Influence of surface traction on soft robot undulation

A pneumatically-driven robot traverses surfaces with different traction by adopting an undulatory mode of locomotion. The robot is composed of soft elastomer (elastic modulus ~100 kPa), an inextensible but flexible neutral plane, and embedded pneumatic channels. In contrast to conventional robots and wheeled vehicles, the robot deforms elastically to make ground contact over a relatively large area, where interfacial tractions have a unique role in controlling both the speed and direction of locomotion. Here, we demonstrate that for the same undulatory gait, the robot will either move forward or backward depending on the ground composition. Building on mathematical principles of elasticity and friction, we introduce a theoretical model that identifies the tribological properties that determine the direction of locomotion. Though overlooked in the past, this tribology-controlled phenomenon represents a central feature of undulation on smooth, soft, and slippery surfaces. These insights provide a starting point for identifying locomotion strategies that allow soft robots, like their natural invertebrate counterparts, to navigate a broad range of surfaces and terrains.