Stretch and twist of soft double-helix chiral rods

The mechanical response of elastic rods is highly nonlinear. For instance, when a rod is subject to combined axial tension and twist at one end, it becomes unstable, coiling into a helix with modulated amplitude. In this study, we explore the nonlinear behavior of naturally straight, elastic double-helix chiral rods. In this single-material structure, transverse bending and axial compression arise from the interaction between the rod’s core and the double-helix architecture under load. Through a combination of analytical models, Finite Element analysis, and experiments, we discover that these rods undergo complex buckling transitions when stretched and twisted. Interestingly, the critical buckling modes are strongly affected by the direction of twisting. We also investigate the evolution of the rod's morphology and the transitions between different buckling modes as a function of material properties and geometry. The observed buckling behavior in elastic double-helix chiral rods offers potential applications in flexible strain sensors, as well as polarization-selective fiber filters and lasers.