Toward a further understanding of the loop formation and elimination in twisted filament: experiments and validation

Motivated by observations of loop formation and elimination phenomena in elastic filaments subjected to torsion and axial end displacement, we develop a tension–torsion tester to study the slack–extension responses of filaments with varied initial twists. The experiments are conducted by initially twisting the filament by a specific degree and subsequently adjusting the axial end displacement. By continuously monitoring the correlation between torque and filament configuration, we can accurately determine the critical points associated with buckling, loop formation and loop elimination. The interconversion of link, twist and writhe is also tracked in the experiment. We establish a relation between torque and end displacement of filament that provides an insight into the conditions leading to instabilities. Three buckling criteria are also evaluated in the context of measurement data. Simulations on the slack and extension of the twisted filaments based on the Cosserat rod theory are performed, providing reliable predictions of the configuration evolution. Our results highlight the importance of twist and slack in torsional buckling processes, providing guidance for the control of systems with twisted slender structures.