for National Geographic News
Walking on water may seem miraculous, but for tiny aquatic snails, it's an everyday activity. Now, scientists have puzzled out the snails' baffling method of propulsion.
"How the snails were dragging themselves across a surface that they could not even grip was absolutely perplexing to us," said lead author Eric Lauga, a professor of mechanical and aerospace engineering at the University of California, San Diego.
"Hanging on to the water's surface is not the issue for the snails. They are naturally buoyant, because they are so small," Lauga said.
Even so, the snails need traction to move across the slippery surface. Think of humans trying to walk on ice—they don't break through, but their feet can't get a grip.
By making small rippling motions with its foot, the snail creates traction for itself, Lauga and his colleagues found after studying videos of the snails. The researchers' observations are detailed this month in the journal Physics of Fluids.
The snails' ability to move depends on water's tendency for its surface to resist disturbance. Water "wants" to stay flat, Lauga said.
When the snail ripples its foot, similar ripples are created on the water's surface. The ripples generate a downward force as the water flattens itself.
These ripples are just the right size for the snail to use to push itself along. "If the ripples were too small, the snail would slip, as on ice," Lauga said.
"If the ripples [were] too big, the snail could not 'grab' them."
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"The overall message is very interesting. I have seen other forms of fluid motion driven by surface undulations, but I do not recall having ever seen any study that demonstrated propulsion [like this] before," said Howard Stone, a professor of engineering and applied mathematics at Harvard University.
"New insights on small- or large-scale propulsion open our minds to thinking about new ways to tackle problems," said Stone, who was not involved in the study.
"The [mechanism] shown in this paper as a means to propulsion is quite appealing."
While the discovery may evoke visions of water-walking shoes, such footwear is unlikely.
For humans to distribute their weight widely enough to not break the water's surface tension, each shoe would have to be several times larger than a football field, Lauga explained.
But fantastic applications are not completely out of the question.
"It would be really interesting to build small-scale robotic versions of these snails to see if our understanding of their locomotion is correct and if we can recreate it," Lauga said.
While he doesn't see immediate practical applications for such devices, "in my wildest dreams I can see James Bond releasing robotic snails on water to spy on his enemies."
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