Meeting Abstract
Elongate, limbless animals from the microscopic C. elegans to eels and snakes move in both fluid and terrestrial habitats using flexural waves of the body. Swimming in fluids is well-understood and dependent on the speed and size of the locomotor and the properties of the media. However, little is known about undulatory motion in materials like mud, rotting flora, and granular matter (GM) where the surroundings provide propulsion while yielding but, unlike fluids, may be permanently deformed by the interaction. We begin the search for principles by studying lateral undulation on the surface of dry homogeneous GM. The desert dwelling snake Chionactis occipitalis travels quickly (30-80 cm/s N=10 individuals, 32 trials) using a stereotyped shape. Surface drag measurements revealed that the ratio of thrust to drag forces, a critical component in undulatory motion, did not depend on speed or depth; like C. elegans the snake motion was non-inertial. We developed a surface resistive force theory (RFT) which revealed the snakes’ waveform maximized center-of-mass speed given a constraint on peak muscle power. We explored the real-world impact of changing waveform using a robophysical model, a 10-link robot snake. When the robot re-encountered previously disturbed material the effectiveness of the motion was reduced-often the robot completely failed to make forward progress-and RFT over-predicted performance. The snakes’ waveform is in the regime where motion is like that in a frictional fluid; by limiting material yield the animal avoids contending with the memory-dependent effects that led to robot failure.
