Slimy snails do the (underwater) locomotion

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?

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: