How gear automated submarine could quickly and securely anchored in the sediment? Engineers have raised the issue and found the answer in kind, drawing the knife, this long shell which plunges deep and high-speed sand beaches. His secret: change the properties of the sediment that surrounds it.

All sailors know. Installing an anchor to hold the boat is a delicate art and must, moreover, consider how we emerge from the bottom to go back to the board. The problem is starkly gear for automated submarines that must arise on the merits without excessive movement.

MIT (Massachusetts Institute of Technology), a team of Hatsopoulos microfluids Laboratory, one led by Anette (known as Peko) Hosoi has taken on the task. Amos Winter, one of the researchers involved, presented the solution to the last congress of the American Physical Society: just imitate the knife. This mollusk bivalve (such as oysters and mussels) lives in the sand and can dip into it at a surprising speed to stay firmly plugged. There are several species, all characterized by a tapered shape, resembling a knife. The MIT researchers were interested in Ensis directus called American knife in France since it shows a marked tendency to invade our shores, to the detriment of local species.

Tall, it measures more than 15 centimeters in length, 2.5 wide. Its performance-hole diggers seemed inexplicable. First, speed. E. directus buried in the sand at a rate of one centimeter per second! About the strength of anchoring, ie that it must implement to the back, Amos Winter asserts that the mollusk is at least ten times better than all existing mechanisms, given its size and its mass.
As a first step, the researchers mimicked the shape of the knife and made a plastic embedded in a substrate of fine glass beads, simulating sand. Unfortunately, this simplistic model escape easily. In short, the knife, the top of its millions of years of existence, must have a secret. A force of observations (a video is available on the magazine MIT), it has finally been pierced.

An expert in fluid mechanics

The U.S. team has installed some of these animals in a sand box with a glass wall and filmed their movements with a camera fast. The pictures showed a brief sequence of two gestures. The mollusk extends down some muscle and tortilla quickly. Almost simultaneously, it opens and closes its shell. The animal then sank suddenly. This series of movements that progress in the sand.

Also inexplicable … Experiments were then allowed the team to imagine a hypothesis and verify it. The rapid movement of the knife shaking the sediment around him and cause a partial liquefaction. The increase becomes much easier and can sink the knife in a lower resistance.

During the summer, Amos Winter has attacked the realization of RoboClam (in English, the knife is called razor clam), a device the size of a lighter capable of movements similar to the animal. The machine is still a prototype but only an experimental model to test hypotheses on the techniques of Burrowing bivalve. The boats they will one day captain a few robots that simply the plunge to take care of themselves anchoring?

Over the past summer, Winter completed the RoboClam itself. Although only about the size of a lighter, it is supported by a large apparatus of pressure regulators, pistons and more that control such things as how hard the robot is pushed in each direction.

“Right now we’re getting it up and running” for tests, Winter said. Among them, “we want to use RoboClam to verify the theory we’ve generated to describe how to dig like a clam.”

This work was sponsored by Bluefin, Battelle and Chevron.

A version of this article appeared in MIT Tech Talk on December 3, 2008 (download PDF).