If you had no arms and no legs, just how would you propose to climb up a hill? Slither straight up like a snake? Ah, but what if the hill were made of sand?
Physicists have unlocked the mystery by studying the mesmerizing motion of sidewinder rattlesnakes on sandy inclines and successfully mimicking this motion in a robot snake nicknamed 'Elizabeth'.
Elizabeth was having sandy trouble. Researchers at Carnegie Mellon University were trying to get her to climb the sand dunes around an archeological dig, but any incline greater than about 10 degrees and she would slide or roll straight back down.
Her struggle is understandable. Ordinary pit vipers, like the diamondback rattlesnake, have the same trouble climbing granular hills, as the research team led by Dan Goldman of the Georgia Institute of Technology describe in the current issue of Science Magazine.
|A sidewinder snake traverses the sand-filled trackway at Zoo Atlanta. Credit: Rob Felt. Courtesy of John Toon.|
"We used the robotic snake as a physical model in order to better understand the sidewinder's movement" said Goldman. And by understanding the sidewinder, the robotic snake's motion also improved.
Sidewinders oscillate side-to-side and up-and-down with respect to the ground, sending two offset waves traveling down the length of their body. Crucially, they do not slide like most snakes but instead lift and displace some segments of their body while keeping others fixed to the surface, maintaining a static friction while in motion. This could be the key adaptation that allows successful navigation through the sand, particularly on hills.
Goldman has been interested in how animals move through granular mediums for a long time, and has previously published research on how lizards 'swim' through the sand. These animals use the fluid-like properties of sand as well as unique shape of their bodies for locomotion.
But under certain conditions, sand can also behave like a solid. This makes both sand castles by the beach and dramatic desert dunes possible.
"For larger animals like the sidewinder, we see them using the more solid properties of the sand. But it's a balancing act because too much force and the sand yields" explained Goldman.
Yield occurs when a material collapses or falls away under a critical amount of force. Surprisingly little is known about how the yield point of sand changes with inclination. By conducting a series of follow-up tests, the research team found that less force is required to create yield on steeper inclines.
To test how the snakes handled this, sidewinders were let loose on an artificial sand dune constructed in Zoo Atlanta, complete with sand shipped all the way from the sidewinder's native Arizona habitat.
Goldman's team varied the inclination angle from 10 to 20 degrees and found that on steeper surfaces, sidewinders effectively decrease the frequency of their oscillations in order to increase contact area with the sand. This provides additional grip without digging deeper into the unstable sand.
By programming Elizabeth to similarly increase her contact length as the inclination angle increased, the team soon had her climbing much steeper hills with ease. "This is a great example of biology meets robotics, mediated by physics" says Goldman, quoting his colleague and co-author Howie Choset.
Applications of versatile, agile robot snakes include search and rescue missions, archeological digs, and maybe even extraterrestrial exploration.
"It's my hope to see this robot technology in the next generations of rovers" confessed Goldman.
Curiosity, you may have some competition in those future Martian hill races.
By Tamela Maciel, also known as "pendulum"