Energy Storage and Release in Twisted, Buckled, and Helical Fibers

The buckling and twisting of slender, elastic fibers is a deep and well-studied field. Recently, there has been great interest in applying this knowledge to the development of miniature linear actuators (artificial muscle), but also to investigate linkages with the formation of helices in nature (e.g. tendrils) as well as DNA supercoiling. A slender rod that is twisted with respect to a fixed end will spontaneously form a hockle, or loop, to relieve the torsional stress that builds. Further twisting results in the formation of plectonemes — a helical excursion in the fiber that extends with additional twisting. Here we investigate the energy stored and subsequently released by hockles and plectonemes as they are pulled apart, in analogy with force spectroscopy studies of DNA and protein folding. Hysteresis loops in the snapping and unsnapping inform the stored energy in the twisted fiber structures.