Coiling of elastic rods on rigid substrates

Abstract

We investigate the deployment of a thin elastic rod onto a rigid substrate and study the resulting coiling patterns. In our approach, we combine precision model experiments, scaling analyses, and computer simulations towards developing predictive understanding of the coiling process. Both cases of deposition onto static and moving substrates are considered. We construct phase diagrams for the possible coiling patterns, e.g. meandering, stretched coiling, alternating loops, and translated coiling, and characterize them as a function of the geometric and material properties of the rod, as well as the height and relative speeds of deployment. The various modes selected and their characteristic length-scales are found to arise from a complex interplay between gravitational, bending, and twisting energies of the rod, coupled to the geometric nonlinearities intrinsic to their large deformations. We give particular emphasis to the first sinusoidal mode of instability, which we find to be consistent with a Hopf bifurcation, and rationalize the meandering wavelength and amplitude. Throughout, we systematically vary natural curvature of the rod as a control parameter, which has a qualitative and quantitative effect on the pattern formation, above a critical value that we determine. Upon establishing excellent quantitative agreement between experiments and simulations with no fitting parameters, we perform a numerical survey to relate the pattern size to the relevant length-scales arising from material properties and the setup geometry, and quantify the typical strain levels in the rod. The universality conferred by the prominent role of geometry in the deformation modes of the rod suggests using the gained understanding as design guidelines, in the original applications that motivated the study. These include the coiling of carbon nanotubes and the deployment of submarine cables and pipelines onto the seabed.