The researchers hope the liquid could be used in sleeves and pants, which are not protected by ballistic vests, and have to stay flexible.
"It's in the works," said Eric Wetzel, a mechanical engineer who heads the project team for the U.S. Army Research Laboratory at the Aberdeen Proving Ground. "We've seen some very interesting things, and it's got a lot of promise."
The liquid, hard particles suspended in a fluid, is soaked into layers of Kevlar, which holds it in place. Scientists recently tested how well the liquid boosted the strength of a Kevlar vest by having an archer shoot arrows at it.
"What we find is that, instead of the arrow going through the Kevlar, it is completely stopped by the Kevlar vest — and sometimes just bounces right off," said Norman Wagner, a chemical engineering professor at the University of Delaware who is working on the project.
Vests treated with the liquid also have blocked stabs from an ice pick.
Researchers are working on further ballistics testing to stop bullets or shrapnel.
"There's opportunities there to, without adding weight or reducing flexibility, to provide protection against these types of threats as well," Wagner said.
The project, which has been underway for about three years, is a joint venture between the U.S. Army Research Laboratory and the University of Delaware's Center for Composite Materials.
Wetzel and Wagner are optimistic the liquid body armor will be useful to local police and prison guards. It also could one day be used to protect people in automobile and airplane crashes, they say.
A key component of the liquid armor is what Wetzel called a "shear thickening fluid." Hard particles are suspended in the liquid, polyethylene glycol. When the fluid is added to the vest, it changes how the "fabric architecture" interacts with an object, Wetzel said.
At low strain rates, the hard particles flow with the fluid, enabling clothing to stay flexible. But when heavily strained, the particles become rigid and "create a structure that can support mechanical stress," Wagner said.
"If it's impacted suddenly by a projectile or a knife, say, it rigidifies and somehow restricts the ability of the fabric to move — keeps the fabric in front of the bullet or the knife," Wetzel said.
The transition happens very quickly, a millisecond or quicker.
"If you relieve the stress it goes back to a liquid," Wagner said. "It's a very interesting material in that way."