Norman Wagner at the University of Delaware and Eric Wetzel at the US Army Research Lab in Aberdeen, MD, have invented a liquid armor that may soon lead to light, flexible full-body armor.
The liquid - called shear thickening fluid is actually a mixture of hard nanoparticles and nonevaporating liquid. It flows normally under low-energy conditions, but when agitated or hit with an impact it stiffens and behaves like a solid. This temporary stiffening occurs less than a millisecond after impact, and is caused by the nanoparticles forming tiny clusters inside the fluid. "The particles jam up forming a log jam structure that prevents things from penetrating through them," Wagner explains.
And for the real geeks out there who want a word to drop into casual conversation, materials with the property are called 'inverse thixotropic' or 'rheopexic'.
You can do a variation on this at home with cornstarch and water. Fill a medium sized bowl with cornstarch and water to a gooey consistency. You can pour it, and dip your hand in slowly. If you punch it, however, all the molecules line up with the agitation and it will be rock solid.
Larry Niven called this "impact armor" in his "Known Space" stories...
"...Louis ran along the fence toward the walkway. The soldiers were close... The roar was disconcerting, even terrifying. A slug slammmed into Louis's ankle; the suit went rigid, and he rolled like a tumbled statue, picked himself up, and ran again. As two soldiers threw themselves at him, he swung over the fence and dropped...
"...Louis ran along the fence toward the walkway. The soldiers were close... The roar was disconcerting, even terrifying. A slug slammmed into Louis's ankle; the suit went rigid, and he rolled like a tumbled statue, picked himself up, and ran again. As two soldiers threw themselves at him, he swung over the fence and dropped...
Louis landed flat on his face, in an impact suit gone rigid as steel. The form-fitting coffin supported him, but he was still stunned..."
--"The Ringworld Engineers" ©1980 by Larry Niven