Twisting, buckling, and tension in elastic helices with multiple perversions

A telephone cord and the coils of cucumber tendrils; these are two examples of elastic beam-like systems that take on the shape of soft, helical springs. By unwinding such a spring and holding its ends fixed, an elastic instability forms, causing the spring to form both chiralities of the helix, meeting at defects called perversions. We investigate this phenomenon in an idealized experiment using highly uniform, microscopic, elastic fibers with cylindrical cross-sections. We measure the force of extension as a function of geometry, material properties, and twist. Previous work has shown that multiple perversions may form as a result of a spring's prismatic cross-section. Here we observe that, surprisingly, multiple perversions still form, despite the uniform cylindrical cross-section of our fibers.