— 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.
“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.
Paying attention to tension
Could the same physics concepts be applied to tiny biomimetic devices designed to crawl beneath the surface? Like the water snails, the devices would have to be buoyant enough to remain near the surface, Lauga said. And like the snails, the application of a slight deformation to the surface would have to be precisely calibrated.
“If it’s too weak, the interface does not deform. It remains flat and you would slip just like on ice,” he said. “If it’s too strong, it’s like you’re trying to walk in yogurt — the interface is not strong enough to push back on you.”
The limiting factor in the whole equation is surface tension, the very thing that makes water not want to deform.
“When you’re taking a bath and splashing with your hand, you’ve easily cut through the surface tension,” he said.
But an ant taking a bath would find it very hard to splash around, because at that level surface tension has become a significant force. If the movement is too feeble, the surface tension wins and the surface barely registers a ripple. If the movement is too strenuous, then the surface tension is no longer relevant and there’s nothing to grip onto.
“The snail has to figure out how to apply the right force — you have to tune yourself to exploit this ‘sweet spot’ of surface tension. If you’re doing too much or too little, it won’t work,” he said.
Howard Stone, an expert on fluid mechanics at Harvard University, described the study’s take-home message as “very interesting and not a theme that I had seen explored before.”
Stone has worked with Lauga and another co-author in the past but wasn’t involved in the latest research. In an e-mail, Stone wrote that he has seen other examples of fluid motion driven by surface undulations. But Stone said he didn’t recall any study that previously showed how a substrate (the snail’s oversized foot in this case) uses a wave-like deformation that couples with the shape of a free surface (the rippling pond) to produce propulsion.
Stone said he could imagine how a small device for cleaning oil slicks or other contaminated surfaces might benefit from similarly skimming just beneath the surface.
I-Ming Chen, a robotic locomotion expert in the School of Mechanical and Production Engineering at Nanyang Technological University in Singapore, agreed that Lauga’s study suggests a possible new propulsion principle for miniature underwater vehicles.
“Though military applications are the immediate thought,” he said in an e-mail, “a more promising one will be in environmental monitoring applications, like autonomous surface water quality monitoring,” as well as observing phenomena right at the boundary separating water from air.
Shrinking the robot could lead to applications within the human body, Chen said, while scaling it up could lead to new gadgets for water recreation. And combining the principle with water strider-inspired mechanics, he noted, could lead to an even more versatile robot capable of traveling along the water-air interface.
For any future application, Lauga figures the devices would have to be likewise tuned to make the most of surface tension and limited to the snails’ inch-long body size. But that will have to come from other labs.
As for Lauga, he said he’d like to take a step back and look at mollusks in general. After all, how many other simple physics problems that have not been well understood might they help illuminate?