Adaptation of Flipper-Mud Interactions Enables Effective Terrestrial Locomotion on Muddy Substrates

Moving on natural muddy terrains, where soil composition and water content vary significantly, is complex and challenging. To understand how mud properties and robot-mud interaction strategies affect locomotion performance on mud, we study the terrestrial locomotion of a mudskipper-inspired robot on synthetic mud with precisely-controlled ratios of sand, clay, and water. We observed a non-monotonic dependence of the robot speed on mud water content. Robot speed was the largest on mud with intermediate levels of water content (25%-26%), but decreased significantly on higher or lower water content. Measurements of mud reaction force revealed two distinct failure mechanisms. At high water content, the reduced mud shear strength led to a large slippage of robot appendages and a significantly reduced step length. At low water content, the increased mud suction force caused appendage entrapment, resulting in a large negative displacement in the robot body during the swing phase. A simple model successfully captured the observed robot performance, and informed adaptation strategies that increased robot speed by more than 200%. Our study is a beginning step to extend robot mobility beyond simple substrates towards a wider range of complex, heterogeneous terrains.