Mechanics of Liquid Crystal Elastomer Actuators for Granular Penetration

Abstract Subsurface penetration by compliant actuators is limited by the competing requirements of force generation and elastic stability. This work investigates the mechanics of granular penetration using thermally actuated liquid crystal elastomer (LCE) structures, focusing on buckling-limited force output and structural design. Reversible shape programming enabled by liquid crystal alignment and Poisson-effect-induced transverse strain produces controlled axial extension and torsional deformation under thermal actuation. Experiments and numerical simulations show that single-pillar LCE actuators are limited by global buckling. To overcome this limitation, multi-pillar architectures are introduced to increase effective bending stiffness while preserving soft actuation. Specifically, a tri-pillar configuration increases the buckling-limited force capacity by more than an order of magnitude compared to a single pillar. Finite element simulations and buckling analyses quantify the dependence of critical load on elastic modulus, pillar geometry, and pillar number, identifying the governing instability modes under laterally constrained conditions. Sequential penetration cycles are enabled through the integration of shape memory polymer supports that mechanically reset actuator geometry, achieving cumulative penetration depths of up to 24 mm. Measurements of granular resistance further demonstrate that imposed tip rotation substantially reduces external loading and delays buckling. Integration of a flexible temperature sensor into the actuator tip shows the feasibility of in situ subsurface measurement during penetration. These results provide mechanics-based design guidelines for compliant actuators operating under compressive loading in granular environments, including the ocean floor.