Dynamics of flexible filaments in granular media

Interactions between flexible structures and their surrounding media underpin many forms of animal locomotion, from the flapping of wings in air to the undulating propulsion of tails and fins in water. However, the coupling between deformable bodies and granular materials, crucial for movement in sandy environments, remains comparatively understudied. The equations governing flexible structures in viscous fluids do not apply to granular media, where resistive forces depend on motion direction rather than speed. We provide the first experimental validation of a predictive theoretical framework for deformable body dynamics in granular media. In this framework, the shape of the filament arises from a balance between the internal elastic and the anisotropic friction forces. We test the model using experiments in which a filament is actuated through prescribed angular oscillations at its head while fully immersed in a granular medium. Across a wide range of driving amplitudes and filament stiffnesses, we observe characteristic localization of deformation near the driven end, consistent with the highly dissipative, friction-dominated medium. Furthermore, significant coupling between filament motion and surface deformation is observed. Quantitative comparisons between experiments and simulations show improved agreement when surface coupling is included, demonstrating that the framework captures the essential physics governing interactions between oscillating flexible structures and granular substrates. These results provide a foundation for predicting and controlling flexible-body motion in granular environments, with implications for biological locomotion and device design in frictional media.