Liquid armor using shear thickening fluid (STF) is being developed at the U.S. Army Research Laboratory at Aberdeen Proving Grounds. STF is made up of hard silica particles suspended in polyethylene glycol, a non-toxic fluid. Kevlar fiber vests soaked with STF are pliable under normal conditions.
When danger looms in the new Matrix movie, the heroes step into flexible armor. The U.S. Army wants something even more amazing for our soldiers—uniforms that turn into lightweight armor on command.
"During normal handling, the fluid is very deformable and flows like a liquid. However, once a bullet or frag hits the vest, it transitions to a rigid material, which prevents the projectile from penetrating the Soldier’s body," said Dr. Eric Wetzel, a mechanical engineer from the Weapons and Materials Research Directorate who heads the project team.
Liquid armor could be used in bomb blankets, and even jump boots, which could be made to stiffen upon impact to support the ankle.
"Shear thickening" fluids (also called "dilatant" fluids) have unusual (that is, non-Newtonian) behavior in response to a shearing force. Their viscosities increase or decrease as the applied shearing stress changes. Cornstarch and water form a shear thickening fluid; when poked hard with a finger, the mixture reacts as a solid. However, if you place a "cube" of it on a table, it slowly flows into a puddle.
Not to be outdone, MIT has been working on a form of liquid armor that uses a class of fluids called magnetorheological fluids. They are made up of nanoparticles of iron in a thick oil or syrup suspension. When a magnetic field is applied, the iron particles align and the fluid becomes extremely stiff. The degree of stiffness varies depending on the strength of the field applied. It would be possible to wear comfortable, flexible armor that would become rigid at the flick of a switch.
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| Oily fluid full of tiny iron particles before being near a magnet (above); and after. |
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Update: Take a look at these recent stories:
Since troops won’t be carrying magnets on the battlefield, Research Director Gareth McKinley says he would wire the fabric of military uniforms with electric current that soldiers could switch on to produce electromagnetic fields. "Ultimately, the way the magnetic field would be applied in a suit, would be that the electromagnets or micro magnets would be part of the actual fabric suit system," says McKinley. "Working with Patrick Doyle, professor of chemical engineering at MIT, we’ve already explored the possibility of using very small magnets as valves to turn these fluids on and off."
McKinley emphasizes that this "instant armor" is definitely not combat-ready yet. "We’ve got five to ten years of research before we can make this material truly bullet-resistant," he says. Right now, he’s working with
T. Alan Hatton, also of MIT’s chemical engineering department, on making his tiny iron particles in different shapes. “The particles that we’re using now look like soccer balls. They’re spherical," says McKinley. "If you were to stack a pile of those together, you don’t get very far. What would be better would be to have faceted particles, particles with flat faces, or particles that look more like donuts. You could stack a pile of donuts on top of each other.”
McKinley and his team also would like their fluid to change even faster, and to remain reversible for years.
See a science fictional take on this subject in an excerpt from an early Larry Niven story; the flexible armor suit is the earliest reference I can find in sf. See also the article on AKROD v2 knee rehabilitation device, which uses a similar fluid. Scroll down for more stories in the same category.
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