Flipper-based locomotion on muddy substrates

Muddy substrates exist widely in nature, from wetlands to nearshore environments. The mechanical behaviors of the mud can vary significantly due to subtle changes in grain size, organic matter, and water content. Due to the limited understanding of the mechanical responses of mud, terrestrial robots still struggle to robustly cope with this wide variation. In this study, using a turtle-inspired robot, we investigated how the performance of flipper-based locomotion depends on mud properties. To systematically vary mud properties, we created synthetic mud with precisely-controlled ratios of sand, clay, and water. Tracking data suggested that in high clay ratio mixtures, the robot's step length first increased from low to intermediate water content, then decreased as water content further increased to near saturation. At low water content, the reduced step length was primarily a result of flipper extraction failures: the cohesive mud mixture could stick to the bottom of the flipper, preventing the flipper from fully lifting off the mud, and resulting in an oscillation in the robot center-of-mass (CoM) along the fore-aft direction within a step. At high water content, the reduced step length was primarily a result of the reduced yield stress, where the mixture yielded upon flipper shear, resulting in a flipper slip.