The shrimp pistol, aka shrimp, is a peculiar contradiction. A few centimeters long, he wields a claw of proportionate size and another massive one that snaps with such force that the resulting shock wave makes the cold prey fall. When the two bits of the claw come together, the bubbles form and then quickly collapse, firing a plasma projectile which in turn produces a flash of light and temperatures of 8,000 degrees Fahrenheit. That's right – an underwater creature that fits into the palm of your hand can, with a claw stroke, arm an explosion of insanely hot bubbles.
Now scientists are learning how to wield this formidable force. Today in the journal Science Advances the researchers describe in detail how they modeled a robotic claw after the plasma gun of the gunmen generated the plasma. This could find a number of underwater uses, once scientists have refined their version of one of the strangest inventions of evolution.
If all the shrimp in the gun have a hammer that blows up the plasma, the whole world really looks like a nail. He uses his claw to hunt, of course, but also to communicate with short shots that measure a mad decibel of 21
Texan mechanical engineer A & M David Staack has realized that versatility could prove useful for humans too. His team started by finding some live pistachio shrimp. Like other arthropods, these animals periodically change, spreading their exoskeletons as they grow. Those exoskeletons gave Staack a beautiful claw cast, which he then scanned to create a detailed 3D model. This sent Shapeways, the commercial 3D printing service, and retrieved a plastic version of the gunmen's plasma gun.
This allowed Staack to experiment with the unique structure of the limb. The upper half of the claw, which the shrimp repel and block, includes a "plunger", which slams into a "grip" in the lower half of the claw. This creates a flow of water that moves quickly and produces bubbles, also known in this situation as cavitation.
"This reminded us of a mousetrap," he says. "So we did some experiments in which we put some mouse traps under water just to see how fast the little arm rotated while you activated it. We got the idea of the mousetrap and we have applied as a way to break the claw. "
In Staack's version of the claw, his upper half rotates quickly on a spring-loaded rod, creating enough force to crush the plunger into the socket . This action generates a flow of water at high speed which in turn produces a cavitation bubble, which is initially low pressure and relatively large. But then it begins to collapse.
"The water enters, and pushes in, and pushes in, and you get very high pressures and temperatures," he adds. The temperatures are so high, in fact, that they create light-emitting plasma, which can also be seen when the crayfish of the gun snaps its claw. "While trying to push back the water, it emits a shock wave." This is how the crustacean spreads its prey in freedom.
In the lab, the researchers used high-speed cameras to observe the jet of water bursting from their claw. They also examine the resulting shock waves, capturing the flash of light while the plasma forms.
Pistol shrimp do not have a monopoly on underwater plasma generation. People weld themselves under water using plasma, known as plasma arc welding, which produces intense heat. And researchers can also do plasma in water with lasers. The problem is that those means are inefficient. The use of the claw to generate plasma is 10 times more efficient than those previously explored, according to Staack. However, it will require more development to scale it down.
It could also become even more efficient, because researchers do not need to faithfully follow the biology of pistol shrimp. In fact, Staack realized that they could cut the size of the upper tip of the claw. In the real shrimp of the gun, it is bulbous because it contains the muscles necessary to operate the limb. But this robotic version is not bound by that biology.
"Replicating what the animal has done is the first step," says Stanford University biologist Rachel Crane, who helped develop Ninjabot, a device that replicates the mantis shrimp shot that similarly produces bubbles of cavitation. "Then you can look at it and understand, yes, I don't need a giant muscle, so I can cut this part. So you can design a better system."
Researchers may also want to look back at nature for ways to modify the system . Hundreds of species of shrimp in the shape of a gun are escaping into the sea, each with its own unique claw. That and even the people within a species varies in their morphology.
"The substrate for the evolution, the only reason why today we have shrimp prawns of all these different varieties, is due to individual variation," says biologist Sheila Patek, who studies the strike of mantis shrimp. Thus, while researchers can make their own modifications to their claw robots, they can also draw inspiration from the intrinsic diversity of pistol shrimp to play with claw morphologies other than the one originally printed in 3D.
That diversity could one day see a device inspired by pistol shrimp used in different fields. One approach would be to use the plasmas generated by the claws to pierce the rock, as the crustacean does in nature to create a house in a coral reef. Or you could use the water purification system by splitting water into its constituent parts, which form a peroxide. "These peroxides can then attack organic contaminants in the water," says Staack. "If you are thinking of cleaning municipal water or cleaning the wastewater, the efficiency becomes very important."
And so the pistol shrimp find some other nail.