What happens when you put snakes on a plane? No, not with Samuel L. Jackson – on a steep inclined plane. Generally, the animals will begin to slide down. But they can halt their fall by actively changing the positioning of their scales to increase friction. Knowing this has allowed robotic engineers to build better search-and-rescue robots whose "feet" function like snake scales.
Why snakes? "They're very cute and very easy to work with," Hamid Marvi, a Georgia Institute of Technology Ph.D candidate, joked at the American Physical Society March Meeting. More seriously, he explained how a snake's ability to crawl through narrow openings and scale heights makes it a "champion animal" that could easily navigate the site of a natural disaster. Robots that move like snakes could locate and bring aid to victims trapped in rubble.
Marvi and other researchers at Georgia Tech started out by studying how snakes' scales let them use anisotropic friction, or friction that is greater in one direction than another, to their advantage. For example, put a sleeping snake on one of those inclined planes. When you orient it so that it slips down head-first, the friction is twice as great as when the snake is oriented to slide down sideways.
This passive mechanism has to do with how the scales are layered on top of each other. You can feel the difference yourself: if you run a hand along a snake's belly from tail to head or side-to-side, the scales will hit sharply against your skin, but if you run your hand in the opposite direction, the scales slip smoothly past. Even the micro-structure of the scale is textured so that the coefficient of friction will be different when moving towards the snake's head or towards its tail.
Beyond the passive mechanisms, a conscious snake has even more control over its slide. "Snakes can actively control their frictional properties," explained Marvi. When an alert snake begins sliding down an inclined plane, its ventral muscles can modify the angle at which its scales hit the surface, increasing its friction.
And without scales to generate friction, a snake can't slither anywhere. The ability to move forward is dependent on the snake having a different coefficient of friction in different directions, which lets it push off of small bumps and rough patches in a rough surface. If you cover the snake's scales in a sock-like cloth "jacket," its friction becomes equal in all directions. The snake can still move its body in the characteristic S-like motion, but the wave doesn't drive the snake forward.
In addition to modifying the position of its scales, a snake can boost its speed by lifting segments of its body off the surface as it moves along. As its body twists into an S, the snake lifts up the curves of the S – removing any friction from the parts of the body that are moving sideways – while keeping the straighter middle segment in contact with the ground – increasing the friction on the part of the body that is moving forward.
To simulate a snake's motion on uneven ground, its scales must be taken into account, and that's exactly what the Georgia Tech team did. Based on their results, they created Scaly-Bot, a robot that can climb slopes. "You need to adjust the scales so you can grip with the substrate very well and the robot doesn't slide down the hill," Marvi says. By changing the angle between its scales and the surface, Scaly-Bot can adjust its friction and climb up.
And Scaly-Bot's successor, Scaly-Bot 2, has even more features, including steering, flexibility, and an accelerometer to sense when it's sliding so its scales can "sweep," seeking the correct angle to prevent further slipping. In fact, two of Scaly-Bot 2's eight motors are dedicated to controlling the angles of its "scales." Of course, a side effect of all these motors is that Scaly-Bot 2 is noisy and a bit ungainly. But then, the Georgia Tech team behind Scaly-Bot 2 is not a team of roboticists.
"Our focus is not on robotics, but on finding mechanisms to help roboticists develop more efficient robots," Marvi said. Further work is ongoing to turn Marvi's work into the materials and mechanisms that could let robots scale heights as easily as snakes do.