Dabiao Liu | Searcharxiv

arXiv2026

Mechanics of hierarchical twisted and coiled polymer artificial muscles: Decoupling force from kinematic limits

Thermally actuated twisted and coiled polymer (TCP) artificial muscles exhibit exceptional specific work capacities but are limited by an inherent competition between load-bearing capacity and actuation stroke. To address this limitation, we investigate a hierarchical helical structure designed to decouple force generation from kinematic limits. We propose a coupled thermo-mechanical model incorporating inter-filamentary contact mechanics and geometric nonlinearities to predict the assembly's equilibrium response. The results indicate that this hierarchical topology significantly amplifies isometric actuation stress compared to monofilament baselines, while maintaining a biological-like contraction stroke of approximately 22%. A critical topological threshold governed by the balance between cooperative load-sharing and geometric confinement is identified. Beyond an optimal bundle complexity, the geometric jamming dominates, as excessive inter-filamentary friction hinders actuation. Furthermore, we elucidate a stiffness-stroke synergy in homochiral configurations, where high helical angles amplify the thermal untwisting torque to overcome increased structural rigidity. Crucially, the volumetric energy density exhibits scale invariance regarding the hierarchical radius, implying that absolute force output can be linearly scaled through geometric upsizing without compromising efficiency. These findings provide a mechanics-based rationale for the structural programming, demonstrating that soft actuator performance limits are dictated by topological order rather than intrinsic material properties.

cond-mat.soft↗

arXiv2025

Morphoelastic ribbons: Differential growth-induced curvature and torsion

Natural slender structures, such as plant leaves, petals, and tendrils, often exhibit complex three-dimensional (3D) morphologies-including twisting, helical coiling, and saddle-bending-driven by differential growth. The resulting internal stresses are partially relieved through the development of intrinsic curvature and torsion. The fundamental challenge lies in effectively correlating microscopic growth fields to the macroscopic shapes and mechanical responses of the ribbon structures. However, existing ribbon or shell models struggle to directly link growth gradients to macroscopic curvature and torsion, necessitating a reduced-dimensional framework. This work establishes a unified one-dimensional (1D) morphoelastic ribbon model derived rigorously from 3D finite elasticity theory via a two-step asymptotic dimension reduction. The reduced-order model captures key geometric nonlinearities and finite rotations while retaining explicit dependence on the growth tensor. We obtain analytical solutions for saddle-bending and twisting configurations induced by specific growth gradients. Furthermore, numerical continuation, based on the reduced model, reveals post-buckling transitions into helical morphologies, identifying bifurcation thresholds and constructing phase diagrams. This framework explicitly links growth fields to ribbon curvature and torsion, providing fundamental mechanics insights into the morphogenesis of slender plant organs and offering the potential for bioinspired soft robotics design.

cond-mat.soft↗