Locomotion of Soft Robots with Flexible Flagella in Granular Medium*

The solid friction analog to the resistive force theory (RFT) in viscous fluids can be applied in the context of granular media to describe the motion of sand lizards and snakes. Inspired by buckling in bacterial flagella in low Reynolds flow, we exploit the deformation of elastic structures for propulsion of limbed robots in granular medium. The centimeter-sized robots that are comprised of a rigid head and multiple elastomeric rods - our analog for flagella. Clamped at one end, the rods are rotated by a motor embedded in the head. This rotation generates a propulsive force that moves the entire robot forward. We combine Discrete Elastic Rods method with RFT to simulate the locomotion of this system. Side by side with simulations, we fabricate soft robots with wireless control and analyze their locomotion in a tank filled with clear beads. The combination of experimental and simulation data allows us to quantify the large deformation in elastic flagella resulting from the drag of granular medium. Finally, flagella of different geometrical shapes are explored to optimize the efficiency of the robot. This model robotic system can potentially help us understand the locomotion of living systems, e.g. bacteria.