Fluid defences

A SUIT of armour that is lightweight and flexible, yet capable of absorbing the impact of a bullet, is an idea that seems to come from the future—a bit like the liquid skin of the cyborg in “The Terminator”. However, for a number of years researchers have been investigating materials that might be able to provide such protection. Now BAE Systems, a British defence contractor, seems close to turning such “liquid armour” into reality.

In recent tests at the company's Advanced Technology Centre, in Bristol, liquid armour has been used to construct a material that is thinner than normal body armour and yet is able to stop a low-velocity bullet. Work is now under way to see if it can also stop more powerful bullets. If these tests are successful, liquid armour could furnish soldiers with protective clothing that is more comfortable and less cumbersome than existing body armour, and provides them with greater protection.

Liquid armour relies on the properties of materials called shear-thickening fluids. The molecules in such liquids are closely packed, but loosely arranged. The result behaves like a liquid in normal conditions, and is able to flow. If subjected to pressure, though, the molecules lock together and the result behaves like a solid. The gooey substance produced by mixing cornstarch with a little water is a commonplace example of such a material. This flows like a liquid when poured yet, if hit with a spoon, reacts like a solid.

Researchers at the University of Delaware developed shear-thickening fluids for use as liquid armour almost ten years ago. Several military laboratories and defence firms have worked on them since then. Among them was Armour Holdings, an American company that BAE took over in 2007.

The fluid that Armour Holdings worked on is a liquid polymer containing nano-engineered particles of silica, though BAE is coy about the details of what it is using. The new armour is not pure liquid, of course. Rather, the fluid is sandwiched between layers of Kevlar, a high-strength polymer commonly employed in protective gear. When the sandwich is struck by a bullet, the locking together of the molecules in the fluid spreads the load across a wide area, allowing the material to absorb more of the impact. It also distorts less than conventional body armour when it is hit. That means it is less likely to cause serious injury by deflecting inward.

Standard body-armour contains around 30 layers of Kevlar. The material being tested by BAE has only ten Kevlar layers. BAE therefore thinks it may be possible to build body-protection suits that are only around half as thick as existing ones, but still able to offer greater protection. Soldiers would find that a welcome relief.