Twisting tubes as soft robotic valves

Soft materials and structures have been blooming with utmost popularity, showing great potential as soft machines for delicate design and new application. Among them, the simple structures of soft tubes are widely applied in engineering. However, a comprehensive understanding of their twisting behaviors is still lacking. In this study, the complete response of hyperelastic tubes under large-scale torsion is explored. The helical-buckling critical conditions and post-buckling characteristics of twisting tubes are revealed via simulations and experiments. Notably, it is found that a sequence of equal-length kinks along the tubing axis appear as the subsequent buckling of twist after the first helical buckling. And the factors determining the geometry of the periodic kink structure are discussed. To efficiently interpret and predict the kink responses, we employ an equivalent plate model based on finite element analysis. Additionally, the self-locking mechanism of kink for gas-blocking purpose and the pre-stretch effect on the twisting behaviours are investigated. Finally, mechanics design of twisting tubes served as soft valves and its application in soft robotics are demonstrated. This study with insights in extreme twist of elastic tubes is hoped to provide a reference to the off-the-shelf tubes widely applied in industry and inspire new flashes of soft machines.