Liquid Armor In Two Flavors: Shear Thickening and Magnetorheological





Liquid armor using shear thickening fluid (STF) is being developed at the U.S. Army Research Laboratory at Aberdeen Proving Grounds. The STF referred to in this article 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: "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.

(From Army Creates Liquid Armor) 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.



(Fluid aligned to magnet [bottom left], from Instant Armor)

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