A fierce shrimp inspires a robot claw that draws plasma

The shrimp with the pistol, aka slamming shrimp, is a particular contradiction. With only a few centimeters long, it uses a proportional sized claw and another massive slam with such force that the resulting shock wave knocks its prey cold. When the two parts of the claw meet, bubbles form and then collapse quickly, throwing a plasma bullet that in turn produces a flash of light and temperatures of 8,000 degrees Fahrenheit. That's right, an underwater creature that fits in the palm of your hand can, with a simple gesture of its claw, turn into a weapon an explosion of extremely hot bubbles.

Scientists are now learning to use this formidable force themselves. Today in the newspaper Progress of science, the researchers explain how they modeled a robotic claw after the plasma pistol of shrimp guns to generate their own plasma. This could lead to many underwater uses, once scientists have tweaked their version of one of the strangest inventions of evolution.

If all that pistol shrimp has is a plasma hammer, the whole world looks like a nail. He uses his claw to hunt, of course, but also to communicate by short clicks that measure 210 decibels. (A pistol shot produces about 150 decibels.) Some species even use plasma explosions to carve pieces of reef for shelter. The result is a seabed so noisy that it can interfere with the sonar.

Texas A & M mechanical engineer David Staack felt that versatility could also be useful for humans. His team began by getting live shrimp with a spray gun. Like other arthropods, these animals moult periodically and lose their exoskeletons as they grow. These exoskeletons provided Staack with a nice little cast of the claw, which he then digitized to create a detailed 3D model. He sent it to Shapeways, the commercial 3D printing service, and retrieved a plastic version of the shrimp plasma gun.

This allowed Staack to experiment with the unique structure of the member. The upper half of the claw, which the shrimp pick up and lock, includes a "diver" who sinks violently into a "socket" located in the lower half of the claw. This creates a rapid flow of water that produces bubbles, also called cavitation in this situation.

"It reminded us of a mousetrap," he says. "So we did experiments in which we put mouse traps under the water just to see how fast the little arm turned when you triggered it. We took this idea of ​​mousetrap and applied it as a way to close the claw. "

In the version of Staack's claw, its upper half rotates quickly on a spring rod, generating enough force to push the plunger into the socket. This action generates a high velocity water stream which in turn produces a cavitation bubble, which is initially low pressure and relatively large. But then it starts to collapse.

"The water enters and grows and grows, and you get very high pressures and temperatures," he adds. The temperatures are so high, in fact, that they create a plasma that emits light, which you can also see when the pistol shrimp slams its own claw. "As he tries to repel the water, he sends a shock wave." This is how the crustacean stuns its prey in the wild.

In the lab, researchers used high-speed cameras to observe the jet of water coming out of their claws. They also imagined the resulting shock waves, capturing the flash of light as the plasma forms.

Spray shrimp does not have a monopoly on submarine plasma production. People weld under the water using plasma, called plasma arc welding, which produces intense heat. And researchers can also make plasma in the water with lasers. The problem is that these means are ineffective. The use of the claw to generate plasma is 10 times more efficient than previously explored methods, according to Staack. However, it will take more development to scale.

This may well become even more effective, as researchers would not need to faithfully follow the biology of the sprayed shrimp. In fact, Staack realized that they could reduce the size of the top edge of the claw. In the real shrimp with the pistol, it is bulbous because it contains the muscles necessary for the functioning of the limb. But this robotic version is not constrained by this biology.

"Reproducing what the animal did is the first step," says Rachel Crane, a biologist at Stanford University, who helped develop Ninjabot, a device that reproduces the shrimp mantis shrimp, which produces also cavitation bubbles. "So you can look at that and understand, yeah, I do not need a giant muscle, so I can cut that part off. Then you can design a better system.

Researchers may even want to come back to nature to find ways to change the system. Hundreds of species of pistol shrimp disappear into the sea, each with its own specially adapted claw. That and even individuals in a species varies in its morphology.

"The substrate of evolution, the only reason we have today caught shrimp of all these varieties, is due to individual variation," said Duke biologist Sheila Patek, who is studying mantis shrimp strike. Thus, while researchers can make their own modifications to their claw robot, they can also draw inspiration from the inherent diversity of pistol shrimp to play with claw morphologies other than the original 3D print.

This diversity may one day see a device inspired by the shrimp gun used in a variety of areas. One approach would be to use plasmas generated by claws to pierce rocks, as the crustacean does in the wild to settle in a reef. You can also use the water purification system by breaking down the water into its components, which forms a peroxide. "These peroxides can then attack organic contaminants in the water," says Staack. "If you plan to clean municipal water or sewage, efficiency becomes very important."

And so, the shrimp with the gun still finds some nails.

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