Slimy snails do the (underwater) locomotion

Bryn Nelson Columnist

How can a snail crawl along the surface of a pond while hanging upside-down underwater, especially when there’s seemingly nothing to grab? The spot-on snail solution, according to a team of engineers, is finding the perfect balance between surface tension and the motion of the snail’s large, slime-trailing foot.

The unique propulsion system, based on distorting the water surface just enough to get a grip, might inspire future applications such as small robotic swimmers that slink beneath the surface while carrying out military or environmental applications.

Snails and other animals have long inspired studies on locomotion. And locomotion, in turn, has given engineers plenty of fresh ideas for lumbering, crawling, swimming and even flying robotic devices based on everything from geckoes and cockroaches to bluegill sunfish and flies.

By observing how snails lubricate and adhere to surfaces, mechanical engineer Anette Hosoi and her team at Massachusetts Institute of Technology created several versions of a battery-powered imitation dubbed RoboSnail a few years ago. Among the potential applications: a device that could help with oil exploration by crawling across viscous petroleum-covered surfaces in hard-to-reach nooks and crannies.

Eric Lauga, an assistant professor of mechanical and aerospace engineering at the University of California at San Diego, has collaborated multiple times with Hosoi and other MIT researchers to study the movement of land snails, though his foray into examining their primarily water-based counterparts began by sheer chance.

About a year and half ago, MIT graduate student David Hu went to a pond a few miles from the Cambridge, Mass., campus to study the mechanism of water strider movement. While there, he also saw water snails moving just beneath the surface and decided to film them and take pictures.

Video   Underwater snail locomotion Engineers have solved the secret of how slime-trailing water snails like this one move upside-down along the water's surface. They solution, they say, is the snail's ability to find the perfect balance between the surface tension and the wrinkling motion of its large foot, permitting it to literally get a grip. MSNBC

“At some point we realized that, well, this movie of these snails crawling underneath the surface is a little bit puzzling,” Lauga said.

What, exactly, were they hanging onto to help them move?

On solid ground, he said, “when you push off with your foot, the ground is able to sustain that force and push back.” The alternating push-off and push-back with an equal but opposite force is how we’re able to walk. Not so much on sheer ice, where a strong push could leave you on your tush.

The problem is even greater in water, for the reason that it’s incredibly difficult to find the “sweet spot” in balancing force and surface tension.

“If the interface that the water makes with the snail is completely flat, then the snail would not be able to move,” said Lauga, a specialist in fluid mechanics who co-authored a study on the water snails for the August issue of the journal Physics of Fluid. “So why aren’t they just slipping and not able to do anything?”

The key, he said, is that the fluid surface doesn’t have to be flat. Water snails have apparently perfected the act of playing with the shape of a deformable interface — in this case, the surface of a pond held intact by surface tension. “It’s like when you’re on an air mattress, and when you move a little to the left or right, the shape moves,” Lauga said.

Every time the snail wrinkles up its foot, it creates small ripples through the slime trail and surrounding water. The slight deformation of that surface allows the snail to “grab” it and push itself onward, even though the grip is to nothing more than a moving fluid.

MORE FROM FRONTIERS

Tourist or terrorist? Computer knows

Tourist or terrorist? Computer knows Climbing no obstacle for snakebots The next 'American Idol'? Ask your computer How a deadly avalanche can be triggered Green alternative to plastics: liquid wood Getting lost for better architecture Turning air into drinking water Revving up the race for fuel efficiency How to be a disagreeable (but likable) robot Can concept of clean coal be salvaged?   Add Frontiers headlines to your news reader: