An active asymmetric surface as the simplest soft locomotor

Soft robots, owing to their inherent compliance, hold the promise of adapting to different environments for effortless locomotion; however they typically require complex actuation sequences and bulky mechanisms, often resulting in low traveling speeds and limited action radii. Here, we introduce a soft, active surface that combines symmetric vibrations with asymmetric geometric elements to achieve propulsion. We present a model that captures the fundamental elastic behaviour of the asymmetric feature, showing that a non-trivial 2D motion emerges from the interaction between the vibration, gravity, elasticity and friction. We also present an experimental validation of our predictions, combining a silicone sawtooth-patterned surface with a vibrating source and achieving traveling speeds of 45 mm/s (0.75 bodylengths/second) on smooth surfaces. This active soft surface could then be added as a propelling module to soft robots, or it can function as a self-standing locomotor, and more complex functionalities could be achieved by combining multiple units or implementing shape-morphing designs (e.g. based on origami and kirigami).