Burrowing dynamics of magnetoelastic robots in shallow granular beds

We will discuss the burrowing dynamics of slender magnetoelastic robots moving with undulating strokes in a water-saturated granular medium. The robot is built with a magnetic head and an elastic tail that are powered by an oscillating magnetic field. Our research draws inspiration from biomimetic soft robot designs that imitate how biological organisms respond to stimuli and their surroundings. The body stroke changes from rotating as a rigid rod to showing anguilliform locomotion as the applied field and the frequency is increased depending on the body elasticity. We measure the effect of magnetic field strength, frequency and sediment depth on the speed and oscillations amplitude of the robot. We demonstrate that, given otherwise similar applied conditions, the swimming speed and oscillation amplitude are systematically lower in sediments than in water. We illustrate the motion of the medium using fluorescence index matching techniques and show that the robot changes the packing density of the medium in its vicinity. The oscillating body is found to result in a non-uniform and lower granular packing fractions in its vicinity as the grains are swept by the oscillating body. We find a pair of counter-rotating vortices are formed in the wake of the burrower, which rapidly close in behind the body, as the medium returns to rest. We will discuss how the size of the vortices depends on the height of the sediment, and its relation to the speed variation with the depth of the sediment bed.