sawtooth variation with twist. We will present a tensional twist-folding model that quantitatively explains the morphologies observed and associated mechanical response and can serve as a guide for fabrication of yarns with precise control of crosssectional architecture.